Abstract:
This invention concerns a method of operating a cryogenic cooling device (1) with a cylinder (4, 5) in which a piston (6, 7) reciprocates and with a gas drive (8, 9) which produces the motion of the piston. In order to reduce the vibrations which occur during operation, the invention proposes that the gas drive (8, 9) is controlled in such a way that the piston (6, 7) is only accelerated for part of the stroke.

Description:
This application is a continuation of application Ser. No. 08/557,053 filed Feb. 26, 1996, now abandoned, which is a 371 of PCT/EP94/01012 filed Mar. 31, 1994. 
    
    
     BACKGROUND OF THE INVENTION 
     Cryogenic cooling devices 1 ) are low-temperature cooling machines in which a thermodynamic cyclic process is running. A single-stage refrigerator comprises a working space in which a displacer reciprocates between two dead centres OT and UT. Related to the displacer is a regenerator through which a working gas also reciprocates corresponding to the motion of the displacer. During the reciprocating motion of the displacer, heat is continually withdrawn from the housing of the refrigerator in the area of one of the two dead centres. With a single-stage refrigerator of this kind it is possible to generate temperatures down to about 30 K. Often refrigerators are of a dual-stage design (refer to DE-A-38 36 884, for example). With dual-stage or three-stage refrigerators it is possible to generate temperatures down to below 10 K. In the refrigerator according to the mentioned document, a gas drive is employed to produce the reciprocating motion of the displacer. For this, a cylinder and piston arrangement is related to the warm side of the displacer whereby said arrangement must be supplied with a driving gas. 
    
     Refrigerators of the kind affected here must be equipped with additional control facilities, through which the supply of the working gas into the working cylinder and also the supply for the gas drive is controlled. It is common to employ helium both as the working gas and the driving gas. In designs of this kind, it is sufficient to equip the refrigerator with two connections, one of which is supplied with high pressure helium (20 bar, for example) and where the other is supplied with low pressure helium (5 bar, for example). 
     During the motion of the displacer, forces occur which are greatest at the dead centres. These forces are transferred to the housing of the refrigerator and thus also to any connected devices. Generally, such devices will be highly sensitive measuring instruments (nuclear magnetic resonance tomographs, electron beam microscopes, for example), the measurement results of which are adversely influenced by the occurring vibrations. In the past, several proposals have been made to dampen these interfering vibrations. From the European Patent 19 426 it is known to arrange a damping arrangement between a refrigerator and an electron beam microscope, but this arrangement is relatively involved. From European Patent Application 160 808 it is known to arrange a flat spring within the working space of a refrigerator. Said flat spring takes up a certain amount of space within the working space so that the efficiency of the refrigerating machine is reduced. In DE-A-38 36 884 it is proposed to separate, in time, the cooling process from the measurement process. This solution requires complex control arrangements and reduces the time available to the measurements. 
     SUMMARY OF THE INVENTION 
     It is the task of the present invention to operate a refrigerator with a gas drive in such a way that an effective reduction of the vibrations can be attained through relatively simple means. 
     According to the present invention, this task is solved by the measures of the patent claims. The present invention relies on the finding that the gas drive in known refrigerators is effective during the entire time of each of the motional phases of the displacer, meaning that the displacer is subjected to a considerable acceleration during the entire time of its motional phases. At the dead centres this leads to relatively strong impacts which are responsible for the vibrations. However, if the displacer is--in line with the present invention--only accelerated at times or only at the beginning of its motional phases, then the final velocities at the dead centres will be lower and the impacts at the housing less hard. By controlling the quantity of gas through which the gas drive is supplied for both motional directions it is possible to control the amount of drive energy and thus attain the desired goal. Through the measures according to the present invention it is possible to reduce the occurring vibrations by a factor of 4 or more, without significantly impairing the efficiency of the refrigerator. 
     DESCRIPTION OF THE INVENTION 
     Further advantages and details of the present invention shall be explained by referring to the design examples presented in drawing FIGS. 1 to 5. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section schematic of a refrigerator of a type which the present invention is applicable; 
     FIG. 2 is a cross-section of an embodiment of a rotating disc of a disc valve in accordance with the invention; 
     FIG. 3 is a cross-section of an embodiment fixed disc of a disc valve in accordance with the invention; 
     FIG. 4 is a cross-section of another embodiment of a rotating disc of a disc valve in accordance with the invention; 
     FIG. 5 is a cross-section of another embodiment fixed disc of a disc valve in accordance with the invention; 
     FIG. 6 is a cross-section of another embodiment of a fixed disc of a disc valve in accordance with the invention; 
     FIG. 7 is a cross-section of another embodiment rotating disc of a disc valve in accordance with the invention; 
     FIG. 8 is a warm-to-cold side motion diagram illustration motional characteristics of an embodiment of the invention; 
     FIG. 9 is a cold-to-warm side motion diagram illustrating motional characteristics of an embodiment of the invention. 
    
    
     The dual-stage refrigerator 1 presented in the drawing figure has a housing which consists of the two parts 2 and 3. The cylindrical working spaces 4 and 5 for the two displacer stages 6 and 7 are housed in housing part 2. 
     The upper displacer stage 6 is equipped, on its warm side, with a drive piston 8, the corresponding cylinder 9 of which is located in a guide bush 10 which terminates the working space 4 against housing part 3. The guide bush 10 is equipped with bore holes 11, 12 and 13. Bore hole 11 opens into working space 4 and serves the purpose of supplying this space with the working gas. Bore hole 13 opens into a cross hole 14 which is connected by a circular groove 15 to the outside wall of guide bush 10. Bore hole 12 is indicated by a dash-dot line and serves the purpose of supplying the drive cylinder 9 with the driving gas. The various bore holes are located in different planes so that they do not cross each other, this being indicated by the dashed or dash-dot lines. They all open into the upper side of the guide bush 10 which in this area is designed as a fixed valve disc 16. 
     Housed in housing part 3 is a motor 17 which actuates a rotating valve disc 19 via the shaft 18. The fixed valve disc 16 and the rotating valve disc 19 which is under the influence of pressure spring 20, form a control valve which serves the purpose of supplying, in a basically known manner, the bore holes with high pressure and low pressure gas. In the design example presented, working gas and driving gas are identical. Preferably helium is employed. 
     The connections for the high pressure and the low pressure gas are marked 21 and 22 respectively. The separating plane 23 between the housing parts 2 and 3 is located at the level of the control valve 16, 19. It is selected in such a way that after removal of the upper housing part 3 with motor 17 and rotating valve disc 19, a flat pot-shaped space 24 is present above the fixed valve disc 16. At the level of this space 24, a bore hole 25 is provided which penetrates the wall of housing part 2 whereby said bore hole connects space 24 with the high pressure connection 21. The low pressure connection 22 is connected to the bore hole 26 in housing part 2 2 ) which opens at the level of the circular groove 15 of guide bush 10. 
    
     During operation of the presented refrigerator, the working gas which is at high pressure flows via connection 21 into chamber 24. From there the various bore holes are supplied with the aid of control valve 16, 19. After its expansion in refrigerator stages 4, 5 the working gas enters bore holes 13, 14 and flows out via circular groove 15 and the low pressure connection 22. The pressure of the working gas which is applied to the high pressure connection 21 is commonly about 20 to 22 bar, whereas the pressure of the working gas at the low pressure connection 22 amounts to about 5 to 7 bar. 
     The speed of the motor 17 and the design of the facing sides of fixed (16) and rotating (19) valve disc are decisive for the gas supply to working space 4 and drive cylinder 9. From DE-A-38 36 884 a design for the control valve is known, where a relatively wide outer recess in the rotating valve disc also supplies, at times, the openings of two bore holes which also have a relatively large diameter with high pressure gas. One of these two bore holes serves the drive cylinder. Due to the selected dimensions, the displacer is accelerated during its entire motional phase from the warm to the cold side of the working space. This equally applies to the motional phase of the displacer from the cold to the warm side of the working space, since the oblong recess which at times connects the openings of the two supply bore holes to the low pressure gas connection, has similarly large dimensions. 
     Drawing FIGS. 2 and 3 show designs for the valve disc 16 and 19, through which the gas can be controlled as desired in the sense of the present invention. The fixed valve disc 16, the top view of which is shown (drawing FIG. 3), permits a view of the openings 27, 28, 29 of the bore holes 11, 12 or 13. As in the case of a state of the art design, the bore hole 13 and its opening 29 are located in the centre whereas the radial distance of the openings 27, 28 of bore holes 11 or 12 is--opposed to the state of the art--no longer the same with respect to the rotational axis of valve disc 19 (circles 31, 32). Moreover, opening 28 of bore hole 12 has a significantly smaller diameter compared to the opening 27 of bore hole 11. 
     The rotating valve disc 19, the design of which is shown in drawing FIG. 2 in a ghosted view through valve disc 19, rotates corresponding to the direction of arrow 33 on the fixed valve disc 16. A bore hole 34 having a relatively wide cross section which connects to high pressure space 24 and which penetrates valve disc 19 is related to circle 31. The recess 35 related to circle 32 is designed as a relatively narrow slot. Moreover, valve disc 19 is equipped with a relatively large recess 36 which extends from the centre to circle 31. Slot-like recess 37 extends from this recess 36 to circle 32. 
     During the rotation of the valve disc 19, the control openings 34 to 37 of valve disc 19 and the control openings 27 to 29 of valve disc 16 pass over against each other. At first slot 35 on circle 32 reaches the opening 28 of bore hole 12. Because of this, the drive cylinder 9 is linked for a relatively short time to space 24 which is at high pressure and the displacer 6 is accelerated in the direction of the cold side of the working space. Thereafter, bore hole 34 on circle 31 of the rotating valve disc reaches the opening 27 of bore hole 11 in the fixed valve disc 16. For the--relatively long--period of time during which these openings 34 and 27 pass over against each other, the working space 4 3 ) is supplied with high pressure gas. Next, the slot 37 on circle 32 reaches the opening 28 of bore hole 12 so that the drive cylinder 9 is linked for a short period of time to the bore hole 13 which is at low pressure. The displacer 6 thus moves to its warm side. Finally, the recess 36 reaches the opening 27 of bore hole 11 on circle 31 so that the gas in the working space 4 4 ) is expanded to a low pressure. Thereafter the cycle repeats itself. 
    
    
     Drawing FIGS. 4 and 5 show a design example where in the valve discs 16, 19 two each bore holes 11, 11&#39;, 12, 12&#39; and 34, 34&#39; which are each offset by 180° and their corresponding openings, are provided. The oblong recess 36 extends on both sides of the centre up to circle 31. Two slots 37, 37&#39; which extend up to circle 32 are present. In this design, the displacer 6 performs twice the number of strokes per turn of valve disc 19. 
     In the presented and described design example, the openings 27, 28 (27&#39;, 28&#39;) of bore holes 11 (11&#39;) 5 ) and 12 (12&#39;) in the fixed valve disc 16 6 ) are located on circles 31, 32 of different radii. Also the bore holes 34 (34&#39;) and recesses 35 (35&#39;), 36 (36&#39;) 7 ), 37 (37&#39;) in the rotating valve disc 19 are located on circles of correspondingly different radii. Thus there exists the possibility of being able to set up large differences with respect to the pass over periods. The supply bore holes for the working space 4 8 ) which must be supplied with high pressure or low pressure for a relatively long periods of time open on circle 31 having the smaller diameter. The supply bore holes for the gas drive open on circle 32 with the larger diameter. Short pass over periods are not attained through smaller bore holes or recesses but instead through the higher circumferential velocity. 
    
    
    
    
     The pass over periods for the gas drive which depend on the speed of the rotating valve disc 19, the position and design of the openings 27 to 29, recesses 35 to 37 as well as bore hole 34, are preferably selected as follows: To effect the motion of displacer 6 towards its cold side, drive cylinder 9 is only linked to the high pressure connection 21 during the first fraction of the time of its entire motion. This fraction is selected--preferably empirically--so that the vibrations caused by the arrival of the displacer at the stops in the housing are considerably reduced and so that the efficiency of the refrigerator is not yet significantly impaired. The order of magnitude of this fraction amounts to about one third of the entire motional period. To effect the motion of displacer 6 in the reverse direction, the pass over time must be selected so short that the link between the drive cylinder 9 and the low pressure connection 22 has already been closed before the displacer attains its dead centre. Due to the further movement of the drive piston, the gas which is still present in the drive cylinder is compressed so that a damping effect (gas cushion) is attained. The pressure increase of the remaining gas is preferably selected in such a manner that a pressure is attained just before the dead centre is reached, which is somewhat higher than the high pressure level. 
     Drawing FIGS. 6 and 7 show design examples of the fixed and rotating valve discs through which an acceleration of displacer 6, 7 is effected several times and for short periods. For this, either the bore hole 28 of the fixed valve disc 16 is (for example) divided into three bore holes 41, 42, 43 or the recesses 35, 37 in the rotating valve disc 19 have been divided into (for example) three recesses 44 to 46 and 47 to 49, respectively. 
     Drawing FIGS. 8 and 9 show the effect of these measures. In each of the upper system of coordinates the path s of the displacer has been drawn vs. the time t. Each of the lower system of coordinates shows the points of time t 1 , t 2  and t 3  at which the high pressure (drawing FIG. 8) or the low pressure (drawing FIG. 9) is applied to the working cylinder 9. 
     Drawing FIG. 8 shows the motion of the displacer from the warm to the cold side. At points of time t 1 , t 2  and t 3  high pressure gas is applied to the drive cylinder for the time t in each case (5% of the time for the entire motion, for example). Thus the displacer is subjected to three brief thrusts which let it move to the cold side in such a manner that the accelerating and decelerating motions alternate (motion curve 51 9 )). The impact of the displacer 6, 7 at housing 2 (dashed line 52 10 )) which substantially causes the vibrations, is relatively small. The motion of the displacer 6, 7 from the cold to the warm side runs correspondingly (refer to motion curve 53 11 ) in drawing FIG. 9 with stop line 54 12 )). The periods of time t are selected in such a manner that in each case a damping gas cushion is generated. 
    
    
    
    
     The designs according to drawing FIGS. 6 to 9 permit the control of the motion sequence of displacer 6, 7 with the aid of short pressure bursts. In comparison to steadily accelerated displacers, the displacer 6, 7 which moves according to the present invention does reach its dead centre at a later point of time; any significant reduction in the efficiency of the refrigerator does, however, not result.