The invention concerns a displacer unit for a Stirling cooling arrangement with a housing, a displacer reciprocating along a displacer axis and an damping mass arranged inside the housing and being connected to the housing via a first spring arrangement.

A Stirling cooling arrangement with such a displacer unit is, for example, known from US 5,895,033. Here, the damping mass is arranged at an inner end face of the housing and is meant to dampen oscillations of the housing. Thus, during operation, an oscillation of the housing caused by the movement of the displacer or the piston is substantially balanced and a quiet operation is achieved. If, however, the housing is retained by a holder, a part of the vibrations caused by the piston or the displacer will be led and transferred directly into the holder, as the counterforces provided by the damping mass can no longer balance this part. This generates undesired noises.

A similar embodiment is known from US 4, 389,849. Here, the damping mass is arranged on the outside of the housing.

Fig. 3 of Veprik, A. M. et al, "Ultra-low vibration split Stirling linear cryogenic cooler with a dynamically counterbalanced pneumatically driven expander", Cy- rogenics 45 (2005)117-122, shows another similar displacer unit.

US 2003/0111311 A1 shows an oscillation damping unit, in which an damping mass is arranged between two helical springs, a bolt being led through each spring. When one helical spring is compressed, the other helical spring expands and vice versa.

The invention is based on the task of keeping oscillations that can be detected at the housing as small as possible. With a displacer unit as mentioned in the introduction, this task is solved in that the damping mass is connected via a second spring arrangement to the displacer, both spring arrangements being more rigid in a direction transversal to the displacer axis than in parallel to the displacer axis, the displacer being con- nected to the housing or the damping mass by a third spring arrangement.

With this embodiment, the damping mass is connected to the displacer via a spring arrangement. The damping mass again is connected to the housing, so that the displacer is also connected to the housing at least via the two spring arrangements. As in a direction transversal to the displacer axis the spring arrangements are more rigid than in a direction in parallel to the displacer axis, both spring arrangements at the same time act as bearings for the displacer, meaning that the displacer can be held in its housing practically without additional bearings. This keeps the friction between the displacer and its housing, or rather a cylinder, in which the displacer moves, small. The smaller the friction, the smaller the wear and the better the efficiency. Further, space is saved, as the bearing of the displacer and the damper can be combined. The third spring arrangement permits an even more accurate alignment of the damper in the housing. In this connection, the displacer can either be connected to the damp- ing mass a second time, or the displacer can be connected directly to the housing via the third spring arrangement. In both cases, it can be achieved that the displacer is supported at two points along the displacer axis, so that it cannot tilt in relation to the housing. This further reduces the friction, thus enabling a long working life. Further, a good efficiency occurs.

Preferably, the first spring arrangement comprises two plate-shaped spring elements arranged in parallel. With a plate-shaped spring element is can easily be achieved that the spring arrangement is substantially more rigid perpendicularly to the displacer axis than in parallel to the displacer axis. The use of two plate-shaped spring elements arranged in parallel and having a predetermined distance to each other ensures that the damping mass can only be moved in the direction of the displacer axis and maintaining a predetermined alignment. If, then, the displacer is supported on the damping mass via the second spring arrangement, it is also ensured that also the displacer can maintain a predetermined alignment to the housing.

Preferably, at least one spring element of the first spring arrangement is ar- ranged between the second spring arrangement and the third spring arrangement. This means that the second spring arrangement and the third spring arrangement must have a certain distance along the displacer axis. The larger the distance, the smaller the tilting tendency of the displacer in relation to the housing.

Preferably, the spring arrangements have spring elements with a linear spring characteristic. Thus, it is possible to initiate a performance change of the Stirling cooling arrangement through a change of the amplitude of the displacer movement. This amplitude change again can be initiated by a change of the ampli- tude of a pressure wave generator supplying the displacer unit. When the springs have linear characteristics, the driving frequency can be maintained, which simplifies the control.

Preferably, the spring arrangements and the damping mass are adapted to each other in such a manner that with a predetermined frequency the displacer and the damping mass perform oppositely directed movements. Thus, the damping mass and the displacer move towards or away from each other. This situation occurs with a resonance frequency. The resonance frequency can easily be set by the selection of the spring constants of the spring arrangements and/or the selection of the damping mass. The corresponding frequency is the set at the expected operation frequency of the Stirling cooling arrangement. This operation frequency is determined by the pressure wave generator. The frequency of the pressure wave generator can be set by a corresponding control of its drive.

It is particularly preferred that with said frequency a product of the damping mass and a maximum deflection of the damping mass is equal to a product of the mass of the displacer and the maximum deflection of the displacer. With such a dimensioning it can be ensured that the centre of gravity of the displacer unit practically remains at a stationary position. If the damping mass is larger than the mass of the displacer, it must be ensured that the maximum deflection of the damping mass is smaller than the maximum deflection of the displacer. However, also this can easily be calculated in advance. When the centre of gravity of the displacer unit is in rest, the displacer unit as a whole cannot be caused to oscillate by a movement of the displacer.

Preferably, the damping mass has at least two partial masses. This has several advantages. Firstly, with a relatively small effort a larger freedom when setting differently large damping masses can be achieved. Secondly, several partial masses can advantageously be used to simplify an assembly, in particular when fixing the spring elements.

It is preferred that at least one partial mass is arranged between two plate- shaped spring elements. Thus, by way of design, it is ensured that these two spring elements must have a certain spatial distance to each other along the displacer axis. As mentioned above, a larger distance is favourable for a stable bearing of the displacer and the damping mass.

It is also advantageous, if at least one plate-shaped spring element is arranged between two partial masses. In this connection, the partial mass can be used to fix the spring element, for example in order to clamp it.

In the following, the invention is explained on the basis of preferred embodi- ments in connection with the drawings, showing:

Fig. 1 a first embodiment of a displacer unit for a Stirling cooling arrangement,

Fig. 2 a second embodiment of a displacer unit,

Fig. 3 a third embodiment of a displacer unit,

Fig. 4 a fourth embodiment of a displacer unit, Fig. 5 a fifth embodiment of a displacer unit,

Fig. 6 a sixth embodiment of a displacer unit,

Fig. 7 a seventh embodiment of a displacer unit

Fig. 8 a eight embodiment of a displacer unit, and

Fig. 9 a ninth embodiment of a displacer unit.

In all figures, the same and corresponding elements have the same reference numbers. All figures show a displacer 1 in a heavily schematic form.

The displacer unit 1 in Fig. 1 is part of a Stirling cooling arrangement, not shown in detail, that preferably has a Gamma configuration. In this kind of configuration, the displacer unit 1 comprises a displacer axis 2, along which a displacer 3 moves, the displacer axis 2 deviating from a movement axis of a pressure wave generator. In particular, the displacer axis 2 is arranged transversely to a paral- IeI on the axis of the pressure wave generator, so that possible oscillations of the pressure wave generator and the displacer unit are decoupled from each other.

The displacer 3 is arranged to reciprocate in a cylinder 4. The cylinder 4 is lo- cated in a housing 5 that surrounds the cylinder 4 approximately concentrically. A regenerator 6 is arranged between the cylinder 4 and the housing 5. A first heat exchanger 7 is provided adjacent to one end of the regenerator 6 and a second heat exchanger 8 is provided adjacent to the other end of the regenerator 6. The first heat exchanger 7 is located between the regenerator 6 and an expansion chamber 9. The second heat exchanger 8 is located between the regenerator 6 and a compression chamber 10. The compression chamber 10 is connected to a chamber 11 that is again connected to a supply channel 12 through which a gas from a pressure wave generator can be supplied and discharged. The displacer 3 is connected to a rod 13. The rod is connected to a damping mass 15 via a spring arrangement 14, in the present case said damping mass comprising a first partial mass 16 and a second partial mass 17.

The damping mass 15 is connected to the housing 5 via a spring arrangement 18 comprising a first spring element 19 and a second spring element 20. For distinction, the spring arrangement 18 is called the "first spring arrangement" and the spring arrangement 14 is called the "second spring arrangement". The rod 13 is also connected to the housing 5 via a third spring arrangement 21.

The second spring arrangement 14 and the third spring arrangement 21 are designed exactly like the two spring elements 19, 20 of the first spring arrangement 18. The spring elements 19, 20 are made to have a plate shape. In a manner not shown in detail, they comprise several spring arms arranged in a helical shape, the radial centre of said spring arms being connected to the damping mass 15 and their radial outside being connected to the housing 5. In this connection, the rotation directions of the helically arranged arms are reversed, so that during a movement the damping mass 15 cannot rotate in rela- tion to the housing 5. Further, the arms are dimensioned so that the spring elements 19, 20 have a linear spring characteristic, that is, the spring elements 19, 20 have a constant spring coefficient. Thus, the return force provided by the spring elements 19, 20 is directly proportional to the deflection, that is, to the movement of the damping mass 15 in relation to the housing 5.

The same applies for the spring elements forming the second spring arrangement 14 and the third spring arrangement 21.

The partial masses 16, 17 have curved bearing surfaces 22-25, so that the space available between the spring elements 19, 20 can be used in a relatively good manner to accommodate the largest possible partial mass 17.

It can be seen that, along the displacer axis 2, the two spring elements 19, 20 of the first spring arrangement 18 have a relatively large distance to each other. At the same time, the spring elements 19, 20 are substantially softer in the direction of the displacer axis 2 than transversely thereto, that is, transversely to the displacer axis 2 the spring elements 19, 20 have a substantially larger rigidity than in the direction of the displacer axis. This does not only ensure that the damping mass 15 can only perform a movement along the displacer axis 2. It is also ensured that the damping mass 15 always maintains a predetermined orientation.

The fact that the rod 13 is on the one side connected via the third spring ar- rangement 21 to the housing 5 and on the other side via the second spring arrangement 14 to the damping mass 15 that is fixed in the housing 5 by the first spring arrangement 18, also ensures that the rod 13 and thus also the displacer 3 can maintain a predetermined alignment to the housing 5 and thus to the cylinder 4. Large frictional forces and thus also a wear are avoided. Further, a good efficiency and an increased working life are ensured.

The damping mass 15 and the spring constants of the spring arrangements 14, 18, 21 are adapted to one another in such a manner that with a predetermined frequency the displacer 3 and the damping mass 15 move in different direc- tions, meaning that they towards move each other or away from each other. In this connection, the design is made so that the product of the maximum deflection of the absorption mass 15 and the absorption mass is equal to the product of the mass of the displacer 3 and the maximum deflection of the displacer 3. In other words, the centre of gravity inside the displacer unit 1 is not changed dur- ing operation, but this mass point of gravity remains fixed at a defined position. Thus, no oscillations can occur, which can propagate to the outside. The frequency chosen is the operation frequency of the Stirling cooling arrangement that can be set by means of the frequency of the pressure wave generator that is not shown in detail. As the spring element 19, 20 and the spring arrange- ments 14 and 21 have linear spring characteristics, a change of the output of the Stirling cooling arrangement by means of a change of the amplitude does not require a new adaptation of the frequency, as, independently of the amplitude, the damping effect of the damping mass 15 remains the same. As can be seen from Fig. 1 , the second spring element 20 is arranged between the two partial masses 16, 17. The second spring element 20 can, for example, be clamped and thus fixed between the two partial masses 16, 17. For fixing the first spring element 19 to the partial mass 17, a fixing element 26 is provided. Thus, not only a partial mass 17 exists between two spring elements 19, 20, but also a plate-shaped spring element 20 between two partial masses 16, 17. In the embodiment according to Fig. 1 , the first spring arrangement 18 is located completely between the second spring arrangement 14 and the third spring arrangement 21. This again causes that the second spring arrangement 14 and the third spring arrangement 21 must have a relatively large distance along the displacer axis 2.

This embodiment is advantageous, but not absolutely necessary.

As appears from a second embodiment that is shown in Fig. 2, the second spring arrangement 14 can also be arranged between the third spring arrangement 21 and the first spring arrangement 18. Otherwise, the dimensioning and the mode of operation of the displacer unit 1 according to Fig. 2 remains the same as according to Fig. 1.

The third embodiment that is shown in Fig. 3 substantially corresponds to that in Fig. 1. Deviating from the embodiment according to Fig. 1 , the second spring arrangement 14 is arranged between the two partial masses 16, 17, and merely the second spring element 20 is located between the second spring arrange- ment 14 and the third spring arrangement 21.

The embodiment of the displacer unit 1 according to Fig. 4 corresponds to that of Fig. 2, apart from the fact that the order of the spring arrangements along the displacer axis has been reversed, that is, the first spring arrangement with the spring elements 19, 20 is arranged next to the displacer 3, whereas the third spring arrangement 21 has the largest distance to the displacer 3. The second spring arrangement 14 is arranged between the third spring arrangement 21 and the two partial masses 16, 17. In the fifth embodiment of the displacer unit 1 according to Fig. 5, the first spring arrangement with the spring elements 19, 20 are again, as in Fig. 1 , arranged between the second spring arrangement 14 and the third spring arrangement 21. However, here the positions of the second and the third spring arrange- ments 14, 21 have been reversed, that is, the second spring arrangement 14 is arranged next to the displacer 3, whereas the third spring arrangement 21 has the largest distance to the displacer 3. Also here a relatively large distance between the second spring arrangement 14 and the third spring arrangement 21 along the displacer axis 2 can be realised.

Fig. 6 shows a sixth embodiment of a displacer unit 1 that substantially corresponds to the third embodiment according to Fig. 3. However, here the third spring arrangement 21 is not, as in the embodiment according to Fig. 3, arranged between the damping mass 15 and the displacer 3, but the damping mass 15 is arranged between the third spring arrangement 21 and the displacer 3. The second spring arrangement 14 is again arranged between the two partial masses 16, 17.

In the embodiments according to the Figs. 1 to 6, the third spring arrangement 21 causes a connection between the rod 13 and the housing 5. This direct connection is abolished in the embodiments according to the Figs. 7 to 9.

In the seventh embodiment of a displacer unit 1 as shown in Fig. 7, a third spring arrangement 27 is arranged between the damping mass 15 and the rod 13. Here, the damping mass 15 has three partial masses, namely the partial masses 16, 17 as known from the Figs. 1 to 6, and a third partial mass 28. This is possible without deteriorating the mode of operation, as the damping mass 15 is guided and aligned along the displacer axis 2 by the two spring elements 19, 20. Due to this alignment, it is sufficient to guide the rod 13 at the damping mass 15 accordingly, which is achieved by the fact that the second spring arrangement 14 and the third spring arrangement 27 are arranged at a distance along the displacer axis 2. Fig. 8 shows an eight embodiment of a displacer unit 1 that differs from the embodiment of Fig. 7 in that the positions of the second spring arrangement 14 and the first spring element 19 of the first spring arrangement 18 have been interchanged.

In the ninth embodiment of the displacer unit 1 as shown in Fig. 9, the displacer mass with the corresponding spring arrangements have been turned by 180° in relation to the embodiment according to Fig. 8, that is, the spring element 19 is now arranged between the partial mass 16 and the displacer 3. The second spring arrangement 14 is again clamped between the two partial masses 16, 17. The second spring element 20 of the first spring arrangement 18 is clamped between the partial masses 17, 28, and the third spring arrangement 27 that connects the displacer 3 to the third partial mass 28 is as far away from the displacer 3 as possible.