NL1043209B1

NL1043209B1 - Cryogenic modular 3 DoF vibration isolator

Info

Publication number: NL1043209B1
Application number: NL1043209A
Authority: NL
Inventors: Hubertus Leonardus Mathias Marie Janssen Ir; Bartholomeus Catharina Thomas Bree Van Ir; Willem Hans Trines Robin
Original assignee: Jpe; Hubertus Leonardus Mathias Marie Janssen Ir
Priority date: 2019-03-28
Filing date: 2019-03-28
Publication date: 2020-10-02

Abstract

The invention concerns a modular vibration isolator that will attenuate base vibrations along 3 orthogonal 5 translational axes, specially designed for use in a cryogenic environment Many experiments in a cryogenic environment need objects to be positioned with high resolution, d own to the nanometer or even fractions of that. Base vibrations, for instance having 10 their origin in seismic noise, cryocooler pumps or machinery, will have a negative effect on the object position stability. The solution is placing the experiment on the platform of a vibration isolator that will start to attenuate base vibrations above the so- 15 called cut-off frequency. In order to reach cryogenic temperatures at the location of the experiment such a vibration isolator must incorporate a path of high thermal conductivity between the cold base and the to be cooled platform. Since many 20 cryogenic experiments involve magnetic fields it is also mandatory for the vibration isolator to be non-magnetic. 1043209

Description

Cryogenic modular 3DoE vibration isolator The invention concerns a modular vibration isolator that will attenuate base vibrations along 3 orthogonal translational axes, specially designed for use in a cryogenic environment. Many experiments in a cryogenic environment need objects to be positioned with high resolution and high position stability, down to the nanometer or even fractions of that. Base vibrations, for instance having their origin in seismic noise, cryocooler pumps or machinery, will have a negative effect on the object position stability. The solution is placing the experiment on the platform of a vibration isolator that will start to attenuate base IS vibrations above the so-called cut-off frequency.

In order to reach cryogenic temperatures at the location of the experiment such a vibration isolator must incorporate a path of high thermal conductivity between the cold base and the to be cooled platform. Since many cryogenic experiments involve magnetic fields it is also mandatory for the vibration isolator to be non-magnetic, The exact nature of this invention, as well as its objectives become clear in the accompanying drawings wherein: Fig.l Is a side view of a single axis vibration isolator module according the invention.

Fig.2 Shows a side view of how 3 single axis modules are configured into a 3 axes vibration isolator system with orthogonal axes according the invention.

Fig.3 Shows a top view of how 3 single axis modules are configured inte a 3 axes vibration isolator system with orthogonal axes according the invention.

A single axis vibration isolator module is depicted in figure 1. The frame 1 is rigidly connected to a cold and vibrating base and is connected via a series of elastic elements Sl and S2 and rigid intermediate body 3 to the table 2. Shaping the springs Sl and 82 as leaf springs will allow the table 2 to translate along the T-axis while constrained in all other directions. The range of motion is limited by the gaps Gl. The table 2 is effectively a mass on the springs Sl and S2, resuliing in a resonance freguency § = /{(Z2ay*sqri{kim}, as described by a standard second order mass-spring system, with m denotes the mass of table 2 and k denotes the combined stiffness of springs 81 and 82.

Base vibrations along the T-axis with freguencies above this frequency T are reduced in amplitude at the location of table 2. For this reason, frequency f is often referred to as the cut-off frequency.

Reducing the cut-off frequency by increasing the mass m or reducing the stiffness k is desired, but in practice limited by boundary conditions from machining, volume, robustness Or costs.

Another method of reducing the cut-off frequency is adding a force F2 on the table 2 that counteracts the force Fl having its origin in the stiffness k. This means that ¥2 must push the table 2 away from T=0 when not at T=0. This effect is also known as a negative stiffness.

Introducing the negative stiffness is achieved by using a spring clement S8 with high axial stiffness but low stiffness in the direction T.

The element S88 is under a compressive load FS from tension spring S3 via the lever L2 having a flexure pivot point S7 with low rotational stiffness k7. This force F8 will have a component along the T-axis when the table is not in position T=0. Keeping the lever ratio {B+A}/A greater than 1 will reduce the influence of the spring stiffness k3, and pivot stiffness k7, thereby also reducing non linearities in the force F2. Introducing the possibility, in general via a screw Tl on S3, to tune the force F2 will allow a near perfect counterbalancing of Fl such that table 2 has virtually no position preference along

1§ the T-axis and thereby no position preference with respect to frame 1, thus effectively having a near zero cut-off frequency.

Any external force F3 acting on table 2 along the T-axis will now move table 2 against one of the end stops at

Gl.

The constant component of such a force along the T- axis can be counterbalanced by introducing a spring element S4 with high axial stiffness.

The element S4 is under an axial load F4 from spring $6 via the lever Ll having a flexure pivot point SS with low rotational stiffness k5. Keeping the lever ratio C/D greater than 1 will reduce the influence of the spring stiffness ké6, and pivot stiffness k5, thereby also reducing non linearities in the force F4, Introducing the possibility, in general via a screw T2 on S6, to tune the force F4 will allow a near perfect counterbalancing of F3 such that table 2 can be positioned in its neutral position T=0,

Combining 3 single axis vibration isolation modules as depicted in figures 2 and 3 will isolate platform 12 along 3 orthogonal translation axes Ti, T2 and T3 with respect to the cold and vibrating base 11. Each of the modules Mi, M2 and M3 is connected via its frame 1 to the vibrating base ll of the system.

The axes Ti, T2 and T3 match the respective Taxis of the 3 identical modules MI, M2 and M3, each of which is uniquely oriented with its axis T perpendicular to one of the 3 planes P.

The planes P are defined by 3 planes of a cube intersecting in one of the cube corner points and this cube is placed with the body diagonal through that mutual corner point collinear with the axis z of the system.

Each of the modules MI, M2 and M3 is

IS connected to the platform 12 via 2 elastic elements R, each being stiff along their longitudinal axis only and connected to table 2 of the module via a connection C, such that the elements R are not colinear and are both parallel to translation axis T of that module,

Many experiments on platform 12 will require electrical connections between the base 11 and platform 12. e.g. for operation of positioners and sensors.

The number of contacts can become significant and the electrical wiring 3 as schematically depicted in figure 2, may become a source of vibrations to the platform 12 for 3 reasons, Firstly, because the wires are typically routed with multiple bends to make a low stiffness coupling to the platform 12 along the axes Ti, TZ and T3 and as a

3D result they also become long, heavy and voluminous.

The practical effect of this is that they will have a low internal resonance frequency, because of their high mass versus stiffness ratio, and will be excited by matching frequencies in the vibrating base Il.

As a result, the platform 2 will experience parasitic vibrations also.

Secondly because the combined wiring will introduce both a stiffness and a load that are

5 undefined and variable, especially when cables are deforming when the platform 12 moves with respect to base 11. The delicate balances that were set to get a low cut-off frequency and to place the modules in their neutral position can easily be disturbed.

Thirdly the individual wires will be in contact and will have relative motion when the platform 12 moves with respect to base 11. The resulting friction forces are undefined and will cause parasitic vibrations of the platform,

A great improvement of all 3 problems can be achieved by implementing a so-called flexible printed circuit board on at least one of the modules, as schematically depicted by FPCB in figure 2. The FPCB with multiple electrical lines has a very thin sheet like cross section and is stiff along its length, The FPCB is connected to frame | of a module, represented by screw T3 and connected to table 2 of ihat module, represented by screw T4. A single and relatively short bend between T3 and T4 is now sufficient to overcome the relative motion along axis T between T3 and T4 without introducing a significant load or stiffness along the T axis.

The practical effect of this is that firstly internal resonances of the electrical connections are greatly increased because of the improved mass versus stiffness ratio, secondly their stiffness and load have become much more predictable because of the well-defined position, shape and deformation of the FPCB and thirdly friction forces are eliminated because of the monolithic behaviour of the FPCB. The described 3 axes vibration isolation system will work in normal ambient conditions but the intended use in an ultra-high vacuum at cryogenic temperatures down to the millikelvin level requires additional specifie requirements. The first is that it is mandatory to have a high thermal conductivity between the platform 12 and the base 11, without compromising the vibration isolation performance. The typical solution is very similar to the above described use of individual wires for the electrical connections. Namely to connect a flexible and conductive copper braid 3, as schematically 1§ depicted in figure 2, made of multiple thin strands or sheets. Such a braid is not a solution here for the same 3 reasons observed for the electrical connections: low internal resonance frequencies, introducing both a stiffness and a load that is undefined and variable, individual strands or sheets will be in contact and will result in undefined friction forces. The solution is to make a high thermal conductivity path in the isolator modules themselves by machining at least the following features as a single monolithic copper part; frame 1, table 2, intermediate body 3 and springs Sl and S82. And at least the following additional parts must be made of copper: connections C, rods R, base 11 and platform 12. The second requirement is that the system is non- magnetic. The abovementioned use of copper is compatible with this requirement. Other parts of the system can be made of non-magnetic materials also.

1043209 eee

Claims

CONCLUSIONS

The invention relates to a device characterized in that: A so-called module comprises a table which can be translated in one direction with respect to a frame in the same module, in such a way that all other degrees of freedom by means of a straight guide, provided with elastic elements preventing friction and backlash in the moving part. the rectification is carried out as 2 parallel guides, constructed with elastic elements, in series, coupled via an intermediate body, placed in such a way that no parasitic displacement perpendicular to the intended translation of the table occurs; the frame comprises an axially rigid, axially biased and in the direction of translation slack elastic element which exerts a force on the table, which compensates for the forces exerted on the table by the straight guide in translation due to its stiffness, with the result that the table does not have a preferred position with respect to the frame in the direction of translation; the aforementioned axially rigid element is biased axially with a spring which transmits its force to the axially rigid element via a lever that reduces the perceived stiffness of the spring at the location of the axially rigid element; the force of the above spring is made adjustable by changing the pretension length with an adjustment mechanism; 1043209 The frame includes a second axially rigid, axially biased elastic element that exerts a force on the table such that it compensates for an external and life-constant force experienced by the table in its direction of translation, with the result that the table is in a certain preferred position relative to of the frame; said second axially rigid element is biased with a second spring which transmits its force to the second axially rigid element via a second lever which reduces the perceived stiffness of the spring at the second axially rigid element; the force of the above spring is made adjustable by changing the pretension length with a second adjustment mechanism; in any case, the frame, straight guide and table are made as a monolithic part made of copper, a material that retains high thermal conductivity in cryogenic temperatures and is non-magnetic; all other parts in the module are made of a non-magnetic material.

A device according to the preceding claim, characterized in that: The 3 modules are rigidly placed via their frame on a common base and the table of each of the 3 modules is provided with two elastic elements which are rigid only in their axial direction , hereinafter referred to as whip, wherein these blades are both oriented parallel to the direction of translation of the module but the blades are not in line with each other; the blades jointly hold a platform in which the translation directions of the three modules are mutually orthogonal to three planes which are defined by planes touching each other at a corner point of a cube; in any case the base, blades and platform are made of copper; all other parts in the system are made of a non-magnetic material.

A device according to claim 1, characterized in that the straight guide is designed as a single parallel guide, with elastic elements,

A device according to claim 1, characterized in that the thermally conductive and non-magnetic copper is replaced by another material.

S, An apparatus according to any one of the preceding claims, characterized in that electrical connections in the system between the moving part and the stationary part are made such that they introduce a low constant force and stiffness in the direction of translation created by the force and stiffness compensation adjustment mechanisms. the modules can be compensated.

A device according to the preceding claim, characterized in that electrical connections in the system between the moving part and the stationary part are made by using a flexible printed circuit board, optionally with the provision of a bend or loop in the printed circuit board between the two fixing points.

A device according to claim 1 characterized in that the force and / or stiffness compensation is performed by an adjusting mechanism operating under cryogenic conditions.

An apparatus according to claim 1, characterized in that: Six modules are placed in the well known hexapod configuration, the frames of the modules being rigidly connected to a common base; the translation directions of these modules coincide with the six supports holding a platform; the tables of these modules are connected, each via an elastic element that is rigid only in the direction of translation of the module, to that platform.

A device according to claim 1 characterized in that a non-contact damping of the movement of the table with respect to the frame is realized by introducing a permanent magnet which moves relative movement of the table with respect to the frame and thereby introduces eddy currents.

A device according to claim 2, characterized in that the elastic blade is optimized for good thermal conduction by designing it as two times two perpendicular line hinges with a thickened middle part.