Field of the invention

The present invention relates to micro- and nanopositioning and to related testing and measurement apparatuses.

Background of the invention

Microscopic measurements and optomechanical instruments require micro- and na- nopositioning solutions where thermal expansions are small, the rigidity is large es- pecially in relation to weight, and further where vibrations are effectively dampened to allow dynamical positioning for good test efficiency.

Main prior art in this regard are motorized positioners where bulky metal or plastic materials are used in solid plate or bar forms as structural elements in the instru- ments and below or in connection to the measured target. This makes them prone to drifting through thermal expansions and to vibrations effecting the system from the outside or generated within the instrument during dynamic positioning.

Publication WO 2017/064354 discloses a linked micromechanical positioning appa- ratus for real-time testing and measurement, which represents very high perfor- mance arrangement of the prior art. Steering signals are part of the concept, and they are used with the help of a positioning controller to control the movement of the actuator, which in turn moves the probe with respect to the measured target. Still thermal drifts and environmental or user induced vibrations may affect the perfor- mance of the apparatus and limit test results. These issues are mentioned as gen- eral characteristics and general problems in the field of micromechanical measure- ments.

In more detail, the problems in prior art comprise issues such that the current micro- and nanopositioning solutions do not completely fulfil the needs of accurate testing and measurement applications in cases where temperature changes are involved in the environment, in the test or measurement target, or in the test or measurement apparatus. Current solutions are also large in size and weight, which prevents meas- uring simultaneously with many probes from many targets and prevents using dy- namic test protocols and thus, lowers test and measurement efficiency. Their typical solid body designs from standard metal or plastic materials do not provide the re- quired stability for changing temperatures. Thus, current solutions may not meet the requirements for increasingly common complex tests involving positioning of many targets, probes or parts within the test and measurement apparatus, and this in turn makes the tests slower and less productive. The large size also typically leads to long mechanical levers arms in the apparatus that makes them more vulnerable to environmental or test and measurement related vibrations that prevent accurate testing and measurements.

There is, therefore, a need for an improved micro- and nanopositioning apparatus, which reduces or practically even eliminates perturbations from temperature changes in the environment or in the parts of the test equipment and which provides high rigidity to weight ratio in compact size. Further, because testing and measure- ment applications are constantly becoming more complex and demanding, this need is common and general, and the solution should thus be compatible with efficient manufacturing methods.

Summary of the invention

The invented solution is to introduce a micro- and a nanopositioner solution, which does not in practice experience perturbing amount of thermal expansion and is rigid as well as compact.

The invented solution includes a sub-solution for the moving part of the positioner that is typically an optical table or measurement platform that is also thermally sta- ble, rigid and compact in size. A new sandwich structure for the invention is pre- sented that has a small thermal expansion coefficient and high stiffness per weight ratio.

In other words, now there has been invented an improved micro- or nanopositioning apparatus by which the above-mentioned problems are alleviated. Various aspects of the invention comprise a new design that is compatible with efficient manufactur- ing. Parts, apparatuses and system according to the invention are characterized by what is stated in the independent claims. Various embodiments of the invention are disclosed in the dependent claims.

In its first aspect, the present invention introduces a micro- or nanopositioning ap- paratus suitable for a testing or measurement system in changing temperature con- ditions. The apparatus is characterized in that it comprises a body having a low coefficient of thermal expansion, a moving stage having a low coefficient of thermal expansion, at least one controllable motor, at least one drive stem driven by the at least one motor, respectively, which drive stem(s) is/are physically connected to the body in at least one location, which drive stem(s) is/are capable to controllably move the moving stage in a micro- or nanometer resolution, so that the apparatus is con- figured to internally mitigate, with the selection of materials and design, the effects of thermal expansions in the position of a tested or measured target, when the target is placed in connection with the moving stage.

In an embodiment of the apparatus, the motor(s) is/are a piezomotor(s), and the drive stem(s) is/are made of a material having a low coefficient of thermal expan- sion.

In an embodiment of the apparatus, there is a single motor which is an electric mo- tor, and the drive stem or drive screw is made of a material having a low coefficient of thermal expansion.

In an embodiment of the apparatus, there is a single motor which is an electric mo- tor, and the apparatus comprises a drive stem or drive screw and a break mecha- nism that is connected to a break stem, where the break stem is made of a material having a low coefficient of thermal expansion, and the break mechanism is movable along the break stem.

In an embodiment of the apparatus, the apparatus is configured to contact the break mechanism in order to lock the break to the break stem, when the motor is not mov- ing the moving stage.

In an embodiment of the apparatus, the apparatus comprises a moving element which is moved by the drive stem, and where the moving stage is fixed to the moving element.

In an embodiment of the apparatus, the low coefficient of thermal expansion means a coefficient that is equal or less than three.

In an embodiment of the apparatus, at least one of the materials with a low coeffi- cient of thermal expansion used in the apparatus is/are Invar.

In an embodiment of the apparatus, at least one of the materials with a low coeffi- cient of thermal expansion used in the apparatus is/are carbon fiber composite.

In an embodiment of the apparatus, there are at least two micro- or nanopositioning apparatuses moving the target in desired directions, where the directions are non- parallel between one another. In an embodiment of the apparatus, the micro- or nanopositioning apparatus acts the target in a vertical direction, where the target is a lens or any other type of optical part or component.

In an embodiment of the apparatus, the moving stage comprises a rigid sandwich structure, which sandwich structure comprises a honeycomb core.

In an embodiment of the apparatus, the sandwich structure comprises a low thermal expansion metal skin sheet with the elastic modulus of at least 130.

In an embodiment of the apparatus, the honeycomb core is made from a carbon fiber composite structure, and the skin sheets are made from metal having a coeffi- cient of thermal expansion less or equal than three.

In an embodiment of the apparatus, the alignment of fibers in a single carbon fiber layer is mutually parallel.

In an embodiment of the apparatus, layers within the carbon fiber composite struc- ture each have a fiber alignment direction differing from one another.

In an embodiment of the apparatus, the carbon fiber composite structure comprises a single or multiple carbon fiber layer(s) where the carbon fibers are cross-woven.

In an embodiment of the apparatus, the moving stage comprises one or more holes, threads, in-cuts or openings.

In an embodiment of the apparatus, the ratio between the honeycomb core thick ness and skin sheet thickness is between one and six.

In an embodiment of the apparatus, at least one surface of the moving stage is a non-planar surface.

In an embodiment of the apparatus, the shape of the moving stage is rectangular or circular.

According to its second aspect, the present invention introduces a manufacturing method for a moving stage of a micro- or nanopositioning apparatus. The manufac- turing method is characterized in that a sandwich structure is manufactured for the moving stage and the manufacturing method comprises the steps of manufacturing a corrugated carbon fiber profile with press molding or pultrusion or other industrial mass production method, and stacking a desired number of the profiles or profile sections onto one another with a horizontal placement difference so that desired dimensions of the honeycomb core with thickness of T are achieved, wherein the stacked structure is either cut afterwards to form the honeycomb core or the profiles are cut into profile sections with width T before the stacking, respectively.

In an embodiment of the manufacturing method, at least one of the outer edges of the honeycomb core are provided with at least one rectangular or circular frame section for further rigidifying the sandwich structure and to protect it from external factors.

In an embodiment of the manufacturing method, the honeycomb core is connected with the two skin sheets by an adhesive, respectively.

According to its third aspect, the present invention introduces a testing or measure- ment system, which is characterized in that the system comprises a micro- or na- nopositioning apparatus according to any of the above-disclosed apparatus embod- iments, where the testing or measurement system forms an optomechanical, opti- cal, microscopic or micromanipulative testing or measurement system.

The advantages of the present invention are various. Presented invention provides solution to problems of prior art by presenting micro- or nanopositioning apparatus that has low thermal expansion properties and can be manufactured with industrially efficient and scalable methods, instead of highly customized and expensive one-off solutions in the prior art. Furthermore, the presented solution also provides high mechanical rigidity to weight ratio in a compact size, as well as high robustness for wear and tear.

Brief description of the drawings

In the following, various embodiments of the invention will be described in more detail with reference to the appended drawings, in which

Figure 1 illustrates a piezomotor based micro- or nanopositioner,

Figure 2 illustrates an electric motor based micro- or nanopositioner,

Figure 3 illustrates a further electric motor based micro- or nanopositioner with an additional break mechanism,

Figures 4 illustrate a carbon fiber composite honeycomb structure manufacturing method that is industrial and efficient to manufacture, and Figure 5 illustrate a sandwich structure with stacked low thermal expansion coeffi- cient material metal plates and a carbon fiber composite honeycomb core that may be attached to one another by an adhesive.

Detailed description of the invention

The present invention relates to the field of micro- and nanopositioning and related testing and measurement apparatus, where in optical measurements it is essential to accurately position and maintain relative position and orientation of the target and optical components in respect to each other. Similarly, it is essential in probing type of applications to accurately position and maintain the stable and accurate test probe position relative to the target over the measurement periods which can be long. Often such measurement targets can be in micrometer scale or nanometer scale or even smaller scale, which makes the tests and measurements highly sen- sitive to even small perturbations to the relative positions or orientations between the target or the probe or the optical components of the measurement apparatus. Test and measurements are constantly evolving to be more complex and, in order to have higher resolution and higher accuracy, the requirements for the testing and for the measurement apparatus are constantly evolving to become more compact and more accurate.

Undesired perturbations in the position can arise for example from a temperature change in the environment or from a temperature change in the micro- or nanopo- sitioner or from the related testing and measurement apparatus itself, for example from a positioner motor or from some other heat source which changes tempera- tures within certain part(s) or structure(s). Such temperature change will change the physical dimensions of the affected parts according to the thermal expansion coef- ficient specific to the materials used in these parts and the physical dimensions; the larger parts expand more than the smaller parts. Sometimes it is possible to com- pensate such effects by clever mechanical design, but this may be often impractical or even impossible considering the application requirements and high manufactur- ing cost of complex compensation designs. Thus, the most effective solution to the thermal expansion related perturbations is to select materials, which have as small coefficient of thermal expansion as possible, as well as to keep the apparatus and its structures as compact (= small) as possible.

Often test and measurement applications require micromechanical positioning of the target or the measurement probe or optics of the measurement apparatus. In such case, fast and dynamic positioning is beneficial to allow efficient test protocols and high productivity. The small weight of the parts is very important for the dynam- ical positioning because it reduces the force and power required from the motors, as well as makes the mechanical settling time to a new position shorter. It is also important to have high rigidity in the positioned parts and in the apparatus to avoid any non-desired change in the dimensions, shape or orientation, or vibrations from the forces generated by the target or the positioned parts. High rigidity and lower weight increases the resonance frequency, which improves dynamic performance and reduces vibration related perturbations from dynamic positioning or from im- pacts coming from the testing and measurement apparatus environment.

Summary of the invented solution is described in the following.

The presented invention may, for example but not limited to, be used in motorized microscope stages or motorized lenses or objective positioners or any other type of positioners for one or more axis for micro- and nanopositioning applications in various types and sizes. In addition to plate (= planar) forms, positioners or their sub-structures, the invention can also incorporate a curved bottom and/or top surface or they can have any other 3D type of shape suitable for the used application. The surfaces may also have one or more holes, threads, in-cuts or openings or any other type of features.

The technical areas, where the present invention is applicable, comprise for example but not limited to: optomechanical parts, components and apparatus, optical measurement systems and accessories, microscope apparatus and accessories, test and measurement apparatus and accessories, and micro- and nanopositioners. Any other further usage areas are not excluded by this listing.

The present invention comprises the following parts and characteristics in its first main embodiment.

The main invention is a micro- or nanopositioning apparatus, which has low thermal expansion properties when temperature changes, and the apparatus comprises in its one embodiment:

- Low thermal expansion property defined as the coefficient of thermal expansion being less or equal than 3.

- Apparatus has a body with such low thermal expansion property.

- Apparatus has a moving element with low thermal expansion property in parts that affect the overall thermal expansion properties of the apparatus.

- Apparatus has a motor which moves the moving element by means of a drive stem or drive screw coupled to the motor, where the drive stem or drive screw has low thermal expansion property, whereas the drive stem or drive screw is fixed at least from one end to the body having also low thermal expansion property. The low thermal expansion property of the drive stem or of the drive screw is however not a necessity; see the next paragraph.

- In case the drive stem or drive screw coupled to the motor does not have a low thermal expansion property, the apparatus can then comprise a separate break stem with a low thermal expansion property, which is fixed at least from one end to the body having also a low thermal expansion property, and the break mechanism element is attached to the moving element which is moving along the break stem. The break is unlocked and allows free movement of the moving element when the motor is moving it and the break may firmly lock the moving element to the break stem, when the motor is not moving the moving element. See the description related to Figure 3.

Further useful features and characteristics of the present invention comprise the following detailed materials and structures.

Low CTE (meaning “Coefficient of Thermal Expansion”) materials used in the apparatus may be for example a carbon fiber composite or Invar (i.e. the nickel-iron alloy also known as“FeNi36”).

The moving element may comprise a rigid sandwich structure (like e.g. a skin-core- skin type of structure), with a low overall coefficient of thermal expansion (“CTE”). The skins are manufactured from low CTE metal (for example, but not limited to Invar). In an exemplary embodiment, the low CTE value can be defined to be less or equal than three, which excludes some prior art materials such as standard aluminum, steel or titanium materials. Metal as a defined type of material excludes other low CTE skin materials used in prior art, like carbon fiber laminates. Furthermore, in an embodiment using the sandwich structure, the core is manufactured from a low CTE material honeycomb type of structure, where low CTE can be defined to be less or equal than three that excludes commonly used metal honeycomb materials.

The body, the moving element and the outer skin materials of the moving element have high mechanical strength. In an example embodiment, the elastic modulus, i.e. Young’s modulus, of these materials can be defined to be over 130 that excludes many prior art materials such as aluminum, titanium and carbon fiber composite laminates. Furthermore, such material is also required to be suitable for machining features, such as threads for screw attachments, which eliminates common composite and ceramic materials of prior art that are typically not machinable with standard procedures. Furthermore, the sandwich skin material is not fragile in an embodiment; it is resistant to wear and tear and various impacts of use. Such a selection excludes e.g. carbon fiber laminates which are sensitive to scratching and wear/tear, as well as ceramics which have high hardness but are fragile to impacts that can make them crack (as glass-like fractures).

Regarding the sandwich structure and its features, the following embodiments are possible.

In addition to high mechanical stiffness and rigidity to weight ratio in direction perpendicular to the skin sheets, the honeycomb core may also have relatively high mechanical stiffness as a stand-alone structure, which enables a sandwich design that is quite low in height with much improved overall rigidity to weight ratio. This is possible because the core contributes significantly to overall rigidity. For example, a carbon fiber honeycomb core can have rigidity that is 20-50 % of the solid body carbon fiber composite design while weight is 8-30 % of the solid body carbon fiber composite weight. Alternatively, a similar fraction of rigidity is obtained with only 5 - 17 % of weight in comparison to a solid body aluminum design. This means that even 6-20 times of improved rigidity to weight ratio may be obtained for the core as a stand-alone structure in comparison to a solid body design.

In addition to high mechanical stiffness in direction perpendicular to the skin sheets, the core may also have similar stiffness in the plane direction of the skin sheet, which enables a sandwich design with low total height and with improved overall rigidity to weight ratio. In this case, the core contributes also to transverse/shear rigidity.

In an example embodiment, it is possible to match the core’s mechanical rigidity and the CTE to be essentially homogeneous in longitudinal and transverse directions i.e. in the skin sheet plane and in the plane perpendicular to it, which is beneficial for the structure, where the core to individual skin thickness ratio is relatively low, and for example between 1 ... 6.

In an example embodiment, a sandwich design may be such that it allows overall thin structures starting from total thickness of e.g. 3 times of an individual skin thickness. There is no upper limit to the maximum structure thickness (and thus, rigidity).

In an example embodiment, the carbon fiber honeycomb core design is such that it may be fabricated using efficient industrial methods by assembling corrugated profiles with an adhesive material. Such profiles may be manufactured using efficient industrial methods for carbon fiber composite manufacturing such as automated press molding or pultrusion processes. These methods differentiate from solutions described in prior art that are based on slow and labor-demanding hand craft methods like vacuum bag forming.

The key advantages of the present invention are the following.

Improved thermal stability improves the accuracy of the tests and measurements. Improved mechanical rigidity to weight ratio enables more dynamic tests and measurements, which increase efficiency and throughput. Improved sandwich structure core rigidity enables sandwich designs with improved rigidity to weight ratio also in low core to skin thickness ratios, which enables designing more compact test and measurement apparatuses. This is an industrially applicable invention, which allows cost-efficient and scalable manufacturing, which further enables offering the invention to common and general applications and market segments. This is in contrast to prior art solutions, where similar problems have been studied in literature or in special applications, which have led to highly customized one-off solutions that do not enable industrial manufacturing and serial production.

In the following, various embodiments of the present invention will be described to- gether with the Figures in the context of micro- or nanopositioning related to testing and measurement apparatus, where optical or electrical testing and measurements are done in micromechanical fashion, which may involve one target, probe or optical system or plurality of them. It is to be noted, however, that the present invention is not limited to the exemplary embodiments described here. In fact, different embod- iments may have applications widely in any environment or context, where such mechanical structures can be generally used.

Figure 1 illustrates a first main embodiment of the invention, comprising a micro- or a nanomover apparatus 10a applied with a piezomotor 14 (i.e. a piezoelectric motor) as a cross-sectional drawing. The apparatus comprises a low CTE body 1 1 as a horizontal structure in the bottom of the apparatus. In the middle of this exemplary structure, there is a horizontally aligned drive stem, which is made of material having a low coefficient of thermal expansion; i.e. this element is called here as a low CTE drive stem 12a. The low CTE drive stem 12a is placed along appropriate support section as the vertically aligned left-hand side part of the body, through which the low CTE drive stem 12a goes through e.g. via a hole. In connection to the low CTE drive stem 12a, there is a moving element 15 around it (shown on top and below it in the cross section). The moving element 15 is in turn fixed to a moving optical stage or sample platform that has a low coefficient of thermal expansion; i.e. this element is called here as a low CTE stage / plate 13. The measured target can be placed on top of the low CTE stage / plate 13. As a relevant movement energy source for the apparatus 10a in this first main embodiment, there is a piezomotor 14 that is functionally connected to the low CTE drive stem 12a. The piezomotor 14 is an element which turns an electric signal (i.e. an applied electric field) into a change of shape of a piezoelectric material within the motor. This can be used e.g. in vibra- tional small movements of a plurality of element heads which controllable can move a stem as a single-axis movement along the axis of the stem, thus acting as a linear motor. Of course, the piezomotor 14 can be a rotary motor as well. This technology can be used to create very small incremental movements of the low CTE drive stem 12a in the direction of its axis, enabling the desired micro- or nano-scale movements as discussed earlier. It is notable that a single micro- or nanopositioner is depicted here for simplification purposes and the present invention may well comprise plural ity of motors and drive stems in parallel or plurality of micro- or nanopositioners can be stacked orthogonally or in some other orientation to allow multidimensional posi- tioning.

The low CTE drive stem 12a is in connection to the low CTE body 1 1 at least from one end, i.e. in the place where it goes through the above-mentioned hole of the vertically aligned left-hand side part. In another embodiment, the low CTE drive stem 12a has a connection to the low CTE body 1 1 in the corresponding right-hand side part which is here shown also as a vertically aligned sub-part (dashed in Figure 1 ).

As a result of the presented structure, the drift resulting from the thermal expansions within the low CTE body 1 1 , the low CTE drive stem 12a and low CTE stage/plate

13, and also from the interactions between these elements, are very small and in practise not meaningful even in situations, where the temperature of the apparatus notably changes within a measurement sequence, for instance. This is a great ad- vantage of the presented micro- or nanopositioner apparatus 10a.

In a further embodiment of the apparatus 10a, there may be used several piezomo- tors 14, which each drive a respective drive stem 12a, so that the drive stems are physically connected to the body 1 1 in at least one location for each stem 12a. In an example, there can be used four piezomotors 14 (not shown) in parallel with respective drive stems 12a in a single micro/nanopositioner within a manipulator. Also in other uses and devices, there can be a need for several parallel piezomotors

14. Such a need usually derives from the fact that a single piezomotor is able to create only a restricted force or power, which might need to be exceeded in certain applications.

Figure 2 illustrates a second main embodiment of the invention, comprising a micro- or a nanomover apparatus 10b applied with an electric motor 17 as a cross-sectional drawing. The low CTE body 1 1 , the low CTE stage / plate 13 and the moving ele- ment 15 are the same as in the first main embodiment. The drive stem 12b in the second main embodiment can be a screw, which acts as a transfer means along an axis for the stem. The screw can be a ball-race screw or a conveyor screw. In the second main embodiment, the driving element in the middle of the apparatus is called as the low CTE drive stem / screw 12b. The stem and the screw 12b are preferably both manufactured from a material having low thermal expanding prop- erties, for example material with a CTE that is less or equal than three. The screw having such thermal expansion properties is a useful way to prevent the thermal effects to propagate via the screw towards the target. The electric motor 17 is func- tionally connected to the low CTE drive stem / screw 12b via a coupler 16.

Figure 3 illustrates a third main embodiment of the invention, comprising a micro- or a nanopositioner apparatus 10c applied with an electric motor 17 and a break 18. The break 18 is provided with a low CTE break stem 19, aligned in parallel with the drive stem 12c. The drive stem or screw 12c can in this embodiment be manufac- tured from a standard material (with non-specified CTE), so therefore it can be called as a standard material drive stem / screw 12c. The low CTE break stem 19 may be placed so that it goes through the moving element 15 or connects to it in some other way that provides firm coupling between the moving element 15, low CTE break stem 19 and the break 18. The micro- or nanopositioner otherwise conforms with the second main embodiment, having the low CTE body 1 1 , the low CTE stage / plate 13, the moving element 15, the coupler 16 and the electric motor 17. The break system now compensates the effects which might happen within the standard ma- terial drive stem / screw 12c. The break 18 is lockable and it can freely move along the break stem 19 made of low CTE material.

When there is no movement of the moving element 15 through the drive stem 12c, the break 18 can be locked to the low CTE material break stem 19. Then the moving element 15 is unable to move even in the presence of thermal expansion effects of the standard material drive stem / screw 12c. This is because the bearings of the ball-race screw or the conveyor screw will act as a counter-force for the forces of the thermally expanding screw. Thus, the moving element 15, along the drive stem 12c, will remain stationary in a very accurate resolution when the break is on, even in the changing temperatures. This is a great advantage of the apparatus with the low CTE break stem 19 and the break 18.

Figures 1-3 present embodiments of the invention, where micro- or nanopositioning apparatus is thermally very stable.

The example embodiments of Figures 1-3 may well be expanded so that there are several drive stems which are aligned e.g. in different directions. In an example, there can be two orthogonally aligned drive stems or three orthogonally aligned drive stems which are manufactured either from low CTE material (as in Figures 1 -2) or from any standard material (as in Figure 3). Of course, the two or more drive stems can also be aligned in a non-orthogonal arrangement between one another. Thus, it is possible to manufacture embodiments of micro- or nanopositioners in either horizontal or vertical configurations or even in both these directions, as well as in stacked configurations in order to form multidimensional micro- or nanopositioners.

In an alternative embodiment, the horizontally aligned system can be changed so that the movements, i.e. the alignment of the drive stem 12, is vertical.

In an embodiment, the target to be moved is a lens or other type of optical part of component. This arrangement can be applied both in vertically directed micro- or nanopositioning or horizontally directed micro- or nanopositioning.

In the following we discuss another aspect of the present invention which describes example embodiments of the low CTE stage or plate 13 that is made with a sand- wich-type of structure to provide high rigidity to weight ratio in compact size. Figures 4 and 5 illustrate the example embodiments of such low CTE metal skin and carbon fiber composite honeycomb structures.

Figure 4 shows an example embodiment on how a carbon fiber composite honey- comb structure 40 can be effectively manufactured. Sheet of, for example, hexago- nally corrugated carbon fiber composite profile 41 may be manufactured with press molding or pultrusion or any other industrially efficient manufacturing method (i.e. an industrially applicable mass production method). In one example embodiment, the profile 41 may be cut to a section 42 with width T 43. The profile sections 42 are then attached to each other with an adhesive to form a honeycomb structure 40 with core thickness T 43, width W 44 and length L 45. In another example embodiment, the profiles 42 may be attached to each other with adhesives to form a very thick honeycomb structure that is then cut to designated core thickness T 43. Generally in the following, we also call the honeycomb part as a core or as a core structure.

Figure 5 shows an example embodiment of an invented low CTE sandwich structure 50 with high rigidity to weight ratio that may be used as low CTE moving stage or plate 13 of Figures 1-3. The structure 50 consists of two low CTE metal skin sheets 51 a, 51 b and a honeycomb core 53 that may be attached to each others with a respective adhesive layer 52a, 52b. The core 53 may be a carbon fiber composite honeycomb core with thickness T 43 as presented in Figure 4. The complete sand- wich structure 50 may also include a solid or hollow rectangularly shaped bar (not shown) which is made from a carbon fiber composite or from a low CTE metal which surrounds the honeycomb core 53 from two or four sides. Such surrounding with the frame may be beneficial to give further rigidity to the overall structure or to protect it from the external, environmental factors.

The profile may consist of only longitudinal fibers in order to have maximum longi- tudinal rigidity (aligned parallel to sandwich thickness as in the sandwich structure 50), which may be beneficial to cores 53 with larger thicknesses and to sandwich structures. Alternatively, a profile may consist of one or two or any other practical number of layers of fibers in different orientations to have equal strength and rigidity in all different directions, which may be beneficial for cores 53 with lower thickness that require higher rigidity in direction parallel to the sheets and for the shear stress. In a further alternative embodiment, a single or multiple layers of carbon fibers of the profile may comprise a cross-woven fabric of the carbon fibers so that their di- rections are non-parallel. This has basically the same advantages as the previous, multi-layered structure with each layer in different fiber orientations.

The presented manufacturing method thus enables efficient manufacturing of any size of honeycomb cores with either especially high rigidity in direction perpendicular to the sheets in cases where the total sandwich thickness to skin sheet thickness ratio is high, or a honeycomb structure with uniform rigidity in all directions in cases where that ratio is low or moderate. It is beneficial to optimize the rigidity to weight ratio for any total rigidity and sandwich size requirements.

In an embodiment of the invention, and by further referring to Figure 5, the honey- comb core 53 can be connected with the skin sheet plates 51 a, 51 b by an adhesive 52a, 52b, like a glue layer. This results in a fixed and internally non-movable struc- ture. By selecting an appropriate adhesive, even the thermal conducting effects via the adhesive can be minimized if the physical structure and the thermally insulating characteristics of the adhesive 52a, 52b are properly taken into account in its selec- tion. Furthermore, the adhesive layer 52a, 52b may be rigid and thin to maximize shear strength of the attached plates (i.e. of the whole sandwich structure 50) or it can be thicker and flexible in cases where it is beneficial for example for vibration damping or any other combination of the physical requirements mentioned earlier.

In the present invention, the materials of the skin 51 a, 51 b and core 53 combinations may vary according to what is presented below with associated advantages and disadvantages over the prior art solid body type or sandwich type of designs.

Concerning at first most basic prior art of solid metal body designs, they are typically made of aluminum or steel and provide simple manufacturability. Their main problems are that metals have a relatively large CTE, which makes the temperature change created dimensional changes large and this phenomenon prevents the performing of accurate tests and measurements. Secondly, solid designs become very heavy and expensive, when high rigidity is required that would lead to large material thickness, which would result in poor rigidity to weight ratio and poor performance to price ratio in such conditions.

Concerning secondly the metal skin, and metal honeycomb sandwich prior art designs that are typically manufactured with an aluminum or a steel skin with aluminum honeycomb, they offer higher rigidity to weight ratio than solid body design. However, similar to standard metal solid body designs, as in the above, the relatively large CTE makes the temperature change created dimensional changes large and this phenomenon prevents the performing of accurate tests and measurements. A second problem of this prior art design is that low stand-alone rigidity of the core leads to large overall thickness to achieve good improvements in the rigidity to weight ratio.

Concerning thirdly the prior art with standard metal skin, but with a composite or ceramic honeycomb design, the main benefit over the solid body design is similar to the metal honeycomb presented above. Such designs may also have some special improvements in properties like corrosion resistance, when comparing to the metal honeycomb core design. However, this prior art still suffers from the thermal expansion related problems like previous two prior art designs.

Concerning fourthly a prior art design with composite or ceramic skin, but with a metal honeycomb, the main benefit in comparison to the previous three prior art designs is low CTE properties that resolve the problem with thermal expansions. In addition, higher rigidity to weight ratio may be achieved than with the prior art solid body design. The main problem with this prior art design is that large overall thickness is required for the improved rigidity to weight ratio due only moderate elastic modulus of the skin sheet and poor transverse/sheer stiffness of the core (like in the second and third prior art designs), a poor manufacturability of the skin, especially machinability of threads, and importantly the skin is also fragile for wear, tear and any impacts that might happen during the use of the apparatus. This limits use of this prior art design to only some special applications that can tolerate large overall thickness, poor robustness of the skin sheets to wear and tear, and high cost due to difficult manufacturing.

In general, the various sandwich structures provide significant rigidity to weight ratio improvement over the solid body design, thus resulting in a great advantage of the sandwich structures. The rigidity with a given skin sheet thickness and core thick ness can be approximated by the equation: where m is sandwich effect multiplier. For example, a core to skin ratio of 3 leads to a multiplier m value of 24.5. This increase of the sandwich effect rigidity is based on the assumption that the core rigidity is large enough to keep the skin sheet distance unchanged under external loading (i.e. forces), whose direction is perpendicular to the sheet. In such a situation, the skin sheets are facing compressive or stretching type of stress instead of bending stress, which leads to an improved overall rigidity because compression/stretching type of stiffness and strength are in general higher than the bending type of stiffness and strength.

In the prior art sandwich structures, the honeycomb cell wall thickness is typically very thin in comparison to the honeycomb cell size, which enables moderate hon- eycomb rigidity in the direction which is perpendicular to the skin sheets but leads to a low rigidity for loads and stress which are parallel to the skin sheet plane or a shearing type of stress. In such a situation, the skin sheet itself must have high rigidity and thus it must be thick enough to meet this requirement. In such a design, the overall rigidity of the sandwich structure results mostly from the sandwich effect and the core rigidity itself does not significantly contribute to it. Altogether this means that in the prior art, the skin sheets as such cannot be very thin and the thickness of the honeycomb core must be relatively large to achieve the required overall rigidity, which leads to a large overall thickness and weight of the sandwich structure.

In an embodiment of the invention, a sandwich structure is manufactured so that its honeycomb core 53 is manufactured from a material having a low coefficient of ther- mal expansion (CTE), and its skin sheets 51 a, 51 b are manufactured from a material having a low coefficient of thermal expansion (CTE) as well. The materials can be the same but they do not need to. In one embodiment, the selected low CTE material is Invar, meaning the nickel-iron alloy comprising 64 % of iron and 36 % of nickel.

In a further embodiment according to the invention, metal skin sheets (skins 51 a, 51 b) from a material that has a low CTE of 3 or less are used with a carbon fiber composite honeycomb (core 53) that has a thin or medium cell wall thickness. For example, 25 mm honeycomb cell size can be combined with a honeycomb wall thickness between 0.5 mm and 2 mm. Cell sizes can be also different and the wall thickness relative to it can have a larger variation range. The overall sandwich can be also very thick in situations where very high rigidity is required, such as for plates having a large size or in high loading conditions. In such a design regime according to the invention, a thin skin sheet thickness is enabled by the high rigidity of the low CTE metal sheet and the high rigidity of the carbon fiber composite honeycomb core also in directions parallel to the skin sheet plane and for shear type of stress. High overall rigidity of the sandwich structure even with low sandwich effect ratios is en- abled by the core characteristics of high rigidity to stress from different directions and for the shearing stress so that the core contributes significantly to the overall rigidity of the sandwich structure.

The main advantages of the invented new sandwich design over the prior art are as follows. At first, temperature changes do not cause significant dimensional changes or changes in shape in the sandwich structure, which enables very stable and ac- curate testing and measurements. This applies especially with the above disclosed nano- or micropositioning apparatus. Secondly, thinner skin thickness and smaller honeycomb core thickness make it possible to construct sandwich structures with high rigidity to weight ratio and with small overall thickness, which is beneficial for current needs for compact and lightweight solutions. Thirdly, the metal skin sheet and carbon fiber composite honeycomb core can be optimized for high resonance frequency and inherent vibration damping properties.

In an embodiment of the invention, the sandwich structure may have other shapes than mere planar forms. The lower surface may be curved, or the upper surface may be curved in an embodiment. In a further embodiment, both the lower and the upper surface of the sandwich structure can be curved, i.e. a non-planar surface. In a fur- ther embodiment, when looking at the sandwich structure from the top side, the sandwiched plate needs not to be a rectangular planar nor a rectangular curved element, but any shapes of the sandwiched plates are possible, together with pos- sible curvatures in given outer surface(s). In an embodiment of the sandwich struc- ture, the shape can be for example circular. In a further embodiment, the plate may comprise one of more holes of single or various sizes, and/or threads for attaching it to the testing or measurement apparatus or for attaching different testing or meas- urement apparatus to the plate. The sandwich structure can be flat with an equal thickness or its lower or upper surface can be curved or have step-like thickness differences or it can have any other three-dimensional shape which can be formed from a low CTE metal sheet used as a skin material. The thickness of the honey- comb core is in that case cut to conform the shape of the skin sheets. The various embodiments of the invention can be implemented based on or with the help of the features of the invention described in the above embodiments that causes the relevant apparatuses or test or measurement systems to carry out the present invention.

The present invention is not limited solely to the above-presented embodiments, but it may vary within the scope of the claims.