Patent Application
20100275717
The present invention relates to an ultra-precision positioning apparatus and more specifically to an ultra-precision positioning apparatus for precisely performing a fine positioning of the submicron order without rotational motion.
The importance of ultra-precision positioning technology has gradually increased in a variety of industrial fields. In particular, the development of ultra-precision measurements fields, such as atomic force microscope (AFM) or scanning electron microscope (SEM), has led to improved precision positioning technology.
Various of ultra-positioning feeding devices are known. For example, the document U.S. Pat. No. 7,239,107 discloses a flexure positioning technique comprising a base, a moveable stage and a positioning mechanism coupled between the base and the stage to move the stage in translation. However, the positioning mechanism is very complex, which leads to increased risks of imprecision.
Furthermore, the rotation motion of the mobile part is not well controlled and may lead to inaccuracy of the translation motion.
There is a need for a simplified ultra-precision positioning apparatus that will eliminate the three degrees of freedom of rotation of the mobile part so that only two or three degrees of freedom of translation remain.
The foregoing shortcomings of the prior art are addressed by the present invention. An object of the present invention is to improve the translation quality and to reduce the undesired rotational motion.
Another object of the present invention is to provide a precision positioning apparatus with simplified design.
In order to achieve the above mentioned objects, the precision positioning apparatus according to the invention comprises:
According to one embodiment, the hinges are circular flexure notch hinges.
According to a preferred embodiment, the rigid base, the moveable r stage and the N linkage mechanisms are machined in a monolithic metallic block, made e.g. from an aluminium alloy, preferably a 7075 aluminium alloy, or from another metallic material such as steel, Invar, copper or titanium.
According to a preferred embodiment, the number N is equal to 4, the rigid base and moveable stage being both rectangular and preferably square.
Advantageously, the moveable stage can be provided with a through hole having an axis perpendicular to the reference plane. This through hole can be used e.g. for illuminating from below a sample positioned on the moveable stage above the through hole. In another application, the through hole can be use to receive a moveable platform, which can move relative to the moveable stage parallel to the axis of the through hole, i.e. perpendicular to the reference plane.
According to an embodiment, the apparatus further comprises a plurality of actuators, each being connected to the moveable rectangular stage to provide motion thereto.
Advantageously, the apparatus according to the invention further comprises at least two position sensors, which can be e.g. interferometers or capacitive, inductive or ultrasonic transducers for measuring the motion of the moveable rectangular stage.
According to still another aspect of the invention, the apparatus according to the invention may further comprises:
The sets of flexure strips are evenly distributed at the periphery of the moveable platform.
The apparatus may further comprise an actuator connected to the moveable platform to provide motion thereto perpendicular to the reference plane.
According to another aspect of the invention, there is provided a monolithic metallic block comprising:
Advantageously, in the block according to the invention:
Advantageously, the central connecting rod is connected to each of the other four connecting rods thanks to two flexure notch hinges.
Advantageously, each connecting rod comprises a rectangular bar.
Advantageously, the double parallelograms are symmetric with respect to an axis perpendicular to the reference plane.
According to a further aspect of the invention, there is provided a micrometric positioning apparatus comprising:
Advantageously, the micrometric positioning apparatus according to the invention further comprises:
Advantageously, the strips are regularly distributed at the periphery of the moveable platform.
According to yet another aspect of the invention, there is provided a precision positioning device comprising:
This unidirectional precision positioning apparatus can be used in connection with the bidirectional positioning apparatus according to the first aspect of the invention, to provide a third degree of freedom of translation. It can also be used independently, to provide one degree of freedom of translation.
The shape and dimensions of the flexure strips are preferably identical. The flexure strips preferably have a constant rectangular cross-section. Each flexure strip realises a beam restrained at its longitudinal ends by the clamping jaws and defines a longitudinal neutral axis extending from the inner plate to the outer annular frame. The neutral axes of the two opposite strips are preferably parallel, and can be aligned.
If necessary, more than two flexure strips, e.g. three or four strips, may be provided between the inner plate and outer annular frame of the at least one guiding sheet. Preferably the strips should be regularly distributed around the inner plate, i.e. the angle between the neutral axes of two adjacent strips should be constant. Multiplying the number of strips increases the guiding accuracy. This is particularly true where the material from which the guiding sheet is made is isotropic.
According to one embodiment, the first guiding sheet is made of an anisotropic metallic material having a longitudinal fibres extending in a longitudinal direction parallel to the reference plane, e.g. a chrysocolla, which is an alloy of copper, tin and zinc exhibiting interesting mechanical properties. In such a case, the guiding sheet is preferably provided with only one pair of opposite flexure strips, “opposite” meaning having parallel, and preferably identical neutral axes, such that the orientation of the fibres in the material of the strips is at the same angle with respect to the neutral axes for the two strips, ensuring identical behaviour of the two strips in particular in terms of material fatigue caused by repeated variations of stress.
It may also prove advantageous, instead of multiplying the number of strips on one and the same guiding sheet, to provide at least a second guiding sheet identical with the first guiding sheet, parallel to the reference plane and oriented at and angle within the reference plane relative to the first guiding sheet. Hence, it is possible to provide more than one pair of flexure strips for guiding the moveable core of the positioning apparatus, while ensuring that the orientation of the fibres with respect to the longitudinal axis of each strip is identical. If the set of guiding sheets consists of P identical sheets, where P is an integer greater than 1, the angle between two consecutive sheets should preferably be 180°/P.
In order to ensure that the orientation of the fibres in each guiding sheet is the same, the guiding sheets should preferably be machined from one and the same sheet by one and the same machine.
Advantageously, the second guiding sheet can be directly laid on the first guiding sheet.
In order to provide a translation of the moveable core, the precision positioning apparatus may further include:
In order to reach the desired accuracy of the deflection of the flexure strips, one major challenge is to ensure that the ends of all flexure strips are restrained in the same way. To ensure this, it may prove advantageous to provide:
This interleaved sheets can be machined (cut) with more accuracy than the clamping jaws, and will define sharp edges for delimiting on the one hand the part of the guiding sheets that is clamped between the clamping jaws and on the other hand the part of the flexure strips that is bendable.
The unidirectional precision positioning defined above ensures that the translation in a direction perpendicular to the reference plane does not generate any parasitic rotation of the moveable core in the reference plane, i.e. about the an axis parallel to the translation axis. In order to eliminate the parasitic rotation out of the reference plane, i.e. about an axis perpendicular to the translation axis, it may be appropriate to duplicate the first guiding sheet with a second guiding sheet located at a distance from the first guiding sheet along the translation axis. Hence, according to one embodiment of the invention: the hollow base comprises an intermediate outer clamping jaw fixed to the lower and upper outer jaws, the inner core comprises an intermediate inner clamping jaw fixed to the lower and upper inner clamping jaw, the first guiding sheet being a lower guiding sheet sandwiched between the lower and intermediate clamping jaws, the set further comprising an upper metallic flat guiding sheet identical with the first guiding sheet sandwiched between the upper and further clamping jaws, whereby flexure of the flexure strips results in translation of the inner core with respect to the hollow support platform in a direction perpendicular to the reference plane without rotation.
Preferably, the distance between the lower and upper guiding sheets should be at least of the same order as the length of the flexure strips, and preferably at least twice this length.
As stated before, the unidirectional precision positioning apparatus described previously can be combined with a bidirectional positioning apparatus according to the first aspect of the invention to provide a tridirectional system. Hence, according to another aspect of the invention, there is provided a precision positioning apparatus comprising:
the precision positioning apparatus further comprising:
Other advantages and features of the invention will become more clearly apparent from the following description of a specific embodiment of the invention given as non-restrictive example only and represented in the accompanying drawings in which:
Hereinafter, a preferred embodiment of an ultra precision positioning apparatus according to the invention will be described in detail with reference to the accompanying drawings.
It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications of the described embodiments, and any further applications of the principles of the invention as described herein, are contemplated as would normally occur to one skilled in the art to which the invention relates.
The base 2 comprises four base sides 21, 22, 23 and 24 forming four inner walls facing the stage. Similarly, the stage 3 comprises four stage sides 31, 32, 33 and 34 forming four walls, each facing one of the walls of the base such that the facing walls are parallel and equidistant. Each stage side 31 is parallel to one base side 21. The stage 3 is connected to the base 2 thanks to four linkage mechanisms 41, 42, 43 and 44.
Each linkage mechanism 41, 42, 43 and 44, as more precisely represented in
The double parallelograms are arranged so that two opposite double parallelograms 41 and 43, or 42 and 44, are symmetric with respect to a vertical axis 5 going through the centre of the stage.
Each double parallelogram is arranged so the connecting rod 413 is placed at equal distance from the stage side 21 and the base side 31. This central connecting rod 413 is parallel to the stage side 21 and to the base side 31. Two pairs of lateral connecting rods 411, 414 and 412, 415 protrude on each side of the central connecting rod 413 and connect the central connecting rod 413 to the stage side 21 and the base side 31, respectively. The angle between the central connecting rod 413 and each of the lateral connecting rods 411, 412, 414, 415 is approximately 45°.
Each lateral connecting rod consists of a bar having a rectangular section and provided with a circular flexure notch hinge at each end. Each circular flexure notch hinge is elastically deformable thereby constituting a pivot joint, which is adapted to small angles of rotation. The circular flexure notch hinge, illustrated more precisely in
There is a total of height circular flexure notch hinges 416, 417, 418a, 418b, 419a, 419b, 420, 421 per double parallelogram. The height circular flexure notch hinges 416, 417, 418a, 418b, 419a, 419b, 420, 421 form eight pivots positioned at one of the vertices 416, 417, 418, 419, 420, 421 of the double parallelogram and has a pivot axis parallel to the axis Z. The deformation of these pivots produces translation of the stage side 31 in the XY reference plane as represented in
Only one double parallelogram is sufficient to provide two degrees of freedom but the chosen over-constrained arrangement, with four double parallelograms, increases guidance performances: the four double parallelograms arrangement increases the equivalent transverse stiffness and reduces the global undesired motion. In the perfect case where all the flexure hinges are identical and parallel, the only degrees of freedom of the XY stage are the two translations parallel to the X and Y axes.
To approach this ideal case, the precision positioning apparatus have been machined in a monolithic alloy by EDM to guarantee the double parallelograms symmetry and above all the position and dimension of each flexure hinge, i.e. the thickness and the stiffness of the circular flexure notch hinges.
The precision positioning apparatus according to the invention further comprises a plurality of actuators connecting the XY stage to the base and providing motion to the XY stage with respect to the base. Preferably, the precision positioning apparatus comprises four actuators, which are disposed symmetrically around the XY stage. Each actuator is able to push on a stage side thereby providing deformation of the six pivots formed by the circular flexure notch hinges, thereby providing translation of the stage side along the axis X or along the axis Y. The over constrained arrangement guarantees that the stage can not rotate during this translation. Moreover, the translation occurs only in the XY reference plane and no motion parallel to Z axis is possible.
The actuators can be, for example, electrostatic, electromagnetic or piezoelectric. The apparatus is provided with displacement sensors to control the motion of the stage.
With this precision positioning apparatus cast in one piece, the undesired rotational motions measured are in the range of 1.3 μrad for a 100 μm displacement with a first resonance at 170 Hz.
When used in an AFM, the precision positioning apparatus according to the invention is also provided with four dual path and differential interferometers for the position measurement of the stage relative to the tip of the AFM.
Moreover, the precision positioning apparatus according to the invention is also provided with a platform or Z-stage 8, which is represented more precisely in
The strips are part of monolithic flat guiding sheets 9 illustrated in
The sandwich of sheets is clamped between the movable stage and an upper outer clamping jaw 30 via a set of bolts. Similarly, the two sheets are sandwiched between an inner cylindrical core 80 of the Z-stage 8 and an upper inner clamping jaw 81 via bolts.
Preferably, two additional pairs of external and internal flat sheets 95, resp. 96 are placed on top and below the two guiding sheets 9, interleaved between the guiding sheets and the massive parts. All the sheets cut from the same basic planar sheet of metal alloy, preferably in one and the same machining operation. Remarkably, each of the sheets is provided with two rectilinear edges 93, 94, 97, 98, which are place directly on top or directly below the end of one of the flexure strips 90A, 90B. Great care is taken during the cutting process to ensure the accuracy of the dimensions of these edges.
The moveable stage 8 of the monolithic metallic bloc is linked to the Z-stage via the flexure strips, which are evenly distributed at the periphery of the cylindrical core.
Similarly, a second set of two guiding sheets and two pairs of interleaved sheets is clamped between the inner core 80 and moveable stage 3 on the one hand, and a set of lower inner and outer clamping jaws 32, 82.
The dimensions of the eight flexure strips are identical and their ends are precisely restrained between the edges 93, 94, 97, 98 of the adjacent sheets. Each flexure strip forms an ideal beam constrained at both ends and presents a rectangular cross-section.
The platform is further coupled to an actuator enabling to translate the platform 8 parallel to the Z-axis. When the actuator translates the platform 8, the flexure strips bend. As the strips are identical and equally distributed at the circumference of the inner core, the bending of the strips leads to the translation of the platform 8 along the Z-axis.
The arrangement of the flexure strips constraints all directions except the movement along the Z axis by minimizing circular deviations because strain is symmetrically distributed around the mobile cylinder. For a 10 μm displacement along the Z axis, which corresponds with a 1 mrad bending angle, elongation of each strip is about five nanometers. Three flexure strips should be sufficient but the four-strip configuration is more symmetrical: elastic forces are balanced between the four flexure strips, minimizing the rotations and straightness errors.
Moreover, the second stack of flexure strips, at the bottom of the mobile cylinder increases the translation guidance because pitch and tilt motions are constrained more efficiently by the tension/compression stiffness of the leaf type strips. For a given unwanted rotation of the mobile cylinder (for example 1 μrad), the equivalent tension of the lower strip is proportional to the cylinder height. Hence, the longer the cylinder is (i.e. the distance between the stacks of sheets, the more constrained the rotations will be. In one experimental prototype, the elongation for a 1 μrad unwanted rotation is about 30 nm, which corresponds to a 210 N force, which is about one thousand times higher than the force necessary for a 10 μm displacement along the Z axis (about 0.2 N). In this respect, it can be considered that this parasitical movement is fully constrained. In other words, this symmetric arrangement of the flexure strips provides an over-constrained configuration, which has only one degree of freedom: the translation parallel to the Z axis, which is perpendicular to the sheets.
It should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the following claims. While the preferred embodiment described above has a rectangular base and a rectangular stage, other polygonal shapes, preferably regular polygons such as an equilateral triangle, a regular pentagon or hexagon, may be contemplated. While the monolithic X-Y stage is combined with a Z-stage, it is also contemplated that both stages can be implemented independently from one another.
