Positioning device having static fluid bearings

Abstract: A positioning device comprising a Ψ-carriage (85) which is guided along a circular cylindrical guide (79) of a transverse carriage (65) by means of a static fluid bearing (87) and which can thus rotate about an axis of rotation which coincides with a centreline (83) of the circular cylindrical guide (79). A working point (107) of a tool (93) fastened to the Ψ-carriage (85) is lying on the axis of rotation (83) so that an angular adjustment of the tool (93) effected by rotation of the Ψ-carriage (85) can take place independently of a position adjustment of the working point (107) effected by a displacement of the transverse carriage (65) and a longitudinal carriage (7). The positioning device can be used in machined for the processing of surfaces, in particular in the manufacture of linear and circular Fresnel lenses for projection television screens by means of a cutting tool.

Description: The invention relates to a positioning device according to the preamble of independent claim 1 for machining a workpiece fastened to a rotatable support, which device is provided with a longitudinal carriage movable in an x-direction, a transverse carriage which is movably guided along the longitudinal carriage in a y-direction perpendicular to the x-direction, and a ϕ-carriage which is rotatably guided along a circular cylindrical guide of the transverse carriage about an axis of rotation of the ϕ-carriage, which is perpendicular to the x-direction and the y-direction and coincides with a centreline of said circular cylindrical guide. A positioning device of the type described in the opening paragraph and functioning as a lathe attachment for lens production is known from AU-A-551,713. The ϕ-carriage of the known positioning device is constituted by an arcuately contoured block 15, which is rotatably mounted in a further block 14 constituting the transverse carriage. The further block is mounted on a so-called transverse table 11 and is guided along a straight guide of the transverse table, which constitutes the longitudinal carriage and which is supposed to be movable in a horizontal direction parallel to the face plate 24. Since the tip of the tool fastened to the ϕ-carriage is lying close to the axis of rotation of the ϕ-carriage, the x-position and the y-position of the tool tip are maintained upon rotation of the ϕ-carriage, so that these positions do not have to be corrected by displacements of the longitudinal and transverse carriage when the ϕ-carriage is rotated. A drawback of the known positioning device is the lack of stability, rigidity and positioning accuracy, which arises as a result of the stacking of the various carriages. In the known device, the forces exerted by the tool on the transverse carriage are transmitted via the longitudinal carriage to the frame of the apparatus. Furthermore, inaccuracies in the position of the longitudinal carriage have a direct, adverse influence on the accuracy of the position of the transverse carriage. The invention has for its object to provide a positioning device which is more stable and rigid, and in which the influence of inaccuracies in the position of the longitudinal carriage have a relatively small effect on the accuracy of the position of the transverse carriage. The positioning device according to the invention is for this purpose characterized in that the longitudinal carriage and the transverse carriage are supported in a direction parallel to the axis of rotation of the ϕ-carriage via respective static fluid bearings by a single common base surface extending parallel to the x-direction and the y-direction, and that the longitudinal carriage and the transverse carriage are guided along said single common base surface by means of said respective static fluid bearings. It is achieved by this, that a main component of a force exerted by the tool on the transverse carriage in a direction parallel to the axis of rotation is directly transmitted to the base surface via the static fluid bearing of the transverse carriage, which results in a high stability and rigidity of the positioning device. Furthermore, inaccuracies in the position of the longitudinal carriage perpendicular to the x-direction, which can arise as a result of inaccuracies in the flatness of the base surface, have no influence on the position of the transverse carriage. It is noted, that CH-A-154 593 discloses a positioning device in which a longitudinal carriage and a transverse carriage are both supported by two common parallel guide rails, which are mounted on a base element. The use of these two distinct guide rails however leads to a kinematically over-constrained support of the carriages, and therefore leads to problems of stability, accuracy and rigidity. A particular embodiment of a positioning device according to the invention is characterized in that the ϕ-carriage is supported in a direction parallel to the axis of rotation of the ϕ-carriage via a separate static fluid bearing by the single common base surface, and that the ϕ-carriage is guided along the single common base surface by means of said separate static fluid bearing. It is achieved by this, not only that the guiding of the ϕ-carriage is substantially frictionless, but also that the main component of the force exerted by the workpiece on the tool in a direction parallel to the axis of rotation is directly transmitted to the base surface via the static fluid bearing of the ϕ-carriage. It is achieved in addition that inaccuracies in the position of the transverse carriage in the direction of the axis of rotation resulting from inaccuracies in the flatness of the base surface have no influence on the position of the ϕ-carriage. A further embodiment of a positioning device according to the invention, which provides a substantially frictionless and very rigid guiding of the transverse carriage and the ϕ-carriage, is characterized in that the transverse carriage is guided along a flat wall of the longitudinal carriage in a direction parallel to the y-direction by means of a static fluid bearing, while the ϕ-carriage is guided, also by means of a static fluid bearing, along the circular cylindrical guide of the transverse carriage. A particular embodiment of a positioning device according to the invention, which provides a substantially frictionless guiding of the longitudinal carriage, is characterized in that the longitudinal carriage comprises a first part, which is guided along the common base surface by means of a static fluid bearing, and a second part, which is guided along a support surface which is perpendicular to the y-direction by means of a static fluid bearing, the first part and the second part being mechanically coupled by means of cross hinges built up from plate springs, which parts of the longitudinal carriage are rotatable relative to one another under spring force of the cross hinges about a pivot axis which coincides substantially with a line of intersection of the base surface and the support surface. It is achieved by the use of the said cross hinges that inaccuracies in the perpendicularity between the support surface and the base surface substantially do not influence the accuracy with which the two said parts of the longitudinal carriage are guided along the base surface and support surface, respectively. A further embodiment of a positioning device according to the invention, which provides a ϕ-carriage drive free from play and hysteresis, is characterized in that the ϕ-carriage is rotatable about the axis of rotation of the ϕ-carriage by means of a drive which comprises a shaft extending in radial direction relative to the axis of rotation of the ϕ-carriage and carrying a friction wheel which bears on a guide rail attached to the ϕ-carriage, which rail has the shape of a circular arc whose centre lies on the axis of rotation of the ϕ-carriage, this friction wheel being driven by a motor attached to the transverse carriage. The invention will be explained in more detail with reference to the drawing, in which: Fig. 1 shows a first embodiment of a positioning device according to the invention, Fig. 2 shows a plan view of the positioning device of Fig. 1, Fig. 3 shows a cross-section of the positioning device taken on the line III-III in Fig. 2, Fig. 4 shows a few details of the cross-section of the positioning device according to Fig. 3, Fig. 5 shows a cross-section of the positioning device taken on the line V-V in Fig. 2, Fig. 6 shows a cross-section of the positioning device taken on the line VI-VI in Fig. 5, Fig. 7 shows a cross-section of the positioning device taken on the line VII-VII in Fig. 2, and Fig. 8 shows a second embodiment of a positioning device according to the invention. The first embodiment of the positioning device illustrated in Figs. 1-6 comprises a base surface 3 belonging to a frame 1, which surface is parallel to an x-direction indicated in Fig. 1 and to a y-direction, which is perpendicular to the x-direction. A first part 5 of a longitudinal carriage 7 has two parallel beams 9 and 11 which are interconnected by a bridge 13 and a plate spring 15. The beams 9 and 11 are guided along the base surface 3 by static fluid bearings 17 and 19, respectively, which are known per se and indicated merely diagrammatically in Fig.3. A few details of the static fluid bearing 17 are shown in Fig.4. Fluid is supplied to a chamber 23 of the beam 9 via a channel 21 which is connected to a pressure source. Fluid is drained off via channels 25. An accurate position of the beam 9 relative to the base surface 3 in a direction transverse to the x- and y-directions is maintained in this case in that the fluid bearing 17 is pre-tensioned by means of a suitable underpressure in a chamber 27 which is provided in the beam 9 in a manner shown diagrammatically in Fig. 4. The chamber 27 is connected to a vacuum source through a channel 29. The static fluid bearing 19 of beam 11 is of a similar type as the bearing 17 shown in Fig. 4, and is consequently also pre-tensioned by means of an underpressure relative to the base surface 3. A second part 31 of the longitudinal carriage 7 comprises, as can be seen in Fig. 6, two beams 33 and 35 which are interconnected by a plate-shaped flexible bridge 37 of aluminium. The beams 33 and 35 are guided along a support surface 43 transverse to the y-direction by means of static fluid bearings 39 and 41, respectively, of a type diagrammatically shown in Fig. 4. An accurate position of the longitudinal carriage 7 in the y-direction relative to the support surface 43 is maintained in this case in that the fluid bearings 39 and 41 are pre-tensioned by means of an underpressure in the known manner shown diagrammatically in Fig. 4. The first part 5 and the second part 31 of the longitudinal carriage 7 are mechanically coupled by means of cross hinges 47 and 49 (see Figs. 1 and 2) which are built up from plate springs 45. The beams 9 and 11 of the first part 5 of the longitudinal carriage 7 are coupled to the beams 33 and 35 of the second part 31 of the longitudinal carriage 7 by means of the cross hinges 47 and 49, respectively. It can be seen in the Figs. 2 and 7 that the plate springs 45 are attached to the beams 9, 11 and 33, 35 in such a way that the first part 5 and the second part 31 of the longitudinal carriage 7 can be rotated relative to one another about a pivot axis 51, which substantially coincides with a line of intersection 53 between the base surface 3 and the support surface 43, under spring pressure of the cross hinges 47 and 49. The coupling by the cross hinges 47 and 49 achieves that an accurate and substantially frictionless guiding of the longitudinal carriage 7 along the base surface 3 and the support surface 43 is even possible in the case of any inaccuracies in the perpendicularity of the support surface 43 relative to the base surface 3. The longitudinal carriage 7 thus guided along the base surface 3 and the support surface 43 can be displaced in the x-direction by means of a drive mechanism, indicated only diagrammatically in Fig. 1, which is of a kind known per se with a straight guide rail 55 attached to the frame 1 and a friction wheel 57 which bears on the guide rail 55 and is driven by a motor 59 attached to the longitudinal carriage 7. The beams 9 and 11 have flat walls 61 and 63, respectively, which are parallel to the y-direction and perpendicular to the base surface 3. A transverse carriage 65 is positioned between the beams 9 and 11 of the longitudinal carriage 7 and is displaceable relative to the longitudinal carriage 7 in the y-direction by means of a drive mechanism of a kind which is known per se , indicated only diagrammatically in Figs. 1 and 2, with a shaft 67 attached to the transverse carriage 65 and a friction wheel 69 which bears on the shaft 67 and is driven by a motor 71 attached to the bridge 13 of the longitudinal carriage 7. In this construction, the transverse carriage 65 is guided along the flat walls 61 and 63 of the beams 9 and 11 of the longitudinal carriage 7 by static fluid bearings 73 and 75, respectively, of the type shown in Fig. 4 and discussed with reference to Fig. 4, which are represented only diagrammatically in Fig. 3. An accurate position in the x-direction of the transverse carriage 65 relative to the beams 9 and 11 is maintained in that the fluid bearings 73 and 75 are pre-tensioned by means of a suitable underpressure as discussed hereinbefore with reference to the fluid bearing 17 shown in Fig. 4. A distance in the x-direction between the beams 9 and 11 and between the beams 33 and 35, connected to the former by means of cross hinges 47 and 49, respectively, is determined by the dimensions of the transverse carriage 65. The bridge 13 between the beams 9 and 11 serves to keep the beams 9 and 11 in position relative to one another when the static fluid bearings 73 and 75 are not operational. Any differences in expansion in the x-direction between the transverse carriage 65 and the bridge 13 during operation are accommodated by bending of the plate spring 15. Furthermore, the transverse carriage 65 is guided along the base surface 3 by means of a static fluid bearing 77 of the type discussed with reference to Fig. 4 and indicated only diagrammatically in Fig. 3. The transverse carriage 65 is also kept in an accurate position relative to the base surface 3 in a direction transverse to the base surface 3 in that the fluid bearing 77 is pre-tensioned by means of an underpressure, as was discussed with reference to the fluid bearing 17 in Fig. 4. The transverse carriage 65 has a circular cylindrical guide 79 which forms a boundary of an imaginary circular cylinder 81 indicated in Fig. 2 with a centreline 83 (see Figs. 1 and 5) perpendicular to the base surface 3. A ϕ-carriage 85 is guided along the circular cylindrical guide 79 of the transverse carriage 65 by means of a static fluid bearing 87, which is indicated only diagrammatically in Fig. 2 and is again of the type shown in Fig. 4. The ϕ-carriage 85 is kept in an accurate position relative to the transverse carriage 65, i.e. a radial position relative to the centreline 83, by pre-tensioning of the fluid bearing 87 by means of permanent magnets 89 indicated diagrammatically in Fig. 5. Furthermore, the ϕ-carriage 85 is guided along the base surface 3 by means of a static fluid bearing 91 of the type shown in Fig. 4 and indicated diagrammatically in Fig. 5. The ϕ-carriage 85 is here kept in an accurate position relative to the base surface 3 in a direction transverse to the base surface 3 in that the fluid bearing 91 is pre-tensioned by means of an underpressure in the same way as with the fluid bearing shown in Fig. 4. The ϕ-carriage 85 is rotatable along the circular cylindrical guide 79 of the transverse carriage 65 about an axis of rotation which coincides with the centreline 83 of the circular cylindrical guide. A cutting tool 93 is fastened to the ϕ-carriage 85, with which tool a plate-shaped workpiece 97 clamped on a vertically positioned rotatable disc 95 can be machined. The disc 95 is rotatable about an axis of rotation 99 which is parallel to the y-direction. The cutting tool 93 has a first edge 101 and a second edge 103, as shown in Fig. 2. A common end of the first edge 101 and the second edge 103 forms a tip 105 of the tool 93. This tip 105 of the tool 93 is shown on an exaggerated scale in Fig. 2. A working point 107 of the tool 93 coincides with the point of intersection of the extension of the edge 101 and the extension of the edge 103, and lies close to the tip 105 of the tool 93. The tool 93 is fastened to the ϕ-carriage 85 in such a way that the working point 107 of the tool as defined above lies on the axis of rotation 83 of the ϕ-carriage 85. It is achieved in this way that an x-position and a y-position of the working point 107 of the tool 93 substantially do not change if the ϕ-carriage 85 is rotated about the axis of rotation 83. Thus an adjustment of an angle at which the tool 93 machines the workpiece 97 can take place independently of an adjustment of an x-position and a y-position of the working point 107 of the tool. The ϕ-carriage 85 is rotated relative to the transverse carriage 65 about the axis of rotation 83 by means of a drive 109. The drive 109 comprises a guide rail 111 attached to the ϕ-carriage 85. The guide rail 111 has the shape of a circular arc whose centre is situated on the axis of rotation 83. A drive motor 113 with an output shaft 115 is attached to the transverse carriage 65, the said output shaft being radially disposed relative to the axis of rotation 83. A friction wheel 117 attached to the shaft 115 bears on the guide rail 111 attached to the ϕ-carriage 85. The drive 109 of the ϕ-carriage 85 is substantially free from play and frictionless, so that an accurate angular adjustment substantially free from hysteresis of the tool 93 is possible. In the case of the positioning device represented in Fig. 1, in which a workpiece 97 rotates about an axis of rotation 99 which is parallel to the y-direction, a main component of a force exerted on the tool 93 by the rotating workpiece 97 is transverse to the x-direction and substantially transverse to the base surface. Since the ϕ-carriage has its bearings directly on the base surface 3, a high degree of rigidity is achieved and the displacement of the tip 105 of the tool 93 under the influence of the said force is minimal. The positioning device shown in Fig. 1 is particularly suitable for processing a so-called master destined for the manufacture of circular Fresnel lenses. In this process, an accurate pattern of concentric lens elements (cutting grooves) is applied at one side of such a plate-shaped master with the aid of the positioning device. A second embodiment of a positioning device according to the invention shown in Fig. 8 differs from the first embodiment described above in that a plate-shaped workpiece 119 is clamped around a drum 121 which is attached to a rotatable support not shown in detail in Fig. 8, rotating about an axis of rotation 123 which is parallel to the x-direction. In the case of the positioning device shown in Fig. 8, again, a main component of a force exerted on the tool 93 by the rotating workpiece 119 is transverse to the x-direction and substantially transverse to the base surface 3, so that here also a high degree of constructional rigidity is obtained by means of a direct bearing of the ϕ-carriage 85 on the base surface 3. The positioning device shown in Fig. 8 is suitable first and foremost for processing a master for the manufacture of linear Fresnel lenses. In this process, an accurate pattern of parallel, linear lens elements (cutting grooves) is applied by the tool 93 at one side of the workpiece 119 clamped around the drum 121. It should be noted that a positioning device in which the longitudinal carriage, the transverse carriage and the ϕ-carriage are positioned relative to one another in a manner other than the one described, or have their bearings in a manner other than the one described, may also possess a single common base surface, by which the longitudinal and transverse carriages are supported via respective static fluid bearings. Thus, for example, the longitudinal carriage and the transverse carriage may be guided along a common base surface by means of static fluid bearings, while the ϕ-carriage is guided along a surface of the transverse carriage perpendicular to the axis of rotation and along the circular cylindrical surface of the transverse carriage by means of static fluid bearings. The fluid used in the static fluid bearings may be a gas or a liquid. Guiding of the ϕ-carriage along the circular cylindrical surface of the transverse carriage may in this case obviously also take place in a conventional manner such as, for example, by means of a guide rail attached to the transverse carriage and roller members attached to the ϕ-carriage. In these cases, generally, a less high constructional rigidity and positioning accuracy are achieved than in the embodiment of a positioning device shown in Fig. 1 or Fig. 8, though sufficiently high for many applications. The two embodiments of a positioning device according to the invention described above make it possible to carry out a cutting operation on a plate-shaped workpiece. This workpiece may in principle be made of any machinable metal such as, for example, copper, aluminium, or brass, the workpiece being machined with a tool suitable for the purpose, such as a diamond chisel, while alternatively it may be made of a machinable synthetic material such as, for example, polymethyl methacrylate or delrin. Besides masters for Fresnel lenses for use as projection television screens, other products may also be manufactured by means of the positioning device. In particular, the positioning device makes it possible to planarize a plate-shaped workpiece with a relatively high accuracy. For this purpose, a horizontal instead of a vertical positioning of the workpiece, which is then machined at its lower side by means of a suitably adapted positioning device according to the first embodiment, may then be used with the advantage that chip disposal is more efficient. Finally, it should be noted that in addition to the cutting operations discussed other types of operations are also possible with the aid of the positioning device. In this connection, grinding and lapping operations may be mentioned, and also laser operations such as, for example, laser welding.

Claims: A positioning device for machining a workpiece (97) fastened on a rotatable support (95), which device is provided with a longitudinal carriage (7) movable in an x-direction, a transverse carriage (65) which is movably guided along the longitudinal carriage in a y-direction perpendicular to the x-direction, and a ϕ-carriage (85) which is rotatably guided along a circular cylindrical guide (79) of the transverse carriage about an axis of rotation (83) of the ϕ-carriage, which is perpendicular to the x-direction and the y-direction and coincides with a centreline (83) of said circular cylindrical guide, characterized in that the longitudinal carriage (7) and the transverse carriage (65) are supported in a direction parallel to the axis of rotation (83) of the ϕ-carriage via respective static fluid bearings (17, 19, 77) by a single common base surface (3) extending parallel to the x-direction and the y-direction, and that the longitudinal carriage and the transverse carriage are guided along said single common base surface by means of said respective static fluid bearings. A positioning device as claimed in Claim 1, characterized in that the ϕ-carriage (85) is supported in a direction parallel to the axis of rotation (83) of the ϕ-carriage via a separate static fluid bearing (91) by the single common base surface (3), and that the ϕ-carriage is guided along the single common base surface by means of said separate static fluid bearing. A positioning device as claimed in Claim 1 or 2, characterized in that the transverse carriage (65) is guided along a flat wall (61, 63) of the longitudinal carriage (7) in a direction parallel to the y-direction by means of a static fluid bearing (73, 75), while the ϕ-carriage (85) is guided, also by means of a static fluid bearing (91), along the circular cylindrical guide (79) of the transverse carriage. A positioning device as claimed in Claim 1, 2 or 3, characterized in that the longitudinal carriage (7) comprises a first part (5), which is guided along the common base surface (3) by means of a static fluid bearing (17, 19), and a second part (31), which is guided along a support surface (43) which is perpendicular to the y-direction by means of a static fluid bearing (39, 41), the first part and the second part being mechanically coupled by means of cross hinges (47, 49) built up from plate springs (45), which parts of the longitudinal carriage are rotatable relative to one another under spring force of the cross hinges about a pivot axis (51) which coincides substantially with a line of intersection (53) of the base surface and the support surface. A positioning device as claimed in Claim 1, 2, 3, or 4 characterized in that the ϕ-carriage (85) is rotatable about the axis of rotation (83) of the ϕ-carriage by means of a drive (109) which comprises a shaft (115) extending in radial direction relative to the axis of rotation of the ϕ-carriage and carrying a friction wheel (117) which bears on a guide rail (111) attached to the ϕ-carriage, which rail has the shape of a circular arc whose centre lies on the axis of rotation of the ϕ-carriage, this friction wheel being driven by a motor (113) attached to the transverse carriage (65).

Inventors: DONA MARINUS JOSEPHUS JAKOBUS, SWINKELS JOHANNES MARTINUS MAR, DONA, MARINUS JOSEPHUS JAKOBUS, SWINKELS, JOHANNES MARTINUS MARIA
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: B23Q1
IPC codes: B23B21, B23Q1