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Approximation of positioning accuracy to nanometer values causes a new level of technical requirements for equipment and processing of drive elements. In this paper an alternative to standard mechanical moving elements and its applicability in various branches of machine building and instrument making is considered.
Теги: machining centers non-contact mechanics precision movements бесконтактная механика обрабатывающие центры прецизионные перемещения
The non-contact magnetic screw-nut pair is a key element of the high-precision linear actuator, promising for use in machining centers, positioning systems, precision mechanics. In this driving gear, it is possible to productively combine a real nanometer level of accuracy with unlimitedly large strokes and exceptionally high power characteristics – stiffness, developed force [1].
The non-contact magnetic screw-nut pair attracted the attention of engineers half a century ago, but the real power characteristics of the created transmissions were hundreds of times lower than the level that could be of practical interest [2, 3].
MAGNETIC LEAD SCREW
AND FEATURES OF ITS WORK
The principle of operation of any magnetic transmission conceptually is extremely simple. It is explained in Fig.1, which depicts two pairs of rectangular opposite polarity magnets (1, 2), placed with a gap (3) and displacement in the lateral direction (4). The poles are acted on by a force whose lateral component (in the figure from left to right) tries to set the poles against each other (for the purpose of the drive it is useful), and the force of attraction that tends to pull the poles towards each other (is harmful because of the threat of loss of contactlessness). In our case, the poles are the crests of threads of the screw (5) and the nut (6), respectively. The lateral force moves the nut while the screw rotates, and the normal force tends to "stick" the nut to the screw. In order to avoid mechanical contact in the pair, the thread grooves of screw (7) and nut (8) are filled with a non-magnetic compound, and compressed gas is throttled into the formed gap (3), on the film of which the screw "floats up", which completely eliminates contact and friction in the magnetic magnetic lead screw.
Despite the simplicity of the principle of the magnetic lead screw's action, its implementation in a real design with competitive characteristics turned out to be far from simple. All work on magnetic lead screws (quite qualitative from the academic point of view) essentially consisted in a detailed study of absolutely unpromising designs, rather as a kind of physical object (not to say, curiosity), in which the values of key characteristics are tens and hundreds of times less than achievable within the framework of adequate design [4, 5].
Some of the exceptions are a series of studies of force and vibration characteristics performed in the US by our ex-compatriot, using a specimen of a rudimentary but based on correct principles magnetic transmission and an aerostatic drive with its use, taken out of Russia [6–8].
Adequate designs suitable for use in high-precision machines appeared only after we had solved and brought to the level of the system of engineering requirements and design recommendations a whole chain of complex and non-obvious problems of mathematical physics, aerodynamics and materials science [1].
The presence of a reliable theoretical foundation allowed to solve the problem of achieving physically limiting characteristics (first of all, power characteristics, and the main one among them – rigidity) and to develop realistic and inexpensive designs and technologies for their production. The result of the work was a drive with a magnetic lead screw, providing practical accuracy in units of nanometers and having a specific rigidity, which for large drives (with screw diameter of about 50 mm) is comparable to a ball screw, and for drives with a screw diameter of about 10 mm is higher on 50 and more percent.
The kinematic (without external load) accuracy of the drive is ensured by the giant averaging of random errors of the screw due to the large working length of the thread crest. The two-way repeatability of all created transmissions was always below the level of measurements available in the laboratory (10 nm). The error estimates were always better than 1 nm. The in-step and accumulated errors of transmission inherit only the low-frequency component of the systematic error of the screw-cutting machine, which makes it possible to compensate them in the control system by adjusting the angle of rotation of the screw with the accuracy of the metrological device used for attestation. Dynamic (with external load) accuracy is provided by high force parameters of the magnetic lead screw.
Thus, an open-loop drive with a relatively simple angle sensor is able to provide practical accuracy of 1 nm with a path length of hundreds of millimeters in the conditions of a hard force mode.
Due to the requirements of the physics of the magnetic circuit, the design of the magnetic lead screw tends to a small step (0.05–0.25 mm for a single thread) that is not typical for lead screws, which obviously limits the maximum linear velocity.
A valuable feature of the magnetic lead screw are extremely low mechanical losses. For example, a screw gear with a thread pitch of 0.1 mm with a screw diameter of 60 mm does not self-brake, and if it is installed vertically, the screw of the magnetic pair under its own weight will be screwed into the nut.
Physical considerations rigidly dictate also some overall characteristics of the pair, for example the length of the magnetic system of the nut can not exceed half the diameter of the screw. There is a well-known case when machine-tool engineers, wishing to double the force of transmission by increasing the length of the nut, arbitrarily made changes in the geometry of the design and... the force naturally decreased several times.
The optimum magnetic lead screw to successfully compete in the force parameters with the ball screw must consist of 4–6 typical identical aeromagnetic modules.
COMPARISON OF ALTERNATIVES
Competitors of the magnetic lead screw are direct drive systems with a linear motor and drives with hydrostatic lead screw. Among them, the drive with a magnetic lead screw occupies a special place, combining the main advantages of competitors, and avoiding their characteristic problems.
Direct drive is very popular, it is produced by many companies, and by some experts it is considered something like an "absolute weapon" in the sphere of linear drive with submicron accuracy. Drive with hydrostatic transmission is a somewhat rarer technology, but it is produced by such German companies as Hyprostatic [9] and Zollern [10], specializing in hydrostatic machine components. Conceptually, the magnetic lead screw is closer to the hydrostatic lead screw: in both cases an open loop drive is used with an exceptionally accurate motion transformation mechanism and monitoring of only the angular coordinate of the screw.
Tables 1 and 2 show a pairwise comparison of the characteristics of the three discussed drives with minimal comments.
Table 1 demonstrates the decisive superiority of the magnetic lead screw over the direct drive in all respects, with the exception, perhaps, of maximum speed. Indeed, for a single-thread with a small pitch, the speed of large machine drives with a magnetic lead screw is limited to about 300 mm/min, and the speed of small drives is 500 mm/min. But this limitation, firstly, is easily managed by using a multiple thread, and secondly for equipment with practical accuracy of a few tens of nanometers, the time savings on free operation is usually not relevant. But the ability to use the motion transformation mechanism with a huge multiplicity can be very productive.
Table 2 reveals two points that require closer comparison. The maximum speed for hydrostatic transmission is significantly lower than for a linear motor, but, nevertheless, it is noticeably higher than for a magnetic lead screw. The discussion of this circumstance repeats the arguments of the commentary to Table 1.
The question of the ratio of the rigidities requires more detailed analysis. The rigidity of the hydrostatic lead screw itself is 3–4 times higher than that of the optimally designed magnetic lead screw. However, in practice it is clearly redundant. In the complete drive system, all other power elements (bearings and, in particular, the screw) have significantly less stiffness, and, as is known, the resultant rigidity of the elastic system is determined by the weakest element. It is the screw in the hydrostatic transmission, which, due to the deeply incised grooves of the thread, greatly reduces the axial rigidity, and even more, the twist rigidity. At high screw speeds, viscous friction creates a torque that strongly attenuates the weakened screw, which leads to an additional and very significant error. The screw of the magnetic lead screw is solid and rigid, and this practically equates its dynamic accuracy in comparison with hydrostatic lead screw in all realistic situations. At the same time, the magnetic lead screw is free of problems with high pressure and overheating of the oil.
SPECIFIC TECHNOLOGICAL PROBLEMS IN PRODUCTION
OF MAGNETIC LEAD SCREW. HISTORY AND CURRENT STATUS
We have pointed out above the subjective problems that, for the time being, prevented the successful use of magnetic lead screw in the industry. However, objective difficulties also played a big role in their development. It was necessary to create technologies for working with rare-earth magnets, magnetically soft alloys, non-magnetic steels, porous materials. A significant difficulty for machine builders was the cutting of a shallow (0.1 mm pitch) thread of unusual profiles in capricious magnetic alloys. The solution of these problems became possible thanks to the efforts of a small number of people. An important role was played by what is called individual personal mastery, often mentioned in a negative aspect, but without which the first steps of many great things are inconceivable.
By now we have working samples of single-module magnetic lead screw. Fig.2 shows pairs of diameters of 20 mm and 25 mm in assembly and a technological sample of a long lead screw with a diameter of 50 mm. We have manufactured and supplied to a foreign customer a drive for recording CD matrices, which has been operating for about 15 years.
Fig.3 shows a demonstration model of the drive with a screw with a diameter of 20 mm and a resolution of 1 nm. We have optimal designs and reproducible industrial technologies for manufacturing single-module magnetic lead screws in a wide range of sizes and characteristics (with a screw diameter from 10 to 60 mm). Magnetic lead screws with an optimal number of modules (4–6) with an updated design of the magnetic circuit and magnetization technology have been developed. Such a solution should set the industrial standard for serial magnetic lead screws.
In the technical policy of the application of magnetic transmissions, two branches are seen: industrial and instrumental. The industrial branch places special emphasis on the high force parameters of the drives (stiffness: 1 000–1 500 N/μm) and large strokes (500 mm and more). In metalworking, this technology, together with advanced metal processing (for example, hard turning), will provide a revolutionary transformation of the production processes of precision engineering products. The highest level of accuracy will allow obtaining details of higher qualities, corresponding to polishing and free abrasive finishing, on turning and vortex milling machines for one mounting of rough blanks.
In the instrument branch, the requirements of practical accuracy will dominate, which, within the framework of this approach, can be brought to 0.1 nm with a stroke of about 25 mm. In this range of tasks, piezo stepper motors can be competitors, in which the stroke is of the same order, and the resolution is also at the nanometer level. Advantages of magnetic lead screws are the use of an open-circuit that does not require an external reference "ruler", as well as the complete absence of vibration. To solve production problems when creating magnetic lead screws of this class, it is necessary to additionally apply the latest technologies of the femtosecond laser micromachining.
When analyzing the characteristics of sub-precision mechanics, the question arises: how and where the application of these products is possible? In the field of mechanical engineering, this can be metalworking complexes used in aerospace, electronic and optical-mechanical fields (the production of components of positioning systems, mirrors, precision couplings, etc.). A huge plus, as mentioned above, is the one-stage production of complex surfaces with high machining accuracy.
For several years, the Advanced Technologies Center has been successfully implementing and manufacturing small-sized machining centers for training and production [11]. A wide range of tasks of ATCNano CNC machines are successfully carried out due to the accuracy (about 0.002 mm in all axes), rigidity (cantilevered design) and high productivity (feeding using ball screws and linear guides, 1 800 mm/min). Fig.4 shows the matrix used to produce microfluidic chips by the method of soft lithography. The dimensions of the matrix are 32 Ч 24 mm, the size of the tracks is 0.2 Ч 0.2 mm.
With the help of modern software, significant accuracy is also achieved when approximating complex surfaces by a single tool. Fig.5 shows the milled relief obtained by approximating using the spherical cutter D4, R2 with a pitch of 0.05 mm (material: Д16T).
The development of the ATCNano CNC milling center project (Fig.6), designed using modern mechanical, electronic and electromechanical components, does not stand still. Mastering in production and replacement of standard mechanical components such as ball screw and lead screw with nut, by precision magnetic lead screws, opens new possibilities and expands the field of application of equipment. Taking into account the development and implementation of new frictionless components, the approach to nanometer accuracy of processing already seems not a fantasy, but a reality.
In addition to material processing complexes, the company is engaged in the development and testing of automated precision movements for laboratory equipment and microscopy [12, 13]. At compact dimensions (200 Ч 200 mm), the installation of tracking sensors and micro ball screws allowed, in conjunction with the advanced electronic (controller, driver) and electromechanical (servo motors) parts, to obtain fully automated imaging with accuracy unattainable for conventional manual movements and stages. Further complete replacement of the movable mechanics by a non-contact magnetic mechanism will allow us to approach the accuracy of displacements, which is comparable to piezoceramic motors and tubes but appreciably benefits in ease of adjustment and speed of movement. ■
The non-contact magnetic screw-nut pair attracted the attention of engineers half a century ago, but the real power characteristics of the created transmissions were hundreds of times lower than the level that could be of practical interest [2, 3].
MAGNETIC LEAD SCREW
AND FEATURES OF ITS WORK
The principle of operation of any magnetic transmission conceptually is extremely simple. It is explained in Fig.1, which depicts two pairs of rectangular opposite polarity magnets (1, 2), placed with a gap (3) and displacement in the lateral direction (4). The poles are acted on by a force whose lateral component (in the figure from left to right) tries to set the poles against each other (for the purpose of the drive it is useful), and the force of attraction that tends to pull the poles towards each other (is harmful because of the threat of loss of contactlessness). In our case, the poles are the crests of threads of the screw (5) and the nut (6), respectively. The lateral force moves the nut while the screw rotates, and the normal force tends to "stick" the nut to the screw. In order to avoid mechanical contact in the pair, the thread grooves of screw (7) and nut (8) are filled with a non-magnetic compound, and compressed gas is throttled into the formed gap (3), on the film of which the screw "floats up", which completely eliminates contact and friction in the magnetic magnetic lead screw.
Despite the simplicity of the principle of the magnetic lead screw's action, its implementation in a real design with competitive characteristics turned out to be far from simple. All work on magnetic lead screws (quite qualitative from the academic point of view) essentially consisted in a detailed study of absolutely unpromising designs, rather as a kind of physical object (not to say, curiosity), in which the values of key characteristics are tens and hundreds of times less than achievable within the framework of adequate design [4, 5].
Some of the exceptions are a series of studies of force and vibration characteristics performed in the US by our ex-compatriot, using a specimen of a rudimentary but based on correct principles magnetic transmission and an aerostatic drive with its use, taken out of Russia [6–8].
Adequate designs suitable for use in high-precision machines appeared only after we had solved and brought to the level of the system of engineering requirements and design recommendations a whole chain of complex and non-obvious problems of mathematical physics, aerodynamics and materials science [1].
The presence of a reliable theoretical foundation allowed to solve the problem of achieving physically limiting characteristics (first of all, power characteristics, and the main one among them – rigidity) and to develop realistic and inexpensive designs and technologies for their production. The result of the work was a drive with a magnetic lead screw, providing practical accuracy in units of nanometers and having a specific rigidity, which for large drives (with screw diameter of about 50 mm) is comparable to a ball screw, and for drives with a screw diameter of about 10 mm is higher on 50 and more percent.
The kinematic (without external load) accuracy of the drive is ensured by the giant averaging of random errors of the screw due to the large working length of the thread crest. The two-way repeatability of all created transmissions was always below the level of measurements available in the laboratory (10 nm). The error estimates were always better than 1 nm. The in-step and accumulated errors of transmission inherit only the low-frequency component of the systematic error of the screw-cutting machine, which makes it possible to compensate them in the control system by adjusting the angle of rotation of the screw with the accuracy of the metrological device used for attestation. Dynamic (with external load) accuracy is provided by high force parameters of the magnetic lead screw.
Thus, an open-loop drive with a relatively simple angle sensor is able to provide practical accuracy of 1 nm with a path length of hundreds of millimeters in the conditions of a hard force mode.
Due to the requirements of the physics of the magnetic circuit, the design of the magnetic lead screw tends to a small step (0.05–0.25 mm for a single thread) that is not typical for lead screws, which obviously limits the maximum linear velocity.
A valuable feature of the magnetic lead screw are extremely low mechanical losses. For example, a screw gear with a thread pitch of 0.1 mm with a screw diameter of 60 mm does not self-brake, and if it is installed vertically, the screw of the magnetic pair under its own weight will be screwed into the nut.
Physical considerations rigidly dictate also some overall characteristics of the pair, for example the length of the magnetic system of the nut can not exceed half the diameter of the screw. There is a well-known case when machine-tool engineers, wishing to double the force of transmission by increasing the length of the nut, arbitrarily made changes in the geometry of the design and... the force naturally decreased several times.
The optimum magnetic lead screw to successfully compete in the force parameters with the ball screw must consist of 4–6 typical identical aeromagnetic modules.
COMPARISON OF ALTERNATIVES
Competitors of the magnetic lead screw are direct drive systems with a linear motor and drives with hydrostatic lead screw. Among them, the drive with a magnetic lead screw occupies a special place, combining the main advantages of competitors, and avoiding their characteristic problems.
Direct drive is very popular, it is produced by many companies, and by some experts it is considered something like an "absolute weapon" in the sphere of linear drive with submicron accuracy. Drive with hydrostatic transmission is a somewhat rarer technology, but it is produced by such German companies as Hyprostatic [9] and Zollern [10], specializing in hydrostatic machine components. Conceptually, the magnetic lead screw is closer to the hydrostatic lead screw: in both cases an open loop drive is used with an exceptionally accurate motion transformation mechanism and monitoring of only the angular coordinate of the screw.
Tables 1 and 2 show a pairwise comparison of the characteristics of the three discussed drives with minimal comments.
Table 1 demonstrates the decisive superiority of the magnetic lead screw over the direct drive in all respects, with the exception, perhaps, of maximum speed. Indeed, for a single-thread with a small pitch, the speed of large machine drives with a magnetic lead screw is limited to about 300 mm/min, and the speed of small drives is 500 mm/min. But this limitation, firstly, is easily managed by using a multiple thread, and secondly for equipment with practical accuracy of a few tens of nanometers, the time savings on free operation is usually not relevant. But the ability to use the motion transformation mechanism with a huge multiplicity can be very productive.
Table 2 reveals two points that require closer comparison. The maximum speed for hydrostatic transmission is significantly lower than for a linear motor, but, nevertheless, it is noticeably higher than for a magnetic lead screw. The discussion of this circumstance repeats the arguments of the commentary to Table 1.
The question of the ratio of the rigidities requires more detailed analysis. The rigidity of the hydrostatic lead screw itself is 3–4 times higher than that of the optimally designed magnetic lead screw. However, in practice it is clearly redundant. In the complete drive system, all other power elements (bearings and, in particular, the screw) have significantly less stiffness, and, as is known, the resultant rigidity of the elastic system is determined by the weakest element. It is the screw in the hydrostatic transmission, which, due to the deeply incised grooves of the thread, greatly reduces the axial rigidity, and even more, the twist rigidity. At high screw speeds, viscous friction creates a torque that strongly attenuates the weakened screw, which leads to an additional and very significant error. The screw of the magnetic lead screw is solid and rigid, and this practically equates its dynamic accuracy in comparison with hydrostatic lead screw in all realistic situations. At the same time, the magnetic lead screw is free of problems with high pressure and overheating of the oil.
SPECIFIC TECHNOLOGICAL PROBLEMS IN PRODUCTION
OF MAGNETIC LEAD SCREW. HISTORY AND CURRENT STATUS
We have pointed out above the subjective problems that, for the time being, prevented the successful use of magnetic lead screw in the industry. However, objective difficulties also played a big role in their development. It was necessary to create technologies for working with rare-earth magnets, magnetically soft alloys, non-magnetic steels, porous materials. A significant difficulty for machine builders was the cutting of a shallow (0.1 mm pitch) thread of unusual profiles in capricious magnetic alloys. The solution of these problems became possible thanks to the efforts of a small number of people. An important role was played by what is called individual personal mastery, often mentioned in a negative aspect, but without which the first steps of many great things are inconceivable.
By now we have working samples of single-module magnetic lead screw. Fig.2 shows pairs of diameters of 20 mm and 25 mm in assembly and a technological sample of a long lead screw with a diameter of 50 mm. We have manufactured and supplied to a foreign customer a drive for recording CD matrices, which has been operating for about 15 years.
Fig.3 shows a demonstration model of the drive with a screw with a diameter of 20 mm and a resolution of 1 nm. We have optimal designs and reproducible industrial technologies for manufacturing single-module magnetic lead screws in a wide range of sizes and characteristics (with a screw diameter from 10 to 60 mm). Magnetic lead screws with an optimal number of modules (4–6) with an updated design of the magnetic circuit and magnetization technology have been developed. Such a solution should set the industrial standard for serial magnetic lead screws.
In the technical policy of the application of magnetic transmissions, two branches are seen: industrial and instrumental. The industrial branch places special emphasis on the high force parameters of the drives (stiffness: 1 000–1 500 N/μm) and large strokes (500 mm and more). In metalworking, this technology, together with advanced metal processing (for example, hard turning), will provide a revolutionary transformation of the production processes of precision engineering products. The highest level of accuracy will allow obtaining details of higher qualities, corresponding to polishing and free abrasive finishing, on turning and vortex milling machines for one mounting of rough blanks.
In the instrument branch, the requirements of practical accuracy will dominate, which, within the framework of this approach, can be brought to 0.1 nm with a stroke of about 25 mm. In this range of tasks, piezo stepper motors can be competitors, in which the stroke is of the same order, and the resolution is also at the nanometer level. Advantages of magnetic lead screws are the use of an open-circuit that does not require an external reference "ruler", as well as the complete absence of vibration. To solve production problems when creating magnetic lead screws of this class, it is necessary to additionally apply the latest technologies of the femtosecond laser micromachining.
When analyzing the characteristics of sub-precision mechanics, the question arises: how and where the application of these products is possible? In the field of mechanical engineering, this can be metalworking complexes used in aerospace, electronic and optical-mechanical fields (the production of components of positioning systems, mirrors, precision couplings, etc.). A huge plus, as mentioned above, is the one-stage production of complex surfaces with high machining accuracy.
For several years, the Advanced Technologies Center has been successfully implementing and manufacturing small-sized machining centers for training and production [11]. A wide range of tasks of ATCNano CNC machines are successfully carried out due to the accuracy (about 0.002 mm in all axes), rigidity (cantilevered design) and high productivity (feeding using ball screws and linear guides, 1 800 mm/min). Fig.4 shows the matrix used to produce microfluidic chips by the method of soft lithography. The dimensions of the matrix are 32 Ч 24 mm, the size of the tracks is 0.2 Ч 0.2 mm.
With the help of modern software, significant accuracy is also achieved when approximating complex surfaces by a single tool. Fig.5 shows the milled relief obtained by approximating using the spherical cutter D4, R2 with a pitch of 0.05 mm (material: Д16T).
The development of the ATCNano CNC milling center project (Fig.6), designed using modern mechanical, electronic and electromechanical components, does not stand still. Mastering in production and replacement of standard mechanical components such as ball screw and lead screw with nut, by precision magnetic lead screws, opens new possibilities and expands the field of application of equipment. Taking into account the development and implementation of new frictionless components, the approach to nanometer accuracy of processing already seems not a fantasy, but a reality.
In addition to material processing complexes, the company is engaged in the development and testing of automated precision movements for laboratory equipment and microscopy [12, 13]. At compact dimensions (200 Ч 200 mm), the installation of tracking sensors and micro ball screws allowed, in conjunction with the advanced electronic (controller, driver) and electromechanical (servo motors) parts, to obtain fully automated imaging with accuracy unattainable for conventional manual movements and stages. Further complete replacement of the movable mechanics by a non-contact magnetic mechanism will allow us to approach the accuracy of displacements, which is comparable to piezoceramic motors and tubes but appreciably benefits in ease of adjustment and speed of movement. ■
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