Abstract:
Provided is a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field and polishing method thereof. The device includes a polishing disc revolution mechanism and a multi-magnetic-pole synchronous rotary drive mechanism, the polishing disc revolution mechanism including a transmission shaft motor, a transmission shaft, a transfer disc, an eccentric shaft fixing disc, a cup-shaped polishing disc and a transmission shaft transmission mechanism, the multi-magnetic-pole synchronous rotary drive mechanism including an eccentric spindle, a synchronous rotary drive disc, flexible eccentric rotating shafts, eccentric sleeves, magnetic poles, the eccentric shaft fixing disc, and a spindle motor, etc. The device does not need a circulating device to renew magnetorheological fluid and does not need to renew the magnetorheological fluid during the finishing process; in fact the entire process from rough polishing to precise polishing can be done at one time. The device maintains a consistent workpiece surface and delivers a low cost and very efficient polishing process that is eminently suitable for the planes of optical elements with large diameter; it is also suitable for studying the material removal mechanism of planar optical materials and detecting sub-surface damage, as well as other experimental studies.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field and polishing method thereof, which would suit the planarization of the planes of an optoelectronic or microelectronic semi-conductor substrate and optical elements. This means it belongs to the technical field of ultra-precision finishing. 
       BACKGROUND 
       [0002]    Since optical elements (lenses, mirrors) are one of the core elements of optical devices, their surface accuracy must be ultra-smooth (roughness: Ra is below 1 nm), and they must also have a relatively high surface figure (the shape accuracy is below 0.5 microns), to achieve excellent optical performance. In the LED field, monocrystalline silicon (Si), monocrystalline germanium (Ge), gallium arsenide (GaAs), monocrystalline silicon carbide (SiC), and sapphire (Al 2 O 3 ) etc., serve as semi-conductor substrate materials, so they must also have an ultra-flat and ultra-smooth surface (roughness of Ra must below 0.3 nm) in order to meet the growth of epitaxial film and, there must be no defects and no damage. Flat optical elements and the semi-conductor substrate both need planarization, but the conventional processes for planarizing flat optical elements and semiconductor substrates are mainly surface grinding, ultra-precision polishing, chemical mechanical polishing, and magnetorheological polishing; this means the quality and precision of the finishing method determines how well optical devices and semi-conductor devices perform. 
         [0003]    Magnetorheological finishing is a new method for finishing an optical surface; it was put forward by KORDONSKI and his collaborators in the 1990s, and is based on a combination of electromagnetics, fluid dynamics, analytical chemistry, and processing technology, etc. Magnetorheological finishing is good for polishing and there is no secondary surface damage, so it is suitable for finishing complex surfaces, unlike traditional polishing processes. Magnetorheological finishing has since developed into a revolutionary finishing method for optical surfaces, particularly for finishing axisymmetric aspheric surfaces, so it is widely used in the final processing of large-scale optical elements, semi-conductor wafers, LED substrates, and liquid crystal display panels, etc. However, current magnetorheological finishing used to finish flat workpieces is mainly using the various models of magnetorheological finishing machines developed by QED, a corporation from the United States. These machines work by placing the workpiece above an arc-shaped polishing disc such that a concave gap is formed between the surface of the workpiece and the polishing disc. An electromagnet pole or a permanent magnet pole with an adjustable magnetic flux density is placed under the polishing disc to form a high-intensity gradient magnetic field at the concave gap. As the magnetorheological fluid moves with the polishing disc to a position adjacent to the concave gap formed by the workpiece and the polishing disc, a flexible protruding “polishing ribbon” is formed. However, contact between the “polishing ribbon” and the workpiece surface belongs to “spots” local contact. During the finishing process, only by controlling the “spots” to perform trajectory scanning along the workpiece surface according to a certain rule, can the entire surface be finished. This trajectory scanning process requires a lot of time which means it is inefficient and it is not easy to guarantee an accurate finishing shape. 
         [0004]    To improve the efficiency of magnetorheological finishing, Patent No. CN200610132495.9 sets forth an abrasive polishing method based on the magnetorheological effect and a polishing device thereof, which works on the principle of magnetorheological finishing and an action mechanism of cluster; this process has already been carried out in a large number of experimental studies. Although this method forms a regional polishing pad using the cluster method, it is difficult to finish the workpiece uniformly, so following a deep analysis, it is found that due to the viscoelasticity of magnetorheological fluid, the workpiece will press down the protruding flexible polishing pad set forth in the patent and make it irrecoverable when passing by the flexible polishing pad. Thus, the flexible polishing pad loses its pressure on the workpiece, which makes a huge difference between the material removal rate at the edge of a workpiece and that in other areas. Moreover it is difficult to renew the abrasive in the viscoelastic polishing pad which further reduces the finishing effect (as shown in  FIG. 1 ). Therefore, based on this deep research, the present invention has a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field and polishing method thereof, which intuitively maintains constant pressure during the finishing process and enables the abrasive to be renewed, whilst simultaneously self-sharpening in real time during this process. This finishing device and finishing method are eminently suitable for high-efficiency ultra-precision finishing of optical elements, semiconductor wafers, ceramic substrates, and other flat materials. 
       SUMMARY OF THE INVENTION 
       [0005]    One object of this invention is to provide a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field, which focuses on the non-uniformity of cluster magnetorheological finishing. This invention is extremely efficient at finishing, it is low cost and there is no surface or sub-surface damage, which makes it suitable for the high-efficiency ultra-precision finishing for the planes of optoelectronic or microelectronic semiconductor substrate and optical elements. 
         [0006]    Another object of this invention is to provide a polishing method of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field. This invention realizes the self-sharpening of abrasive gathered on the surface of the magnetorheological flexible polishing pad, while recovering the shape of a magnetorheological flexible polishing pad via the regular movement of the magnetic pole array forming a dynamic magnetic field; this will maintain and improve the finishing performance of the magnetorheological flexible polishing pad, improve the efficiency of magnetorheological polishing, and realizes uniform finishing of the workpiece. 
         [0007]    The technical solution of the present invention is that: a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention, comprises a polishing disc revolution mechanism and a multi-magnetic-pole synchronous rotary drive mechanism, the polishing disc revolution mechanism comprising a base, a transmission shaft motor, a transmission shaft, a transfer disc, an eccentric shaft fixing disc, a cup-shaped polishing disc and a transmission shaft transmission mechanism, the multi-magnetic-pole synchronous rotary drive mechanism comprising an eccentric spindle, a synchronous rotary drive disc, flexible eccentric rotating shafts, eccentric sleeves, magnetic poles, the eccentric shaft fixing disc, a spindle motor, a spindle transmission mechanism, wherein the transmission shaft motor is fitted onto the base, a driving transmission member of the transmission shaft transmission mechanism is fitted onto an output shaft of the transmission shaft motor, a driven transmission member of the transmission shaft transmission mechanism is connected to the transmission shaft, the transfer disc is fitted coaxially onto an upper end face of the transmission shaft, the eccentric shaft fixing disc is fitted coaxially onto an upper end face of the transfer disc, the cup-shaped polishing disc is fitted coaxially onto an upper end face of the eccentric shaft fixing disc, the spindle motor of the multi-magnetic-pole synchronous rotary drive mechanism is fitted onto the base, a driving transmission member of the spindle transmission mechanism is fitted onto an output shaft of the spindle motor, a driven transmission member of the spindle transmission mechanism is connected to the eccentric spindle, the eccentric spindle is mounted in a hollow cavity inside the transmission shaft, the synchronous rotary drive disc is fitted onto an upper end of the transmission shaft, the flexible eccentric rotating shaft is installed on an upper end of the synchronous rotary drive disc, the eccentric sleeve is fitted onto the flexible eccentric rotating shaft, the magnetic pole is fitted inside the eccentric sleeve, and the flexible eccentric rotating shaft is mounted inside a shaft hole provided in the cup-shaped polishing disc. 
         [0008]    A polishing method of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention, comprises steps of: 
         [0009]    1) selecting magnetic poles with appropriate diameter and magnetic field strength based on characteristic of the object to be finished, installing the magnetic poles in the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field, adjusting the angle of the eccentric sleeves based on requirements such that all the magnet rotating eccentric distances are consistent; 
         [0010]    2) installing a workpiece onto a tool head, with a lower surface of the workpiece being parallel to an upper end face of the cup-shaped polishing disc, adjusting a gap between the lower surface of the workpiece and the cup-shaped polishing disc to range from 0.5 mm to 5 mm; 
         [0011]    3) adding at least two of the following three abrasives the into deionized water, wherein the three abrasives are micron-grade abrasive with a concentration ranging from 2 wt % to 15 wt %, sub-micron abrasive with a concentration ranging from 2 wt % to 15 wt % and nanoscale abrasive with a concentration ranging from 2 wt % to 15 wt %, adding sub-micron carbonyl iron powder with a concentration ranging from 2 wt % to 20 wt % and micron-grade carbonyl iron powder with a concentration ranging from 3 % wt to 15 wt % into the deionized water, and adding a dispersing agent with a concentration ranging from 3 wt % to 15 wt % and anti-rusting agent with a concentration of ranging from 1 wt % to 6 wt %, stirring the deionized water thoroughly, and then ultrasonically vibrating the deionized water for 5 to 30 minutes to form magnetorheological fluid; 
         [0012]    4) pouring the magnetorheological fluid into the cup-shaped polishing disc, starting the spindle motor to drive the eccentric spindle to rotate, the rotation of the drive bearing forcing the synchronous rotary drive disc to swing, the swing of the synchronous rotary drive disc forcing each flexible eccentric rotating shaft to realize rotate simultaneously, the rotation of the flexible eccentric rotating shaft forcing the magnetic pole to rotate under the magnet rotating eccentric distance so as to realize the transition from the dynamic magnetic field to the static magnetic field at the end face of the magnetic pole, the magnetorheological fluid forming a flexible polishing pad with abrasive real-time renewing and self-sharpening and shape recovering under the effect of the dynamic magnetic field; 
         [0013]    5) starting the transmission shaft motor to drive the cup-shaped polishing disc to rotate at a high speed, driving the tool head to rotate at a high speed and swing in low speed to realize the high-efficiency, ultra-smooth and uniform polishing of surface material of the workpiece. 
         [0014]    This self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention transforms the static magnetic field into a dynamic magnetic field by means of the eccentric rotation of magnetic poles; this rearranges the magnetic chain in the flexible polishing pad and the abrasive becomes self-sharpening and the polishing pad recovers in real time. These actions solve the core problem whereby a polishing pad formed by a static magnetic field loses its finishing pressure on the workpiece during operation because the polishing pad deforms due to viscosity and magnetism of the magnetorheological fluid. This invention allows the magnetic pole to dynamically adjust its rotating eccentric distance by the cooperation between the eccentric hole in the flexible eccentric rotating shaft and the eccentric sleeve, and use the multi-magnetic-pole synchronous rotary drive mechanism to enable a close arrangement of the numerous synchronous rotary magnetic poles. Theoretically, this invention can form a large, flexible and compact polishing pad that can polish the plane of optical elements with large diameter. Another advantage of this invention is using a dynamic magnetic field to renew the magnetorheological fluid; it does not need to use a circulating device to renew magnetorheological fluid or renew it during the finishing process. This not only saves space due to not needing finishing equipment, it also solves the problem with conventional magnetorheological finishing where residue adheres to the circulating device and contaminates the magnetorheological fluid. Furthermore, this invention will not affect the internal structure of the self-sharpening polishing device when the cup-shaped polishing disc is installed and removed. In fact removing the cup-shaped polishing disc for cleaning is easy because there is no effect from magnetism. The magnetorheological fluid prepared for this invention belongs to mixed fluid flow with mixed thickness. The fluidity and material removal capacity of the magnetorheological fluid is realized by adjusting the gap between the upper surface of the workpiece and the cup-shaped polishing disc, in fact the entire process from rough to precise polishing can be done at one time. Furthermore, this invention can maintain a consistent finish of the workpiece surface because it is efficient and low cost, and there is no surface or sub-surface damage, which makes it suitable for polishing optical elements with large diameter. This invention is also suitable for studying the material removal mechanism of planar optical materials and detecting sub-surface damage, as well as other experimental studies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic view showing how a conventional static magnetic field polishing pad operates. 
           [0016]      FIG. 2  is a schematic diagram of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention. 
           [0017]      FIG. 3  is a cross-sectional view of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention. 
           [0018]      FIG. 4  is a cross-sectional view of the flexible eccentric rotating shaft the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention. 
           [0019]      FIG. 5  is a partially enlarged view of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention. 
           [0020]      FIG. 6  is a schematic view of the installation of the magnet of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention. 
           [0021]      FIG. 7  is a schematic view of the finishing process of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of the present invention. 
       
    
    
       [0022]      FIGS. 1-7  show the following: 
         [0023]      1 . cup-shaped polishing disc,  2 . first fixing screw,  3 . eccentric shaft fixing disc,  4 . second fixing screw,  5 . drive disc end cap,  6 . radial-thrust bearing,  7 . outer spacer bushing,  8 . synchronous rotary drive disc,  9 . shaft end cap,  10 . third fixing screw,  11 . transfer disc,  12 . fourth fixing screw,  13 . transmission shaft,  14 . bearing end cap,  15 . fifth fixing screw,  16 . bearing block,  17 . spindle motor,  18 . sixth fixing screw,  19 . flexible eccentric rotating shaft,  20 . eccentric sleeve,  21 . magnetic pole,  22 . deep groove ball bearing,  23 . seventh fixing screw,  24 . spindle end cap,  25 . drive bearing,  26 . separation sleeve,  27 . eighth fixing screw,  28 . eccentric spindle end cap,  29 . ninth fixing screw,  30 . spindle bearing,  31 . inner sleeve,  32 . outer sleeve,  33 . transmission shaft bearing,  34 . inner fixing sleeve,  35 . outer fixing sleeve,  36 . bearing block,  37 . transmission shaft motor,  38 . tenth fixing screw,  39 . base,  40 . spindle driving belt wheel,  41 . first flat key,  42 . spindle transmission belt,  43 . eccentric spindle,  44 . eleventh fixing screw,  45 . spindle driven belt wheel,  46 . twelfth fixing screw,  47 . transmission shaft driven belt wheel,  48 . transmission shaft transmission belt,  49 . second flat key,  50 . transmission shaft driving belt wheel,  51 . eccentric distance of the eccentric spindle,  52 . eccentric distance of the flexible eccentric rotating shaft,  53 . magnet rotating eccentric distance,  54 . thin notch,  55 . eccentric hole,  56 . boss,  57 . eccentricity of the eccentric hole,  58  small eccentric shaft,  59 . lower flange block,  60 . upper flange block,  61 . workpiece,  62 . tool head,  63 . magnetorheological fluid,  64 . flexible polishing pad,  65 . circulating device. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0024]    This invention will be further described with reference to the accompanying drawings and embodiments, but the actual process that can be realized is not limited to these embodiments: 
       Embodiment 1 
       [0025]      FIG. 3  shows a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field which comprises a polishing disc revolution mechanism and a multi-magnetic-pole synchronous rotary drive mechanism. The polishing disc revolution mechanism comprises a base  39 , a transmission shaft motor  37 , a transmission shaft  13 , a transfer disc  11 , an eccentric shaft fixing disc  3 , a cup-shaped polishing disc  1  and a transmission shaft transmission mechanism. The multi-magnetic-pole synchronous rotary drive mechanism comprises an eccentric spindle  43 , a synchronous rotary drive disc  8 , flexible eccentric rotating shafts  19 , eccentric sleeves  20 , magnetic poles  21 , the eccentric shaft fixing disc  3 , a spindle motor  17 , a spindle transmission mechanism wherein the transmission shaft motor  37  is fitted onto the base  39 , a driving transmission member of the transmission shaft transmission mechanism is fitted onto an output shaft of the transmission shaft motor  37 , a driven transmission member of the transmission shaft transmission mechanism is connected to the transmission shaft  13 , the transfer disc  11  is fitted coaxially onto fitted upper end face of the transmission shaft  13 , the eccentric shaft fixing disc  3  is fitted coaxially onto the upper end face of the transfer disc  11 , the cup-shaped polishing disc  1  is fitted coaxially onto an upper end face of the eccentric shaft fixing disc  3 , the spindle motor  17  of the multi-magnetic-pole synchronous rotary drive mechanism is fitted onto the base  39 , a driving transmission member of the spindle transmission mechanism is fitted onto the output shaft of the spindle motor  17 , a driven transmission member of the spindle transmission mechanism is connected to the eccentric spindle  43 , the eccentric spindle  43  is mounted in a hollow cavity inside the transmission shaft  13 , the synchronous rotary drive disc  8  is fitted onto the upper end of the transmission shaft  13 , the flexible eccentric rotating shaft  19  is installed onto an upper end of the synchronous rotary drive disc  8 , the eccentric sleeve  20  is fitted onto the flexible eccentric rotating shaft  19 , the magnetic pole  21  is fitted inside the eccentric sleeve  20 , and the flexible eccentric rotating shaft  19  is mounted in a shaft hole inside the cup-shaped polishing disc  1 . 
         [0026]    In this embodiment, said spindle transmission mechanism comprises a spindle driving belt wheel  40 , a spindle transmission belt  42 , and a spindle driven belt wheel  45 , wherein the spindle driving belt wheel  40  is mounted on the output shaft of the spindle motor  17 , the spindle driven belt wheel  45  is mounted on the eccentric spindle  43 , and the spindle transmission belt  42  is wound around the spindle driving belt wheel  40  and the spindle driven belt wheel  45 . 
         [0027]    In this embodiment the transmission shaft transmission mechanism comprises a transmission shaft driving belt wheel  50 , a transmission shaft driven belt wheel  47 , and a transmission shaft transmission belt  48 , wherein the transmission shaft driving belt wheel  50  is mounted on an output shaft of the transmission shaft  13 , the transmission shaft driven belt wheel  47  is mounted on the transmission shaft  13 , and the transmission shaft transmission belt  48  is wound around the transmission shaft driving belt wheel  50  and the transmission shaft driven belt wheel  47 . 
         [0028]    In this embodiment the transmission shaft motor  37  is fitted onto the base  39  by ten fixing screws  38 , the transmission shaft driving belt wheel  50  is fitted onto the transmission shaft motor  37  by a second flat key  49 , a bearing block  16  in which a pair of transmission shaft bearings  33  are installed is installed vertically at the centre of the base  39 , a bearing end cap  14  is mounted on an end face of the bearing block  16  by fifth fixing screws  15  such that it presses against an outer ring of the transmission shaft bearing  33 , an inner fixing sleeve  34  and an outer fixing sleeve  35  support and separate the transmission shaft bearings  33  on which the transmission shaft  13  is supported, the transfer disc  11  is fitted coaxially onto the upper end face of the transmission shaft  13  by fourth fixing screws  12 , the eccentric shaft fixing disc  3  is fitted coaxially onto the upper end face of the transfer disc  11  by second fixing screws  4 , the cup-shaped polishing disc  1  is fitted coaxially onto the upper end face of the eccentric shaft fixing disc  3  by first fixing screws  2 , a transmission shaft driven belt wheel  47  is fitted onto a lower end face of the transmission shaft  13  by twelfth fixing screws  46 , the eccentric spindle  43  of the multi-magnetic-pole synchronous rotary drive mechanism is fitted inside the hollow cavity in the transmission shaft  13  by a pair of spindle bearings  30 , an inner sleeve  31 , and an outer sleeve  32  position inner rings and outer rings of the spindle bearings  30 , an eccentric spindle end cap  28  is fitted onto the upper end of the transmission shaft  13  by ninthby ninth fixing screws  29 , such that it presses against the outer ring of the spindle bearing  30 , a drive bearing  25  is fitted onto an end of an eccentric shaft of the eccentric spindle  43 , a spindle end cap  24  is fitted onto an upper end of the eccentric shaft of the eccentric spindle  43  by seventh fixing screws  23  such that it presses against an inner ring of the drive bearing  25 ; the synchronous rotary drive disc  8  is fitted onto an outer ring of the drive bearing  25 , radial-thrust bearings  6  are installed in arrayed holes of the synchronous rotary drive disc  8 ; outer spacer bushings  7  separate outer rings of the radial-thrust bearings  6 , the flexible eccentric rotating shaft  19  is fixed by the radial-thrust bearing  6 , a shaft end cap  9  is fitted onto a smaller end of the flexible eccentric rotating shaft  19  by third fixing screws  10 , a drive disc end cap  5  is fitted onto the upper end of the synchronous rotary drive disc  8  by eighth fixing screws  27  such that it presses against an outer ring of the radial-thrust bearing  6 , a deep groove ball bearing  22  is installed at a large upper end of the flexible eccentric rotating shaft  19 , the eccentric sleeve  20  into which the magnetic pole  21  is fixed, is fitted into the eccentric hole of the large upper end of the eccentric rotating shaft  19 , the deep groove ball bearings  22  are installed in the eccentric shaft fixing disc  3  by means of arrayed holes, the spindle driven belt wheel  45  is fitted onto a lower end of the eccentric spindle  43  by eleventh fixing screws  44  such that it presses against thespindle bearing  30 , the spindle motor  17  is fitted onto the base  39  with a sixth fixing screw  18 , and the spindle driving belt wheel  40  is fitted onto the spindle motor  17  by a first flat key  41 . 
         [0029]      FIGS. 3 and 6  show that an eccentric distance  51  of the eccentric spindle and an eccentric distance  52  of the flexible eccentric rotating shaft  19  are equal in numerical value, and the eccentric directions of all the flexible eccentric rotating shafts  19  are consistent and are opposite to the eccentric direction of the eccentric spindle  43 . 
         [0030]    A rule of arrangement of the arrayed holesin the synchronous rotary drive disc  8  is equal to that of the arrayed holes in the eccentric shaft fixing disc  3 ; and a pitch-row of the arrayed holes in the synchronous rotary drive disc  8  is equal to that of the arrayed holes in the eccentric shaft fixing disc  3 . 
         [0031]      FIGS. 3 and 4  show that the outer cylinder of the flexible eccentric rotating shaft  19  has a boss  56 , the outer cylinder an eccentric hole  55  inside, the eccentric distance  52  of the flexible eccentric rotating shaft is twice of an eccentricity  57  of the eccentric hole, and three or more staggered thin notches  54  is provided between the outer cylinder of the flexible eccentric rotating shaft  19 , and a small eccentric shaft  58  of the flexible eccentric rotating shaft  19  to compensate for manufacturing error between the arrayed holes in the synchronous rotary drive disc  8  and the arrayed holes in the eccentric shaft fixing disc  3 . 
         [0032]      FIGS. 2 and 3  show that an eccentricity  57  of the eccentric hole of the flexible eccentric rotating shaft  19  is equal to an eccentricity of the eccentric sleeve  20 ; it can change from 0 to twice of the eccentricity of the eccentric sleeve  20  by adjusting the angle of rotation of the eccentric sleeve  20 , the angle of rotation of each eccentric sleeve  20  is consistent with that of each flexible eccentric rotating shaft  19 ; the rotation of the eccentric spindle  43  forces the synchronous rotary drive disc  8  to swing, the swing of the synchronous rotary drive disc  8  forces each flexible eccentric rotating shaft  19  to realize synchronizing rotation; and the rotation of the flexible eccentric rotating shaft  19  forces the magnetic pole  21  to rotate under a magnet eccentric distance  53  so as to realize the transition from a dynamic magnetic field to a static magnetic field at the end face of the magnetic pole  21 . 
         [0033]      FIG. 5  shows that the transmission shaft  13  has a lower flange block  59  at the upper end, and the bearing end cap  14  has an upper flange block  60  in clearance fit with the lower flange block  59  to keep the transmission shaft bearing  33  waterproof and dustproof. 
         [0034]    The magnetic poles  21  are cylindrical flat ends permanent magnet with and a minimum magnetic field strength of 500 Gs and a diameter ranging from 5 mm to 50 mm; the minimum number of magnetic poles  21  is one, the number of magnetic poles  21  is determined by the size of the object to be finished and the size of the cup-shaped polishing disc  1 ; the magnetic poles  21  are arranged in the eccentric shaft fixing disc  3  according to a certain rule such that the end faces of the magnetic poles  21  being kept in the same plane. 
         [0035]    The cup-shaped polishing disc  1 , the eccentric shaft fixing disc  3 , the flexible eccentric rotating shafts  19 , and the eccentric sleeves  20  can be made from diamagnetic materials, i.e., stainless steel, magnalium alloy, or ceramic. 
         [0036]      FIG. 7  shows a polishing method of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field of present invention, comprises steps of: 
         [0037]    1) selecting 48 magnetic poles  21  with a diameter of 20 mm and a magnetic field strength of 3200 Gs based on the characteristics of a single crystal silicon with a diameter of 150 mm, installing the 48 magnetic poles  21  into the self-sharpening polishing device magnetorheological flexible polishing pad formed by dynamic magnetic field by dividing them into three equi-distant annular rows, adjusting the angle of the eccentric sleeves  20  such that all the magnet rotating eccentric distances  53  are 3 mm; 
         [0038]    2) installing the single crystal silicon with a diameter of 20 mm onto a tool head  62 , with a lower surface of the workpiece  61  being parallel to an upper end face of the cup-shaped polishing disc  1 , adjusting a gap between the lower surface of the workpiece  61  and the cup-shaped polishing disc  1  to be 1.5 mm; 
         [0039]    3) adding aluminium abrasive with a particle size of 5 microns and a concentration of 3 wt % and aluminium abrasive with a particle size of 0.5 microns and a concentration of 2 wt % into deionized water, adding carbonyl iron powder with a particle size of 0.8 microns and a concentration of 4 wt % and carbonyl iron powder with a particle size of 0.8 microns and a concentration of 3 wt % into deionized water, and adding a dispersing agent with a concentration of 4 wt % and an anti-rusting agent with a concentration of 3 wt %, stirring the deionized water thoroughly, and then ultrasonically vibrating the deionized water for 20 minutes to form a magnetorheological fluid  63 ; 
         [0040]    4) pouring the magnetorheological fluid  63  into the cup-shaped polishing disc  1 , starting the spindle motor  17  and adjusting rotating speed of the spindle motor  17  to 20 rpm to drive the eccentric spindle  43  to rotate, the rotation of the drive bearing  25  forcing the synchronous rotary drive disc  8  to swing, the swing of the synchronous rotary drive disc  8  forcing each flexible eccentric rotating shaft  19  to rotate simultaneously, the rotation of the flexible eccentric rotating shaft ( 19 ) forcing the magnetic pole  21  to rotate under the magnet rotating eccentric distance  53  so as to realize the transition from a dynamic magnetic field to a static magnetic field at the end face of the magnetic pole  21 ; the magnetorheological fluid forming a flexible polishing pad  64  with abrasive real-time renewing and self-sharpening and shape recovering under the effect of the dynamic magnetic field; 
         [0041]    5) starting the transmission shaft motor  37  and adjusting the rotating speed of the transmission shaft motor  37  to 400 rpm to drive the cup-shaped polishing disc  1  to rotate at high speed; adjusting the rotating speed of the tool head  62  to −300 rpm, the swinging speed of the tool head  62  to 10 times/min and the swinging of the tool head  62  to 20 mm; finishing the single crystal silicon for 60 minutes to completing high-efficiency polishing of surface material of the single crystal silicon and obtaining an ultra-smooth and uniform surface with a roughness of Ra 0.3 nm. 
       Embodiment 2 
       [0042]      FIG. 3  shows a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field which comprises a polishing disc revolution mechanism and a multi-magnetic-pole synchronous rotary drive mechanism. The polishing disc revolution mechanism is composed of a base  39 , a transmission shaft motor  37  being fitted onto the base  39  by tenth fixing screws  38 , a transmission shaft driving belt wheel  50  being fitted onto the transmission shaft motor  37  by a second flat key  49 , a bearing block  16  being installed vertically at the center of the base  39 , a pair of transmission shaft bearings  33  being installed into the bearing block  16 , a bearing end cap  14  being installed on an end face of the bearing block  16  by fifth fixing screws  15  such that it presses against an outer ring of the transmission shaft bearing  33 , an inner fixing sleeve  34  and an outer fixing sleeve  35  supporting and separating the transmission shaft bearings  33 , a transmission shaft  13  cooperating with the transmission shaft bearings  33 , a transfer disc  11  being fitted coaxially onto an upper end face of the transmission shaft  13  by fourth fixing screws  12 , an eccentric shaft fixing disc  3  being fitted coaxially onto an upper end face of the transfer disc  11  by second fixing screws  4 , a cup-shaped polishing disc  1  being fitted coaxially onto an upper end face of the eccentric shaft fixing disc  3  by first fixing screws  2 , a transmission shaft driven belt wheel  47  being fitted onto a lower end face of the transmission shaft  13  by twelfth fixing screws  46 , and a transmission shaft transmission belt  48 . The multi-magnetic-pole synchronous rotary drive mechanism consists of an eccentric spindle  43  being fitted into the transmission shaft  13  by a pair of spindle bearings  30 , an inner sleeve  31  and an outer sleeve  32  positioning inner rings and outer rings of the spindle bearings  30 , an eccentric spindle end cap  28  being fitted onto the upper end of the transmission shaft  13  by ninth fixing screws  29  such that it presses against the outer ring of the spindle bearing  30 , a drive bearing  25  being fitted onto an end of an eccentric shaft of the eccentric spindle  43 , a spindle end cap  24  being fitted onto an upper end of the eccentric shaft of the eccentric spindle  43  by seventh fixing screws  23  such that it presses against an inner ring of the drive bearing  25 , a synchronous rotary drive disc  8  being fixed by an outer ring of spindle end cap  24 , radial-thrust bearings  6  being installed in arrayed holes of the synchronous rotary drive disc  8 , outer spacer bushings  7  separating outer rings of the radial-thrust bearings  6 , flexible eccentric rotating shafts  19  being fixed by the radial-thrust bearings  6 , shaft end caps  9  being fitted onto smaller ends of the flexible eccentric rotating shafts  19  by third fixing screws  10 , a drive disc end cap  5  being fitted onto the upper end of the synchronous rotary drive disc  8  by eighth fixing screws  27  such that it presses against an outer ring of the radial-thrust bearing  6 , deep groove ball bearings  22  being installed at larger upper ends of the flexible eccentric rotating shafts  19 , eccentric sleeves  20  being fitted into eccentric holes at the larger upper ends of the flexible eccentric rotating shafts  19 , magnetic poles  21  being fitted into the eccentric sleeves  20 , an eccentric shaft fixing disc  3  in which the deep groove ball bearings  22  are installed by arrayed holes, a spindle driven belt wheel  45  being fitted onto a lower end of the eccentric spindle  43  by eleventh fixing screws  44  such that it presses against the spindle bearing  30 , a spindle motor  17  being fitted onto the base  39  by a sixth fixing screw  18 , a spindle driving belt wheel  40  being fitted onto the spindle motor  17  by a first flat key  41 , and a spindle transmission belt  42 . 
         [0043]      FIGS. 3 and 6  shows that an eccentric distance  51  of the eccentric spindle, and an eccentric distance  52  of the flexible eccentric rotating shaft  19  are equal in numerical value, and the eccentric directions of all the flexible eccentric rotating shafts  19  are consistent and are opposite to the eccentric direction of the eccentric spindle  43 . 
         [0044]    A rule of arrangement the arrayed holes in the synchronous rotary drive disc  8  is equal to that of the arrayed holes in the eccentric shaft fixing disc  3 ; and a pitch-row of the arrayed holes in the synchronous rotary drive disc  8  is equal to that of the arrayed holes in the eccentric shaft fixing disc  3 . 
         [0045]      FIGS. 3 and 4  show that an outer cylinder of the flexible eccentric rotating shaft  19  has a boss  56 , and the outer cylinder has an eccentric hole  55  inside, the eccentric distance  52  of the flexible eccentric rotating shaft is twice eccentricity  57  of the eccentric hole, three or more staggered thin notches  54  is provided between the outer cylinder of the flexible eccentric rotating shaft  19  and a small eccentric shaft  58  of the flexible eccentric rotating shaft  19  compensate for the manufacturing errors between the arrayed holes in the synchronous rotary drive disc  8  and those in the eccentric shaft fixing disc  3 . 
         [0046]      FIGS. 2 and 3  show that an eccentricity  57  of the eccentric hole of the flexible eccentric rotating shaft  19  is equal to an eccentricity of the eccentric sleeve  20 ; it can change from 0 to twice of the eccentricity of the eccentric sleeves  20  by adjusting the angel of rotation of the eccentric sleeve  20 , the angle of rotation of each eccentric sleeve  20  is consistent with that of each flexible eccentric rotating shaft  19 , the rotation of the eccentric spindle  43  forces the synchronous rotary drive disc  8  to swing, the swing of the synchronous rotary drive disc  8  forces each flexible eccentric shaft  19  to rotate simultaneously; the flexible eccentric rotating shaft  19  forces the magnetic pole  21  to rotate under a magnet rotating eccentric distance  53  so as to realize the transition from a dynamic magnetic field to a static magnetic field at the end face of the magnetic pole  21 . 
         [0047]      FIG. 5  shows that the transmission shaft  13  has a lower flange block  59  at the upper end, and the bearing end cap  14  has an upper flange block  60  in clearance fit with the lower flange block  59  to keep the transmission shaft bearing  33  waterproof and dustproof. 
         [0048]    The magnetic poles  21  are cylindrical flat-end permanent magnet with a minimum magnetic field strength of 500 Gs and a diameter ranging from 5 mm to 50 mm; the minimum number of magnetic poles  21  is one, the number of the magnetic poles  21  is determined by the size of the object to be finished and the size of the cup-shaped polishing disc  1 ; the magnetic poles  21  are arranged in the eccentric shaft fixing disc  3  according to a certain rule with the end faces of the magnetic poles  21  being kept in the same plane. 
         [0049]    The cup-shaped polishing disc  1 , the eccentric shaft fixing disc  3 , the flexible eccentric rotating shafts  19 , and the eccentric sleeves  20  may be made of diamagnetic materials, i.e., stainless steel, magnalium alloy, or ceramic. 
         [0050]      FIG. 7  shows a polishing method of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field, which consists of the following steps: 
         [0051]    1) selecting 12 magnetic poles  21  with a diameter of 15 mm and a magnetic field strength of 2800 Gs based on the characteristic of a single crystal silicon carbide with a diameter of 100 mm, installing the 12 magnetic poles  21  into the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field by arranging them into one equi-distant annular row, adjusting the angle of the eccentric sleeves  20  such that all the magnet rotating eccentric distances  53  are 1 mm; 
         [0052]    2) installing the single crystal silicon carbide with a diameter of 100 mm onto a tool head  62 , with a lower surface of the workpiece  61  being parallel to an upper end face of the cup-shaped polishing disc  1 , adjusting a gap between the lower surface of the workpiece  61  and the cup-shaped polishing disc  1  to be 1 mm, with the center of the single crystal silicon carbide facing toward the center of the annular magnetic poles  21 ; 
         [0053]    3) adding diamond abrasive with a particle size of 4 microns and a concentration of 4 wt %, and diamond abrasive with a particle size of 200 nanometers and a concentration of 2 wt %, into deionized water, adding carbonyl iron powder with a particle size of 500 nanometers and a concentration of 3 wt % and carbonyl iron powder with a particle size of 4 microns and a concentration of 3 wt % into the deionized water, and adding a dispersing agent with a concentration of 3 wt % and an anti-rusting agent with a concentration of 3 wt %; stirring the deionized water thoroughly and then ultrasonically vibrating the deionized wate for 25 minutes to form the magnetorheological fluid  63 ; 
         [0054]    4) pouring the magnetorheological fluid  63  into the cup-shaped polishing disc  1 , starting the spindle motor  17  and adjusting rotating speed of the spindle motor  17  to 25 rpm to drive the eccentric spindle  43  to rotate, the rotation of the drive bearing  25  forcing the synchronous rotary drive disc  8  to swing, the swing of the synchronous rotary drive disc  8  forcing each flexible eccentric rotating shaft  19  to rotate simultaneously the rotation of the flexible eccentric rotating shaft  19  forcing the magnetic pole  21  to rotate under the magnet rotating eccentric distance  53  so as to realize the transition from a dynamic magnetic field to a static magnetic field at the end face of the magnetic pole  21 ,the magnetorheological fluid forming a flexible polishing pad  64  with abrasive real-time renewing and self-sharpening and shape recovering under the effect of the dynamic magnetic field; 
         [0055]    5) starting the transmission shaft motor  37  and adjust ing the rotating speed of the transmission shaft motor  37  to 350 rpm to drive the cup-shaped polishing disc  1  to rotate at a high speed, adjusting the rotating speed of the tool head  62  to 0 rpm, the swinging speed of the tool head  62  to 0 times/min, finishing the single crystal silicon carbide for 100 minutes to complete annular polishing of surface material of the single crystal silicon carbide; observing the polishing ring with optical microscopy to determine if there is any sub-surface damage to the single crystal silicon carbide. 
       Embodiment 3 
       [0056]    The difference between embodiment 3 of the present invention and embodiment 1 lie in that: embodiment 3 describes a 100 mm single crystal sapphire being polished. A polishing method of the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field comprises steps of: 
         [0057]    1) selecting one magnetic pole  21  with a diameter of 15 mm and a magnetic field strength of 3000 Gs based on the characteristic of a single crystal sapphire with a diameter of 100 mm, installing place the magnetic pole  21  into the self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field adjusting the angle of the eccentric sleeve  20  such that the magnet rotating eccentric distance  53  is 1.5 mm, as shown in  FIG. 7 ; 
         [0058]    2) installing the single crystal sapphire with a diameter of 100 mm onto a tool head  62  with a lower surface of a workpiece  61  being parallel to an upper end face of the cup-shaped polishing disc  1 , adjusting a gap between the lower surface of the workpiece  61  and the cup-shaped polishing disc  1  to be 1 mm, with the center of the single crystal sapphire facing toward the centre of the magnetic pole  21 ; 
         [0059]    3) adding diamond abrasive with a particle size of 5 microns and a concentration of 3 wt %, diamond abrasive with a particle size of 0.8 microns and a concentration of 3 wt %, and diamond abrasive with a particle size of 200 nanometers and a concentration of wt 3%, into the deionized water, adding carbonyl iron powder with a particle size of 500 nanometers and a concentration of 4 wt %, and carbonyl iron powder with a particle size of 5 microns and a concentration of 3 wt %, into deionized water, and adding a dispersing agent with a concentration of 3 wt % and an anti-rusting agent with a concentration of 4 wt %; stiring the deionized water thoroughly and then ultrasonically vibrating the deionized water 25 minutes to form a magnetorheological fluid  63 ; 
         [0060]    4) pouring the magnetorheological fluid  63  into the cup-shaped polishing disc  1 , starting the spindle motor  17  and adjusting rotating speed of the spindle motor 17 to 50 rpm to drive the eccentric spindle  43  to rotate, the rotation of the drive bearing  25  forcing the synchronous rotary drive disc  8  to swing, the swing of the synchronous rotary drive disc  8  forcing the flexible eccentric rotating shaft  19  to rotate simultaneously, the rotation of the flexible eccentric rotating shaft  19  forcing the magnetic pole  21  to rotate under the eccentric magnet rotating eccentric distance  53  so as to realize the transition from a dynamic magnetic field to a static magnetic field at the end face of the magnetic pole  21 , the magnetorheological fluid forming a flexible polishing pad  64  with abrasive real-time renewing, and self-sharpening and shape recovering under the effect of the dynamic magnetic field; 
         [0061]    5) starting the transmission shaft motor  37  and adjusting the rotating speed of the transmission shaft motor 37 to 0 rpm to drive the cup-shaped polishing disc  1  to rotate at a high speed, adjusting the rotating speed of the tool head  62  to 400 rpm and the swinging speed of the tool head  62  to 0 times/min, finishing the single crystal sapphire for 60 minutes to complete fixed-point polishing of the surface material, observing the ring formed by polishing via optical microscopy, detecting the material removal rate and establishing the model using the single-point magnetic pole  21  to remove material from the single crystal sapphire. 
         [0062]    These embodiments explain how a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field and polishing method thereof according to the present invention, transforms a static magnetic field into a dynamic magnetic field by means of the eccentric rotation of a magnetic pole which rearranges the magnetic chain of the polishing pad so that the abrasive can renew itself, self-sharpen itself, and renew its shape in real time, thus solving the core problem that a polishing pad formed by a static magnetic field loses its finishing pressure on the workpiece due to deformation caused by viscosity and magnetism in the magnetorheological fluid. 
         [0063]    The use of a multi-magnetic-pole synchronous rotary drive mechanism enables the close arrangement of numerous synchronous rotating magnetic poles into a large, flexible and compact polishing pad which can polish the plane of optical elements with large diameter. At the same time, by selecting magnetic poles with different magnetic field strengths, as well as different diameters and different quantities, it can realize single-point polishing, annular polishing, and regional polishing of the workpiece according to different arranging rules; all of which are suitable for studying the material removal mechanism of planar optical materials and sub-surface damage detection and for other experimental studies to meet the needs of scientific researches and practical industrial applications. Moreover, this invention does not need to renew the magnetorheological fluid during the finishing process, which saves the space of equipment and the cost of finishing. As can be seen, this invention is a clever concept that is convenient, easy to use, and delivers an extremely high surface finishing; this is a revolutionary high precision and high efficiency method for polishing optical elements with large diameter. 
         [0064]    It should be noted that the above embodiments are only detailed description of the present invention and should not be construed as limitations to this invention. For the person skilled in the art, various changes in form and detail may be made without departing from the spirit and scope of the claims.