Abstract:
An objective lens actuator which drives an objective lens in a focusing direction and a tracking direction, so as to focus lights on a recording surface of an optical disc, as a recording medium of information. The objective lens actuator includes a magnet, a lens holder which is configured to hold the objective lens thereon, and four pieces of tracking coils which are disposed in a line on a side surface of the lens holder for holding the objective lens thereon, facing to the magnet.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an objective lens actuator to be used in an optical disc apparatus for reproducing or recording information onto or from a recording surface of an optical disc. 
     In an optical disc apparatus for recording information onto a disc-like information recording medium or reproducing information recording thereon, there is mounted an objective lens actuator for actuating or driving an objective lens in a focusing direction and a tracking direction, so as to record or reproduce the information, correctly, following a tack on the optical disc. 
     In general, such the objective lens actuator is built up with, a magnetic circuit made of a yoke and a permanent magnet, a moving part attached with an objective lens thereon, a holder for holding this moving part thereon, and a support member for elastically supporting the moving part with respect to the holder. Onto the moving part are attached a focusing coil and a tracking coil, wherein the moving part is actuated or driven in the focusing direction through an electromagnetic force generated due to an effect of flux from the permanent magnet attached on a yoke when supplying current into the focusing coil. In the similar manner, the moving part is driven in the tracking direction through an electromagnetic force generated due to the effect of flux from the permanent magnet attached on the yoke when supplying current into the tracking coil. 
     The density of magnetic flux effecting upon the coil is almost uniform at a central portion of the permanent magnet, but it becomes small, gradually, in a peripheral portion of the permanent magnet. If the moving part is operated or actuated to locate in such an area where the magnetic flux density is not uniform, then a change is caused in a magnitude of the electromagnetic force generating on the coil, then a moment of rotation is produced around a center of gravity of the moving part. 
     Explanation will be given on this principle, by referring to  FIG. 10  for showing the typical structures of the tracking coils  737   r  and  737   l  and the permanent magnet  736   c  of the objective lens actuator. 
       FIG. 10  is a side view for showing the force, which is generated on the tracking coil, and also the moment around an “x”-axis due to that force, when the moving part is actuated in a “z” direction, within the objective lens actuator of the conventional art. 
     As is shown in  FIG. 10 , when supplying current into the tracking coils  737   r  and  737   l  under the condition that the moving part is actuated by “Δz” into the “z” direction, an unbalance is generated in the magnetic flux density between the position on upper sides of the tracking coils  737   r  and  737   l  and lower sides thereof, and there are remaining the magnetic forces, respectively, i.e., (Ftl 3 -Ftl 2 ) downwards in the “z” direction on the tracking coil  737   l  and (Ftr 3 -Ftr 2 ) upwards in the “z” direction on the tracking coil  737   r . As a result thereof, the moment “M” acts onto the moving part, rotating around the “x”-axis. 
     This rotation moment comes into a cause of reason of tilt of the objective lens. Since the tilt of the objective lens results into generation of an aberration, and thereby deterioration of recording quality and a reproduced signal, therefore it is necessary to suppress the tilt of the objective lens. 
     For such the problem of the tilt of the objective lens, there is already known an objective lens actuator for acting on the tracking coils and/or the focusing coil, wherein the magnet (hereinafter, being called, “a permanent magnet”) is determined to be in such a configuration that the moments due to the electromagnetic force to tilt the objective lens substantially offset or cancel each other (i.e., an example shown in the following Patent Document 1). With this, the moment rotating around the center of gravity of the moving part, which is produced due to the electromagnetic force generated in the tracking coils, and the moment rotating around the center of gravity of the moving part, which is produced due to the electromagnetic force generated in the focusing coil, are cancelled with each other, and thereby preventing or protecting the objective lens from the tilt thereof. 
     [Patent Document 1] Japanese Patent Laying-Open No. 2001-101687 (2001) (see claim 1 and FIGS. 1 to 5). 
     BRIEF SUMMARY OF THE INVENTION 
     With the conventional art mentioned above, however, since it uses both, i.e., the moment rotating around the center of gravity of the moving part, which is produced due to the electromagnetic force generated in the tracking coils, and the electromagnetic force, which is generated in the focusing coil, there is a possibility that the rotation moments cannot be cancelled with each other, each trying to tilt the objective lens, in particular, when the moving part operates into only one of the tracking direction and the focusing direction. 
     Then, an object of the present invention is to provide an objective lens actuator having a small tilt of the objective lens thereof. 
     The object mentioned above, according to the present invention, is accomplished by an objective lens actuator, for driving an objective lens in a focusing direction and a tracking direction, so as to focus lights on a recording surface of an optical disc, as a recording medium of information, comprising: a magnet; a lens holder, which is configured to hold said lens thereon; and four (4) pieces of tracking coils are disposed in a line on a side surface of said lens holder for holding said objective lens thereon, facing to said magnet. 
     Also, the object according to the present invention is accomplished by an objective lens actuator, for driving an objective lens in a focusing direction and a tracking direction, so as to focus lights on a recording surface of an optical disc, as a recording medium of information, comprising: magnets, which are disposed on two (2) surfaces putting said objective lens therebetween; a lens holder, which is configured to hold said lens thereon; and four (4) pieces of tracking coils are disposed in a line on each of two (2) side surfaces of said lens holder for holding said objective lens thereon, facing to said magnets. 
     Also, the object according to the present invention is accomplished by the objective lens actuator described in the above, wherein said magnet is disposed one by one at positions inner than outer sides of two (2) pieces of tracking coils lying on an outside far from said objective lens among said four (4) pieces of tracking coils, and outer than inner sides of two (2) pieces of tracking coils lying in an inside near to said objective lens. 
     Also, the object according to the present invention is accomplished by the objective lens actuator described in the above, wherein said four (4) pieces of tracking coils are connected in series, lengths of coil wires per one (1) turn of said four (4) pieces of tracking coils are equal to each other, and the number of turns of two (2) pieces of tracking coils lying on an outside far from said objective lens among said four (4) pieces of tracking coils is less than the number of turns of two (2) pieces of tracking coils lying in an outside near to said objective lens among said four (4) pieces of tracking coils. 
     Also, the object according to the present invention is accomplished by the objective lens actuator described in the above, wherein said four (4) pieces of tracking coils are connected in series, lengths of coil wires per one (1) turn and the number of turns of said four (4) pieces of tracking coils are equal to each other, and distance between two (2) pieces of tracking coils lying on an outside far from said objective lens among said four (4) pieces of tracking coils and the magnet facing to those is longer than distance between two (2) pieces of tracking coils lying in an inside near to said objective lens among said four (4) pieces of tracking coils and the magnet facing to those. 
     Also, the object according to the present invention is accomplished by the objective lens actuator described in the above, wherein said four (4) pieces of tracking coils are connected in series, length of coil wire locating in said focusing direction of two (2) pieces of tracking coils lying on an outside far from said objective lens among said four (4) pieces of tracking coils is longer than length of coil wire locating in said focusing direction of two (2) pieces of tracking coils lying in an inside near to said objective lens among said four (4) pieces of tracking coils, and the number of turns of two (2) pieces of tracking coils lying on the outside far from said objective lens among said four (4) pieces of tracking coils is less than the number of turns of two (2) pieces of tracking coils lying in the inside near to said objective lens among said four (4) pieces of tracking coils. 
     Also, the object according to the present invention is accomplished by the objective lens actuator described in the above, wherein said four (4) pieces of tracking coils are divided into two groups, two (2) pieces of tracking coils on an outside far from said objective lens among said four (4) pieces of tracking coils and two (2) pieces of tracking coils in an inside to said objective lens among said four (4) pieces of tracking coils, said two (2) groups of tracking coils are connected with, in parallel, lengths of coil wires per one (1) turn and numbers of turns of said four (4) pieces of tracking coils are equal to each other, and diameter coil wire of two (2) pieces of tracking coils lying on an outside far from said objective lens among said four (4) pieces of tracking coils is smaller than diameter coil wire of two (2) pieces of tracking coils lying in an inside near to objective lens among said four (4) pieces of tracking coils. 
     According to the present invention, since the rotation moment, which is generated by two (2) pieces of tracking coils lying in an inside near to the objective lens among four (4) pieces of tracking coils is in opposite to the rotation moment, which is generated by two (2) pieces of tracking coils lying in an outside far from the objective lens among four (4) pieces of tracking coils, and therefore the rotation moment acting upon the four (4) pieces of tracking coils as a whole comes to be small. Accordingly, it is possible to provide the objective lens actuator having small tilt of the objective lens thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       Those and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a perspective view of an objective lens actuator, according to an embodiment of the present invention; 
         FIG. 2  is an upper view of the objective lens actuator, according to the embodiment of the present invention; 
         FIG. 3  is a side view of a moving part of the objective lens actuator, according to the embodiment of the present invention; 
         FIGS. 4A to 4C  are upper views for showing forces, which are generated in tracking coils when the moving part operates in a “y” direction, and also moments rotating around a “z”-axis, which are generated due to those forces, within the objective lens actuator of the present invention; 
         FIGS. 5A and 5B  are upper views for showing forces, which are generated in tracking coils when the moving part operates in a “z” direction, and also moments rotating around an “x”-axis, which are generated due to those forces, within the objective lens actuator of the present invention; 
         FIG. 6  is an upper view of the moving part and permanent magnets of the objective lens actuator, according to other embodiment of the present invention; 
         FIG. 7  is a side view of the moving part of the objective lens actuator, according to the other embodiment of the present invention; 
         FIG. 8  is a perspective view for showing an optical head, applying the objective lens actuator therein, according to the present invention; 
         FIG. 9  is an exploded perspective view for showing a disc apparatus, applying the objective lens actuator therein, according to the present invention; and 
         FIG. 10  is a side view for showing forces, which are generated in the tracking coils when the moving part operates in the “z” direction, and a moment rotating around the “x”-axis, which is generated due to those forces, in the objective lens actuator of the conventional art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments according to the present invention will be fully explained by referring to the attached drawings. 
     Embodiment 1 
       FIG. 1  is a perspective view of an objective lens actuator  73 , according to the present invention. 
     In  FIG. 1 , the “x”-axis direction thereof indicates a jitter direction, i.e., a tangential direction of an optical disc not shown in the figure, the “y”-axis direction thereof a tracking direction, i.e., a radial direction of the optical disc, and “z”-axis direction thereof a focusing direction, i.e., a direction of an optical axis of an objective lens  731 , respectively. Also, definitions are made therein, the direction for the objective lens  731  to approach to the optical disc not shown in the figure is “up” while “down” far away from that. 
     The objective lens actuator  73  comprises a magnetic circuit, a moving part, onto which the objective lens  731  is attached, a holder  733  for holding this moving part therein, support members  734  for elastically supporting the moving part with respect to the holder  733 , a yoke  735 , and permanent magnets  736 . The wire-like support members  734  are fixed in the vicinity of an end surface of the holder  733  at one ends thereof, and are fixed on both ends of a lens holder  732  at the other ends thereof, with using a solder or the like, for example. The objective lens  731  is attached on an upper surface of the lens holder  732 . Also, onto the lens holder  732  are attached tracking coils  737   a  to  737   h  (however, those  737   a  to  737   h  are not shown in the figure since they cannot be seen in  FIG. 1 ), lying between the objective lens  731 , and a focusing coil  738 . 
     Herein, the objective lens  731 , the lens holder  732 , the tracking coils  737   a  to  737   h  and the focusing coil  738  build up the moving part. Thus, this moving part movers with respect to the holder  733 . Explanation will be made on the more detailed structures of the embodiment 1 of the present invention, by referring to  FIGS. 2 and 3 . 
       FIG. 2  is an upper view for showing the moving part and the permanent magnets  736  building up the objective lens actuator  73 , for showing the example 1 of the present invention. 
       FIG. 3  is a side view for showing the moving building up the objective lens actuator  73 , also for showing the example 1 of the present invention. 
     A first feature of the present embodiment lies in that, as is shown in  FIG. 2 , four (4) pieces of the tracking coils are arranged or disposed in a line, on each of the side surfaces of the lens holder  732 , facing to the permanent magnets  736 , respectively. A second feature of the present embodiment lies in that, on each side surface is disposed one (1) piece of the permanent magnets  736 , each having a single pole and the same characteristic, i.e., four (4) pieces in total thereof at positions, being inner than the outer sides of two (2) pieces of the tracking coils  737   a  and  737   d  or  737   e  and  737   h  on an outside far from the objective lens, among the four (4) pieces of the tracking coils, and outer than the inner sides of two (2) pieces of the tracking coils  737   b  and  737   c  or  737   f  and  737   g  in an inside near to the objective lens, among the four (4) pieces of the tracking coils. 
     Though N-poles of all the permanent magnets  736  are facing to the coils in the figure, but since it is possible to control the direction of the driving forces depending upon the direction of currents flowing in the coils, then S-poles of all the permanent magnets may be direct to face to the coils. And a third feature of the present embodiment lies, as is shown in  FIG. 3 , the four (4) pieces of the tracking coils are connected in series on each side surface, wherein length of coil wire per one (1) turn is equal to each other for four (4) pieces of the tracking coils, and the number of turns of the outer two (2) pieces of the tracking coils  737   a  and  737   d  far from the objective lens is set to be smaller than the number of turns of the inner two (2) pieces of the tracking coils  737   b  and  737   c  near to the objective lens. 
     Explanation will be given about the mechanisms for suppressing an tilt of the objective lens  731  with the objective lens actuator  73 , which is built up in such the structures, by referring to  FIGS. 4A to 5B . 
       FIG. 4A  illustrates therein the directions of magnetic fields in the vicinity of the tracking coil, under the condition where the moving part of the objective lens actuator  73  operates by Δy in “y” direction from a neutral condition thereof, and also electromagnetic forces Ft 1  to Ft 8 , which are produced on a side in parallel with “z”-axis of the tracking coil when driving the tracking coil in this condition.  FIG. 4B  illustrates therein only a component in “y” direction of the electromagnetic force in the tracking direction, which is produced along a side of the tracking coil shown in  FIG. 4A  in parallel with “z”-axis, and  FIG. 4C  illustrates therein only a component in “x” direction of the electromagnetic force in the tracking direction, which is produced along the side of the tracking coil shown in  FIG. 4A  in parallel with “z”-axis. 
     As shown in  FIG. 4A , i.e., when the objective lens  731  is operated by Δy into “y” direction, the side in parallel with the “z”-axis of the tracking coil is disposed at a position coming out from a center of the permanent magnet  736 . At the position coming out from the center of the permanent magnet  736 , also the direction of magnetic field is not perpendicular to “x”-axis, but has an tilt of a certain angle. The electromagnetic forces Ft 1  to Ft 8  are generated along the sides of the tracking coils in parallel with “z” axis, when supplying current into the tracking coil in this condition, have components in “x” direction in addition to the components in “y” direction. 
     Among of those, the electromagnetic forces Ft 1   y  to Ft 8   y  in “y” direction, as shown in  FIG. 4B , are driving forces for driving or actuating the moving part in the tracking direction. With those electromagnetic forces Ft 1   y  to Ft 8   y  in this “y” direction, since the tracking coils  737   a  to  737   d  and the tracking coils  737   e  to  737   h  are disposed at positions, being symmetric with respect to a line to the axis in parallel with “y” axis passing through a center of gravity of the moving part, then the rotation moment due to the electromagnetic forces Ft 1   y  to Ft 4   y  in “y” direction, which are generated in the tracking coils  737   a  to  737   d , are balanced with the rotation moment due to the electromagnetic forces Ft 5   y  to Ft 8   y  in “y” direction, which are generated in the tracking coils  737   e  to  737   h , and therefore there is hardly generated moment for rotating the moving part. 
     In the similar manner, as is shown in  FIG. 4C , also with the electromagnetic forces Ft 1   x  to Ft 8   x  in “x” direction, since the tracking coils  737   a ,  737   b ,  737   e  and  737   f  and the tracking coils  737   c ,  737   d ,  737   g  and  737   h  are disposed at positions, being symmetric with respect to a line to the axis in parallel with “x” axis passing through a center of gravity of the moving part, then the rotation moment due to the electromagnetic forces Ft 1   x , Ft 2   x , Ft 5   x  and Ft 6   x  in “x” direction, which are generated in the tracking coils  737   a ,  737   b ,  737   e  and  737   f , are balanced with the rotation moment due to the electromagnetic forces Ft 3   x , Ft 4   x , Ft 7   x  and Ft 8   x  in “x” direction, which are generated in the tracking coils  737   c ,  737   d ,  737   g  and  737   h , and therefore there is hardly generated moment for rotating the moving part. In this manner, according to the first and the second features of the present embodiment, since it is possible to reduce the rotation moment produced on the tracking coils accompanying the driving forces in the tracking direction even when the objective lens is operated, therefore it is possible to bring the tilt of the objective lens to be small. 
       FIGS. 5A and 5B  illustrate therein a distribution condition of magnetic flux density in “x” direction due to the permanent magnets  736  of the objective lens actuator  73  according to the present embodiment, the electromagnetic forces Ft 9 , Ft 20 , Ft 14  and Ft 17  in the tracking direction, which are produced along the sides of the tracking coils in parallel with “z”-axis, when driving the tracking coils under the condition that the moving part is operated by Δz in “z” direction from the neutral condition, and the electromagnetic forces Ft 10 , Ft 11 , Ft 13 , Ft 12 , Ft 16 , Ft 15 , Ft 19  and Ft 18  in the focusing direction, which are produced along the sides of the tracking coils in parallel with “y”-axis accompanying with those, and further rotation moments Mo and Mi around “x”-axis, which are produced due to those electromagnetic forces. 
     In particular,  FIG. 5A  illustrates the electromagnetic forces Ft 9  and Ft 20 , which are produced along the sides of the outer two (2) pieces of the tracking coils  737   a  and  737   d  in parallel with “z”-axis far from the objective lens, among the four (4) pieces of the tracking coils  737   a  to  737   d , the electromagnetic forces Ft 10 , Ft 11 , Ft 12  and Ft 13  in the focusing direction, which are produced along the sides of the tracking coils in parallel with “y”-axis accompanying with those, and only the rotation moment “Mo” around the “x”-axis, which is produced by those electromagnetic forces Ft 10 , Ft 11 , Ft 12  and Ft 13 , while  FIG. 5B  illustrates the electromagnetic forces Ft 14  and Ft 17  in the tracking direction, which are produced along the sides of two (2) pieces of tracking coils  737   b  and  737   c  in parallel with “z”-axis in the inside near to the objective lens  731  among the four (4) pieces of the tracking coils  737   a  to  737   d , the electromagnetic forces Ft 15 , Ft 16 , Ft 18  and Ft 19  in the focusing direction, which are produced along the sides of the tracking coils in parallel with “y”-axis accompanying with those, and only the rotation moment Mi around the “x”-axis, which is generated due to those electromagnetic forces Ft 15 , Ft 16 , Ft 18  and Ft 19 . 
     As illustrated in the figures, when supplying current into the tracking coils in the condition that the objective lens  731  is operated only by Δz in “z”-direction, then the tracking coils generate the electromagnetic forces Ft 9 , Ft 20 , Ft 14  and Ft 17  along the sides in parallel with “z”-axis thereof, and at the same time, accompanying with this, also generate the electromagnetic forces Ft 10 , Ft 11 , Ft 13 , Ft 12 , Ft 16 , Ft  15 , Ft  19  and Ft 18  in “z” direction, i.e., the focusing direction, along the sides of the tracking coils in parallel with “x”-axis. 
     Herein, the electromagnetic force “F” can be expressed as below, when current “I” flows in the coil having a length “L” lying within the magnetic flux density “B”.
 
 F=B×I×L   (Equation 1)
 
     Firstly, in relation to the electromagnetic forces Ft 9 , Ft 20 , Ft 14  and Ft 17  in “y” direction, the influences thereof will be mentioned. 
     In the present embodiment, since four (4) pieces of the tracking coils  737   a  to  737   d  are connected in series, current flowing through the coils is equal to each other. Also, since the characteristics of the permanent magnets  736  are same, then the magnetic flux density is also equal to each other at the position along the side of each tracking coil in parallel with “z”-axis facing to the permanent magnets  736 . In this case, the electromagnetic force in “y” direction, which is generated in each of the tracking coils  737   a  to  737   d  is determined by the sum total of lengths of the sides of each tracking coil in parallel with “z”-axis facing to the permanent magnets  736 , from the equation (1). 
     Further, with the present embodiment, since the total sum of lengths of sides of each tracking coil in parallel with “z”-axis facing to the permanent magnets  736  is equal to the length of the coil wire of four (4) pieces of the tracking coils per one (1) turn thereof, and the number of turns of the outer two (2) pieces of tacking coils  737   a  and  737   d  far from the objective lens  731  is less than the number of turns of two (2) pieces of the tracking coils of the inside near to the objective lens  731 , among the four (4) pieces of the tracking coils  737   a  to  737   d , the total sum of lengths of the sides of the two (2) pieces of tracking coils  737   a  and  737   d  on the outside in parallel with “z”-axis facing to the permanent magnets  736  is shorter than the total sum of lengths of the sides of the inner two (2) pieces of tracking coils  737   b  and  737   c  in parallel with “z”-axis facing to the permanent magnets  736 . As a result thereof, the electromagnetic forces Ft 9  and Ft 20 , which are generated in the outer two (2) pieces of tracking coils  737   a  and  737   d , comes to be smaller than the electromagnetic forces Ft 14  and Ft 17 , which are generated in the two (2) pieces of tracking coils  737   a  and  737   d  in the inside. Since this electromagnetic force in “y” direction is a driving force for driving the moving part in the tracking direction, and a point of action of this force is located almost on a straight line in parallel with “y”-axis passing through the center of gravity of the moving part on a “y-z” plane, then there is hardly generated a moment for rotating the moving part. Next, the influences thereof will be mentioned, in relation to the electromagnetic forces in “z” direction. 
     Although the magnetic flux density in “x” direction produced by the permanent magnets  736  is almost uniform or flat in a central portion of the permanent magnets  736 , as is shown on the graphs at the right-hand side in  FIGS. 5A and 5B , respectively, but it is reduced to be small in the peripheral portions, i.e., the upper and the lower ends of the permanent magnets  736 , then unbalance is produced in the magnetic flux density between the position on the upper side and the position on the lower side of each tracking coil in parallel with “x”-axis. Therefore, the electromagnetic force generated on each of the tracking coils  737   a  to  737   d  in “z” direction can be determined, when currents flowing in the coils are equal as in the present embodiment, by the total sum of the lengths of each tracking coil on each side thereof in parallel with “x”-axis facing to the permanent magnets  736 , and also the magnetic flux density at that position, from the equation (1). As a result thereof, the electromagnetic forces act, respectively, i.e., downwards in “z” direction in balance (i.e., Ft 10 -Ft 11 ), upon the outer tracking coil  737   a , upwards in “z” direction in balance (i.e., Ft 12 -Ft 13 ), upon the outer tracking coil  737   d . In the similar manner, the electromagnetic forces act, respectively, i.e., upwards in “z” direction in balance (i.e., Ft 15 -Ft 16 ), upon the tracking coil  737   b  in the inside, downwards in “z” direction in balance (i.e., Ft 18 -Ft 19 ), upon the tracking coil  737   c  in the inside. 
     Due to the magnetic forces generated on those tracking coils in “z” direction, the rotation moment acts on the moving part around “x” axis. However, since the rotation moment “Mo” due to the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) generated on the outer tracking coils  737   a  and  737   d  is in opposite in the direction thereof to the rotation moment “Mi” due to the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) generated on the tracking coils  737   b  and  737   c  in the inside, therefore it is enough to bring the rotation moments Mo and Mi to be equal to each other in the magnitude thereof, for the purpose of reducing the rotation moments acting upon the tracking coils  737   a  to  737   d  to tilt the moving portion, to be small. 
     Herein, the magnitude of the rotation moment can be presented by the product of the electromagnetic force and the distance from the point of action of that electromagnetic force up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane. The distance from the point of action of that electromagnetic force, up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part, on this “y-z” plane, is “Do”, for the outer two (2) pieces of tracking coils  737   a  and  737   d  on, being larger than “Di” of the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     Accordingly, it is enough to determine the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated on the outer two (2) pieces of tracking coils  737   a  and  737   d  lying far from the center of gravity of the moving part, to be smaller than the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated on the inner two (2) pieces of tracking coils, and thereby bringing a ratio between the magnitude of the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated on the tracking coils on the outside, and the magnitude of the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated on the inner tracking coils, to be equal to a ratio between the distance “Di”, from the point of action of the electromagnetic force in “z” direction generated on the inner tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and the distance “Do”, from the point of action of the electromagnetic force in “z” direction generated on the outer tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane. 
     In the present embodiment, the length of coil wire per one (1) turn is equal to, among the four (4) pieces of tracking coils, and the number of turns of the outer two (2) pieces of tracking coils  737   a  and  737   d  is determined to be smaller than the number of turns of the inner two (2) pieces of tracking coils  737   b  and  737   c , and thereby, the total sum of lengths of the sides of the outer two (2) pieces of tracking coils  737   a  and  737   d , in parallel with “y”-axis facing to the permanent magnets  736 , is smaller than the total sum of lengths of the sides of the inner two (2) pieces of tracking coils  737   b  and  737   c , in parallel with “y”-axis facing to the permanent magnets  736 . 
     As a result thereof, the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated in the outer two (2) pieces of tacking coils  737   a  and  737   d , comes to be smaller than the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated in the inner two (2) pieces of tacking coils  737   b  and  737   c . With this, it is possible to bring the ratio between the magnitude of the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated in the outer tacking coils, and the magnitude of the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated in the inner tacking coils, to be equal to the ratio between the distance “Di”, from the point of action of the electromagnetic force in “z” direction generated on the tracking coils in the inside up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and the distance “Do”, from the point of action of the electromagnetic force in “z” direction generated on the outer tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and thereby bringing the magnitude of the rotation moment “Mo” by the outer two (2) pieces of tracking coils  737   a  and  737   d  to be equal to the rotation moment “Mi” by the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     Accordingly, the rotation moments “Mo” and “Mi” negate each other, since they are almost same in the magnitude and are opposite to each other in the directions thereof, and therefore the rotation moments come to be small, acting upon the tracking coils  737   a  to  737   d , to try to tilt the moving part. 
     This effect can be obtained, in the similar manner, with the tracking coils  737   e  to  737   h , which are attached on a side surface of the opposite side of the lens holder  732 , facing to the permanent magnets  736 . As was mentioned above, it is possible to make the tilt of the objective lens  731  small even when the objective lens operates. 
     Embodiment 2 
       FIG. 6  is an upper view for showing the moving part and the permanent magnets  736 , building up the objective lens actuator  73 , according to a second embodiment (i.e., an embodiment 2) of the present invention. 
     In  FIG. 6 , although as the third feature of the embodiment 1 mentioned above lies in that the four (4) pieces of tracking coils are connected in series, wherein the length of the coil wire per one (1) turn is equal to each other among the four (4) pieces of tracking coils, and the number of turns of the outer tracking coils  737   a  and  737   d  far from the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d  is smaller than the number of turns of the inner tracking coils  737   b  and  737   c  near to the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d : however, according to the present embodiment, it is characterized in that the four (4) pieces of tracking coils are connected in series, wherein the length of the coil wire per one (1) turn is equal to each other among the four (4) pieces of tracking coils, a distance “G” between the outer two (2) pieces of tracking coils  737   a  and  737   d  far from the objective lens  731 , among the four (4) pieces of tracking coils  737   a  to  737   d , and the permanent magnets  736  facing thereto is determined to be larger than a distance “g” between the inner two (2) pieces of tracking coils  737   b  and  737   c  near to the objective lens  731 , among the four (4) pieces of tracking coils  737   a  to  737   d , and the permanent magnets  736  facing thereto. 
     The other structures of the present embodiment are similar to those of the embodiment 1, and therefore the detailed explanation thereof will be omitted herein. 
     With such the structures in this manner, it is possible to bring the magnetic flux density, at the position along the sides of the outer two (2) pieces of tracking coils  737   a  and  737   d  in parallel with “x”-axis facing to the permanent magnets  736 , to be smaller than the magnetic flux density, at the position along the sides of the inner two (2) pieces of racking coils  737   b  and  737   c  in parallel with “y”-axis facing to the permanent magnets  736 . 
     From the equation (1) mentioned above, as shown in the present embodiment, the current flowing through the four (4) pieces of tracking coils  737   a  to  737   d , and the electromagnetic forces in “z” direction, which are generated on the tracking coils  737   a  to  737   d , respectively, in case when the total sums of lengths of the sides of the four (4) pieces of the tracking coils  737   a  to  737   d  in parallel with “z”-axis facing to the permanent magnets  736  are equal to each other, are determined by the magnetic flux density at portion of the sides in parallel with “x”-axis facing to the permanent magnets  736  on each of the tracking coils  737   a  to  737   d . Therefore, it is possible to bring the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated on the outer two (2) pieces of tracking coils  737   a  and  737   d , to be smaller than the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated on the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     With this, it is possible to bring the ratio between the magnitude of the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated on the outer tacking coils, and the magnitude of the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated on the inner tacking coils, to be equal to the ratio between the distance “Di”, from the point of action of the electromagnetic force in “z” direction generated on the inner tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and the distance “Do”, from the point of action of the electromagnetic force in “z” direction generated on the outer tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and thereby bringing the magnitude of the rotation moment “Mo” by the outer two (2) pieces of tracking coils  737   a  and  737   d  to be equal to the rotation moment “Mi” by the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     Accordingly, similar to the embodiment 1, the rotation moments “Mo” by the outer two (2) pieces of tracking coils  737   a  and  737   d  and “Mi” by the inner two (2) pieces of tracking coils  737   b  and  737   c  negate each other, since they are almost same in the magnitude and are opposite to each other in the directions thereof, and therefore the rotation moments come to be small, acting upon the tracking coils  737   a  to  737   d , to try to tilt the moving part. This effect can be obtained, in the similar manner to that of the embodiment 1, with the tracking coils  737   e  to  737   h , which are attached on a side surface of the opposite side of the lens holder  732 , facing to the permanent magnets  736 . With this, it is possible to make the tilt of the objective lens  731  small even when the objective lens operates. 
     Embodiment 3 
     Although as the third feature of the embodiment 1 mentioned above lies in that the four (4) pieces of tracking coils are connected in series, wherein the length of the coil wire per one (1) turn is equal to each other among the four (4) pieces of tracking coils, and the number of turns of the outer tracking coils  737   a  and  737   d  far from the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d  is smaller than the number of turns of the inner tracking coils  737   b  and  737   c  near to the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d : however, according to the present embodiment, it is characterized in that the four (4) pieces of tracking coils are divided into two (2) of an outside and two (2) of an inside, wherein a pair of tracking coils of the outside one or the inside one are connected in parallel, the length of the coil wire per one (1) turn and the number of turns are equal to each other among the four (4) pieces of tacking coils, and the diameter of the coil wire of the outer two (2) pieces of tacking coils  737   a  and  737   d  far from the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d  is determined to be thinner than the diameter of the inner two (2) pieces of tracking coils  737   b  and  737   c  near to the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d . The other structures of the present embodiment are similar to those of the embodiment 1, and therefore the detailed explanation thereof will be omitted herein. 
     With such the structures, it is possible to bring the current flowing into the outer two (2) pieces of tracking coils  737   a  and  737   d  to be smaller than the current flowing into the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     From the equation (1) mentioned above, the magnetic flux density on an upper and a lower sides of the tracking coil in parallel with “x”-axis due to the permanent magnets  736  having the same characteristics is equal to each other, respectively, on the upper and lower sides of the inner and the outer ones, and in case where the total sums of lengths of the sides in parallel with “x”-axis facing to the permanent magnets  736  are equal to each other among the four (4) pieces of tracking coils  737   a  to  737   d , the electromagnetic force in “z” direction, which is generated on each of the tracking coils  737   a  to  737   d , is determined by the current flowing in each of the tracking coils  737   a  to  737   d , and therefore it is possible to bring the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated on the outer two (2) pieces of tracking coils  737   a  and  737   d  to be smaller than the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated on the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     With this, it is possible to bring the ratio between the magnitude of the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated in the outer tacking coils, and the magnitude of the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated in the inner tacking coils, to be equal to the ratio between the distance “Di”, from the point of action of the electromagnetic force in “z” direction generated on the inner tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and the distance “Do”, from the point of action of the electromagnetic force in “z” direction generated on the outer tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and thereby bringing the magnitude of the rotation moment “Mo” by the outer two (2) pieces of tracking coils  737   a  and  737   d  to be equal to the rotation moment “Mi” by the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     Accordingly, similar to the embodiment 1, the rotation moments “Mo” by the outer two (2) pieces of tracking coils  737   a  and  737   d  and “Mi” by the inner two (2) pieces of tracking coils  737   b  and  737   c  negate each other, since they are almost same in the magnitude and are opposite to each other in the directions thereof, and therefore the rotation moments come to be small, acting upon the tracking coils  737   a  to  737   d , to try to tilt the moving part. This effect can be obtained, in the similar manner to that of the embodiment 1, with the tracking coils  737   e  to  737   h , which are attached on the side surface of the opposite side of the lens holder  732 , facing to the permanent magnets  736 . With this, it is possible to make the tilt of the objective lens  731  small even when the objective lens operates. 
     Embodiment 4 
     In the present embodiment mentioned above, though only one is changed, i.e., the configurations, the positions, or the characteristics of the outer two (2) pieces of tracking coils  737   a  and  737   d  and the inner two (2) pieces of tracking coils  737   b  and  737   c : however, as is shown in  FIG. 7 , it is also possible to connect the four (4) pieces of tacking coils in series, wherein the number of turns of the outer two (2) pieces of tracking coils  737   a  and  737   d  far from the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d  is set to be smaller than the number of turns of the inner two (2) pieces of tracking coils  737   b  and  737   c  near to the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d , and also the lengths of the sides of the outer two (2) pieces of tracking coils  737   a  and  737   d  far from the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d , in parallel with “z”-axis facing to the permanent magnets  736  are set to be longer than the lengths of the sides of the inner two (2) pieces of tracking coils  737   b  and  737   c  near to the objective lens  731  among the four (4) pieces of tracking coils  737   a  to  737   d , in parallel with “z”-axis facing to the permanent magnets  736 . 
     In this case, similar to the embodiment 1, in addition to reducing the number of turns of the outer two (2) pieces of tracking coils  737   a  and  737   d  less than the number of turns of the inner two (2) pieces of tracking coils  737   b  and  737   c , the length “H” of the sides of the outer two (2) pieces of tracking coils  737   a  and  737   d , in parallel with “z”-axis and facing to the permanent magnets  736  is set to be longer than the length “h” of the sides of the inner two (2) pieces of tracking coils  737   b  and  737   c , in parallel with “z”-axis and facing to the permanent magnets  736 , and thereby brining the sides of the outer two (2) pieces of tacking coils  737   a  and  737   d , in parallel with “z”-axis and facing to the permanent magnets  736  to be far away from the central portion of the permanent magnet  736 , to be disposed at the position where the magnetic flux density in “x” direction is small, as is shown in the right-hand side in  FIG. 7 , and thereby it is possible to bring the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated on the outer two (2) pieces of tracking coils  737   a  and  737   d , to be smaller than the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated on the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     With this, it is possible to bring the ratio between the magnitude of the electromagnetic forces (Ft 10 -Ft 11 ) and (Ft 12 -Ft 13 ) in “z” direction, which are generated in the outer tacking coils, and the magnitude of the electromagnetic forces (Ft 15 -Ft 16 ) and (Ft 18 -Ft 19 ) in “z” direction, which are generated in the inner tacking coils, to be equal to the ratio between the distance “Di”, from the point of action of the electromagnetic force in “z” direction generated on the inner tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and the distance “Do”, from the point of action of the electromagnetic force in “z” direction generated on the outer tracking coils up to the straight line in parallel with “z”-axis passing through the center of gravity of the moving part on the “y-z” plane, and thereby bringing the magnitude of the rotation moment “Mo” by the outer two (2) pieces of tracking coils  737   a  and  737   d  to be equal to the rotation moment “Mi” by the inner two (2) pieces of tracking coils  737   b  and  737   c.    
     Accordingly, the rotation moments “Mo” by the outer two (2) pieces of tracking coils  737   a  and  737   d  and “Mi” by the inner two (2) pieces of tracking coils  737   b  and  737   c  negate each other, since they are almost same in the magnitude and are opposite to each other in the directions thereof, and therefore the rotation moments come to be small, acting upon the tracking coils  737   a  to  737   d , to try to tilt the moving part. This effect can be obtained, in the similar manner to that of the embodiment 1, with the tracking coils  737   e  to  737   h , which are attached on the side surface of the opposite side of the lens holder  732 , facing to the permanent magnets  736 . With this, it is possible to make the tilt of the objective lens  731  small even when the objective lens operates. 
     Embodiment 5 
       FIG. 8  is a perspective view of an optical head  7 , applying the objective lens actuator  73  therein, according to the present invention. 
     In  FIG. 8 , the optical head  7  is the device for recording or reproducing information onto/from a disc, and is mainly built up with three elements, i.e., the objective lens actuator  73 , an optic system, and a flexible printed-circuit board  74 . The optic system is built up with a semiconductor laser  71  for emitting a laser light, a lens or a mirror, not shown in the figure, for dividing the light from the semiconductor laser or condensing it on the disc, and a photo detector  72  for receiving the reflection light from the disc thereon, etc. The flexible printed-circuit board  74  electrically connects the objective lens actuator  73  and the optic system  9  with a circuit board  9  through plural numbers of wiring patterns. 
     With the present embodiment, by applying the objective lens actuator  73  therein, according to the preset invention, it is possible to suppress generation of an optical aberration accompanying with the tilt of the objective lens  731 , and thereby to increase optical characteristics of the optical head  7 . 
     Embodiment 6 
       FIG. 9  is an exploded perspective view of a disc apparatus  1  applying the objective lens actuator  73  therein, according to the present invention. 
     In  FIG. 9 , the disc apparatus  1  is mainly built up with a bottom case  10 , a disc tray  4  for transferring the disc, i.e., the recording medium of information, into/from the apparatus, and a circuit board  9  mounting thereon semiconductor parts for conducting driving control and signal processing of electronic parts mounted within the disc apparatus. On an upper surface and a front surface of the bottom case  10  are provided a top case  2  and a front panel  3 , respectively, and they cover the upper surface and the front surface of the bottom case  10 . 
     On the disc tray  4  is attached a unitized mechanism portion (hereinafter, being called a “unit mechanism”)  6 , and the lower surface of which is covered by an under cover  8 . On the unit mechanism  6  are mounted a spindle motor  5  for rotating the disc, the optical head  7 , and an optical head transfer mechanism for moving the optical head  7  in the radial direction of the disc along with a guide shaft not shown in the figure. 
     In the present embodiment, with mounting the optical head  7  applying the objective lens actuator  73 , according to the present invention, in the disc apparatus  1 , because of no necessity of excessive electric power for compensating the tilt of the objective lens  731 , it is possible to achieve electric power saving. Also with an increase of the optical characteristics of the optical head  7 , it is possible to increase reliability in the operation of recording information on the disc and of reproducing of the disc information therefrom. 
     While we have shown and described several embodiments in accordance with our invention, it should be understood that disclosed embodiments are susceptible of changes and modifications without departing from the scope of the invention. Therefore, we do not intend to be bound by the details shown and described herein but intend to cover all such changes and modifications that fall within the ambit of the appended claims.