Linear motor and linear motor cogging reduction method

Provided is a linear motor capable of reducing cogging. The linear motor has a field magnet part 5 having a plurality of permanent magnets 21 arranged to form N and S poles alternately; a core 14 having a plurality of salient poles 14a, 14b and 14c arranged facing the field magnet part 5; and a three-phase coil 16 wound around the salient poles 14a, 14b and 14c of the core 14. At respective sides in the moving direction of an armature having the three-phase coil 16 and the core 14, auxiliary cores 18 made of a magnetic material are provided to sandwich the armature 10. The distance P1 between a center of each auxiliary core and a center of a center salient pole 14b is set to be substantially ¼×(2N+1)×a magnetic pole pitch between N poles of the field magnet part 5 (N: an integer equal to or greater than 1).

TECHNICAL FIELD

The present invention relates to a linear motor having moving a mover which moves linearly relative to a stator and, more particularly, to a linear motor having an auxiliary core so as to reduce cogging of the linear motor and a cogging reduction method thereof.

BACKGROUND ART

In a linear motor, a mover is moved linearly relative to a stator. In the stator of the linear motor, a plurality of permanent magnets is arranged so as the N and S magnetic poles are formed alternately. On the stator, the mover is arranged via a gap. In order to maintain the gap constant between the stator and the mover, linear movement of the mover is guided by a guide device such as a linear guide or a bearing.

In the mover, a magnetic body core is provided facing the permanent magnets. The core has a plurality of salient poles projecting toward a field magnet part. The plural salient poles are wounded with three-phase coils of U, V and W phases, respectively. When a three-phase AC (alternate current) having a phase difference of 120 degrees is passed through the three-phase coils of U, V and W phases, a moving magnetic field is generated in the three-phase coils. By the action of the moving magnetic field produced by the three-phase coils and the magnetic field produced by the permanent magnets, the mover moves linearly.

The core is provided in order to strengthen the magnetic field generated by the coils. The core is made of a magnetic material such as silicon steel. Therefore, even while current is not passed through the coils, magnetic attraction is generated between the salient poles of the core and the permanent magnets. When the mover moves along the stator, the salient poles of the core are attracted by front permanent magnets or attracted back by rear permanent magnets due to the magnetic attraction. Therefore, the magnetic attraction added to the mover varies periodically per magnetic pole pitch of permanent magnets. This periodic variation in attraction is called cogging. Even if a current is passed through the coils, there remains cogging, which acts as disturbance.

As an approach to cancel cogging, as illustrated inFIG. 11, there is known a linear motor having auxiliary magnetic poles2aand2bof magnetic bodies provided at respective ends in the moving direction of the core1of the mover (see Patent documents 1 and 2). In this linear motor, the auxiliary magnetic poles2aand2bare provided to strengthen the magnetic flux of salient poles1aand1bat respective ends in the moving direction of the core1. If the auxiliary magnetic poles2aand2bare not provided, a magnetic circuit of the salient poles1aand1bat the respective ends is difficult to form and the magnetic flux of the salient poles1aand1bat the respective ends becomes weaker than that of the center salient pole1c. When the magnetic flux of the salient poles1aand1bat the respective ends becomes weak, the magnetic flux of the salient poles1aand1band the magnetic flux of the center salient pole1care unbalanced to cause cogging. The auxiliary cores2aand2bare provided to strengthen the magnetic flux of the salient poles1aand1bat the respective ends and solve the problem of unbalancing.[Patent Document 1] Japanese Utility Model Laid-Open No. 7-53427[Patent Document 2] Japanese Patent Application Laid-Open No. 55-68870

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, when the auxiliary cores are provided to strengthen the magnetic flux of the salient poles at the respective ends of the core, the newly provided auxiliary cores cause cogging. In order to reduce cogging of the auxiliary cores, there is a need to take a new measure against the cogging. Thus, the conventional cogging reduction method is difficult to adopt as a general measure against cogging due to various intertwined factors.

The inventors have noted that the magneto-resistance at the salient pole in the center of the core is low, the magnetic flux is easily to pass, and a waveform of a cogging force generated in the whole core (seeFIG. 7, the horizontal axis indicates the core phase and the vertical axis indicates the cogging force) is synchronous with a waveform of a cogging force generated in the center salient pole. Then, they have learned that the cogging force of the whole core can be reduced by generating a cogging force at the auxiliary cores having such a waveform as to cancel the waveform of the cogging force generated at the center salient pole.

The present invention was made in view of the foregoing and has an object to provide a linear motor and a linear motor cogging reduction method that are new and capable of reducing cogging.

Means for Solving the Problems

In order to solve the above-mentioned problems, for example, a linear motor includes a field magnet part having a plurality of permanent magnets arranged to form N and S poles alternately; a core having a plurality of salient poles arranged facing the field magnet part; a three-phase coil wound around the salient poles of the core; an armature having the three-phase coil and the core and moving linearly relative to the field magnet part; an auxiliary core of magnetic body provided on at least one side of the armature in a relative moving direction of the armature; and a distance between a center of the auxiliary core and a center of a center salient pole among the salient poles in the relative moving direction of the armature is set to be substantially ¼×(2N+1)× a magnetic pole pitch between N-N poles of the field magnet part (N: an integer equal to or greater than 1).

In the linear motor of the example above, the auxiliary core and the core are separate components so as to form a gap between the auxiliary core and the core or to interpose a non-magnetic material therebetween.

In the linear motor of the example above, the auxiliary core has a tip end part and a base part, and a thickness of the tip end part in the moving direction is smaller than a thickness in the moving direction of the base part.

In the linear motor of the example above, the tip end part of the auxiliary core is cut off at a side facing the core so that the thickness in the moving direction becomes smaller.

In the linear motor of the example above, the auxiliary core is provided at each side of the armature in the moving direction in such a manner that the armature is sandwiched between the auxiliary cores.

In the linear motor of the example above, the three-phase coil is a coil set having coils of U, V and W phases wound around the salient poles, respectively, arranged in the moving direction and the center salient pole is a salient pole positioned in center in the moving direction among the three salient poles.

In another exemplary embodiment of the invention a linear motor includes a field magnet part having a plurality of permanent magnets arranged to form N and S poles alternately; a core having a plurality of salient poles arranged facing the field magnet part; a three-phase coil wound around the salient poles of the core; an armature having the three-phase coil and the core and moving linearly relative to the field magnet part; an auxiliary core of magnetic body provided on at least one side of the armature in a relative moving direction of the armature; and the auxiliary core and the core being separate components so as to make a gap between the core and the auxiliary core or to interpose a non-magnetic material therebetween.

In yet another exemplary embodiment of the invention a linear motor cogging reduction method having a field magnet part having a plurality of permanent magnets arranged to form N and S poles alternately, a core having a plurality of salient poles arranged facing the field magnet part, a three-phase coil wound around the salient poles of the core, an armature having the three-phase coil and the core and moving linearly relative to the field magnet part, and an auxiliary core of magnetic body provided on at least one side of the armature in a relative moving direction of the armature, the linear motor cogging reduction method comprising: placing the auxiliary core in such a manner that a distance between a center of the auxiliary core and a center of a center salient pole among the salient poles in the relative moving direction of the armature is substantially ¼×(2N+1)× a magnetic pole pitch between N-N poles of the field magnet part (N: an integer equal to or greater than 1).

EFFECTS OF THE INVENTION

In some exemplary embodiments of the invention, as the auxiliary core is arranged substantially at the position an odd multiple of a magnetic pole pitch×¼ away from the center salient pole, it is possible to generate such a cogging force at the auxiliary core as to cancel the cogging force generated by the center salient force. This means that it is possible to reduce the cogging of the whole core.

In some exemplary embodiments of the invention, as the gap is made between the core and the auxiliary core or the non-magnetic material is interposed therebetween, it is possible to prevent the salient poles at the respective ends of the core and the auxiliary core from forming a magnetic circuit. Hence, it is possible to reduce the influence of the conventional measure against cogging that strengthens the magnetic flux of the salient poles at the respective ends of the core, thereby to reduce the cogging reliably. In addition, as the core and auxiliary core are separate components, even when there is an error in design values in dimensions of the manufactured auxiliary core, or besides the components of the linear motor, a component that causes cogging is provided between the stator and the mover, there is no need to manufacture the core again, of which manufacturing process is complicated, and only need to change the auxiliary core.

In some exemplary embodiments of the invention, the auxiliary core can be separated from the core while maintaining the pitch between the auxiliary core and the center salient pole constant. Hence, it is possible to prevent the salient poles at the respective ends of the core and the auxiliary core from forming a magnetic circuit. Besides, the base part is thicker than the tip end part, the auxiliary core can be easily attached to the table or the like. Further, as the attraction applied to the auxiliary core can be reduced, it becomes possible to reduce a load on a guide part for guiding linear movement of the mover.

In some exemplary embodiments of the invention, it is possible to shorten the entire length of the core including the auxiliary core while the gap is made between the salient poles at the respective ends of the core and the auxiliary core.

In some exemplary embodiments of the invention, it is possible to evenly reduce the cogging force generated by the center salient pole, in a balanced manner, with use of the auxiliary cores provided at the respective sides of the armature.

In some exemplary embodiments of the invention, it is possible to reduce the cogging force at the center salient pole among the three salient poles effectively.

In some exemplary embodiments of the invention, as the gap is made between the core and the auxiliary core or the non-magnetic material is interposed therebetween, it is possible to prevent the salient poles at the respective ends of the core and the auxiliary core from forming a magnetic circuit. Hence, it is possible to reduce the influence of the conventional measure against cogging that strengthens the magnetic flux of the salient poles at the respective ends of the core, thereby to reduce the cogging reliably. In addition, as the core and auxiliary core are separate components, even when there is an error in design values in dimensions of the manufactured auxiliary cores, or besides the components of the linear motor, a component that causes cogging is provided between the stator and the mover, there is no need to manufacture the core again, of which manufacturing process is complicated, and only need to change the auxiliary cores.

In some exemplary embodiments of the invention, as the auxiliary core is arranged substantially at the position an odd multiple of a magnetic pole pitch×¼ away from the center salient pole, it is possible to generate such a cogging force at the auxiliary core as to cancel the cogging force generated by the center salient pole. This means that it is possible to reduce the cogging of the whole core.

REFERENCE NUMERALS

MODE FOR CARRYING OUT THE INVENTION

With reference to the attached drawings, exemplary embodiments of the present invention will be described in detail below.FIG. 1is a perspective view of a linear motor according to an exemplary embodiment of the present invention (including a cross sectional view of a table) andFIG. 2is a front view thereof. On an elongating base4, a field magnet part5is mounted as a stator of the linear motor. On the base4, linear guides9for guiding linear movement of a table3are mounted. The table3is mounted on the upper surfaces of moving blocks7of the linear guides9. On the lower surface of the table3, an armature10is suspended as a mover of the linear motor between the linear guides9of both sides. As illustrated in the front view ofFIG. 2, a gap g is made between the armature10and the field magnet part5. The linear guides9maintain this gap constant irrespective of movement of the table3.

The base4has a bottom wall4aand a pair of side walls4bprovided at the respective sides in the width direction of the bottom wall4a. On the upper surface of the bottom wall4a, raceway rails8of the linear guides9are mounted. On each raceway rail8, moving blocks7are mounted slidably. Between the raceway rail8and each moving block7, a plurality of balls (not shown) is interposed rollably. In the moving block7, a circuit-shaped ball circulation passage is provided for circulating the balls. When the moving block7slides relative to the raceway rail8, the plural balls rolls therebetween and circulate in the ball circulation passage. This enables smooth sliding of the moving block7relative to the raceway rail8.

On the upper surface of each moving block7of the linear guide9, the table3is mounted. The table3is made of a non-magnetic material such as aluminum. On the table3, a moving object is mounted. Also, on the table3, a position detecting unit12such as a liner scale is mounted for detecting the position of the table3relative to the base4. A position signal detected by the position detecting unit12is sent to a driver that drives the linear motor. The driver controls a current to be supplied to the armature10so as to move the table3in accordance with a position instruction from an upper-level controller.

FIG. 3is a cross sectional view taken along the moving direction of the armature10. On the lower surface of the table3, the armature is provided via an insulating material13. The armature10has a core14made of a magnetic material such as silicon steel and a three-phase coil16wound around salient poles14a,14band14cof the core14. The core14has a base plate14dmounted on the lower surface of the table3and the comb teeth shaped salient poles14a,14band14cprojecting downward from the base plate14d. The number of the salient poles14a,14band14cis a multiple of 3 and in this exemplary embodiment, it is 3. The salient poles14a,14band14care arranged in the moving direction of the armature10with a fixed pitch kept therebetween. The three salient poles14a,14band14care wound with coils16a,16band16cof U, V and W phases. The three-phase coil16carries three-phase AC having a phase difference of 120 degrees. After the three-phase coil16is wound around the salient poles14a,14band14c, it is sealed with resin.

On the lower surface of the table3, a pair of auxiliary cores18is mounted sandwiching the armature10. The auxiliary cores18and the core14of the armature10are separate components. And, a gap W is created between the core14and each of the auxiliary cores18. The auxiliary core18is made of a magnetic material such as silicon steel or rolled steel of general structure. As no coil is wound on each auxiliary core18, the auxiliary core18does not function as an electromagnet.

FIGS. 4A and 4Bare detailed view of the auxiliary core18.FIG. 4Ais a plan view of the auxiliary core18andFIG. 4Bis a side view of the auxiliary core18. The auxiliary core18is approximately plate shaped as a whole. The lateral width of the auxiliary core18is almost equal to that of the core14. The auxiliary core18has a base part18amounted onto the table3and a tip end part18bprovided closer to the field magnet part5. In the base part18a, a screw hole18cis formed for mounting the auxiliary core18onto the table3. A side of the tip end part18bfacing the armature10is cut over the entire length in the width direction. This cut part18dis provided to make the tip end part18bthinner than the base part18a.

As illustrated inFIG. 3, as the gap W is given between the core14and the auxiliary cores18, it becomes possible to prevent the salient poles14aand14cat the respective ends of the core14and the auxiliary cores18from forming a magnetic circuit. Hence, it is possible to reduce the influence of the conventional cogging reduction method that strengthens the magnetic flux of the salient poles14aand14cat the respective ends of the core14.

In addition, as the tip end part18bof each auxiliary core18is made thin, it is possible to separate the auxiliary core18from the core14as much as possible while keeping the pitch P1between the auxiliary core18and the center salient pole14bconstant. Hence, it is possible to prevent the salient poles14aand14cat the respective ends of the core14and the auxiliary cores18from forming a magnetic circuit.

FIG. 5illustrates the field magnet part5mounted on the upper surface of the base4. The field magnet part5has a thin-plate-shaped yoke20and a plurality of permanent magnets21arranged in a line on the yoke20. Each permanent magnet21is a rare earth magnet such as neodymium magnet having high coercive force. Either one of N pole and S pole is formed at the front side of the plate-shaped permanent magnet21, and the other is formed at the back side thereof. The permanent magnets21are arranged on the yoke20in such a manner that N and S poles are formed alternately in the longitudinal direction. The permanent magnets21are fixed to the yoke20by adhesion.

The yoke20is made of a magnetic material such as silicon steel or rolled steel of general structure. The yoke20is formed like an elongating plate. The permanent magnets21fixed onto the yoke20are covered with a cover plate22(indicated by the chain double-dashed line). The cover plate22is also fixed to the yoke20by adhesion. The yoke20to which the permanent magnets21and the cover plate22are fixed is mounted on the base4with use of a fixing part such as a bolt23. The field magnet part5is unitized and a plurality of field magnet parts5is unitized in accordance with the length of the base4and mounted on the base4. The base4to which the field magnet parts5is fixed is fixed to a bed (not shown) with use of a fixing part such as a bolt24.

FIG. 6is a plan view of the field magnet part5. In this exemplary embodiment, the plan shape of each permanent magnet21is parallelogram. The distance from the center of an N-pole permanent magnet21ato the center of another N-pole permanent magnet21ais a magnet pole pitch P2between N-N poles of the field magnet part5. Needless to say, the magnetic pole pitch P2of N-N poles of the field magnet part is twice as long as the magnetic pole pitch P3between N-S poles and equal to the magnetic pole pitch between S-S poles.

With reference toFIGS. 7 to 9, a cogging reduction method according to the present invention will be described. When the core14made of magnetic material is moved over the permanent magnets21of the field magnet part5, magnetic attraction is caused between the permanent magnets21and the core14. Out of the magnetic attraction, a component that is generated in the moving direction of the armature10is relevant to cogging. A component perpendicular to the moving direction of the armature10(attraction in the vertical direction) is received by the linear guides9and is irrelevant to the cogging.

While no current is passed through the three-phase coil16, the armature10is moved linearly relative to the field magnet part5. Then, the salient poles14a,14band14cof the core14are attracted by front permanent magnets21or rear permanent magnets21in the moving direction. This periodic variation in attraction is cogging.

FIG. 7is a graph showing the cogging force generated at each of the salient poles14a,14band14cwhen the armature10is moved from −180 to 0 electrical degrees (½ of the magnetic pole pitch between N poles). The cogging forces generated at the salient poles14a,14band14cof U, V and W phase are represented as sine curves of which the phases are 120-degree different from each other, like currents passing through the three-phase coils of U, V and W phases. If the amplitudes of the three sine curves are the same, the cogging force of the whole core obtained by combining cogging forces of the three salient poles14a,14band14calways becomes zero irrespective of the position of the armature10. That is, no cogging is generated.

However, the magneto-resistance of the center salient pole14bof W phase is the lowest and the magnetic flux can pass easily. When cogging forces of the salient poles of U, V and W phases are compared, the cogging force of the center salient pole14bof W phase is the greatest and the cogging forces of the salient poles14aand14cat the respective ends are smaller. In view of this, the cogging force of the whole core is generated in synchronization with the cogging force of the center salient pole14bof W phase. If the auxiliary cores18can generate such a cogging force that can cancel the cogging force of the salient pole of W phase, the cogging force of the whole core can be reduced.

FIG. 8is a graph showing comparison between the waveform of the cogging force generated at the whole core and the waveforms of the cogging forces generated at the auxiliary cores18. The waveforms of the cogging forces generated at the auxiliary cores (1) and (2) are phase-shifted by 90 electrical degrees from the waveform of the cogging force of the whole core and serves as waveforms that can reduce the cogging force of the whole core. Then, the cogging force waveform obtained by combining the waveforms of the auxiliary cores (1) and (2) is an inversion of the whole core waveform. Therefore, the cogging force obtained by combining the cogging forces of the whole core and the auxiliary cores (1) and (2) always becomes close to zero irrespective of the electrical angle of the armature10.

Here, in order that the cogging force for canceling the cogging force of the whole core is generated by the auxiliary cores18, the phase of the center salient pole14bof W phase has only to be shifted by 90 electrical degrees from the phase of the auxiliary core18. In other words, as illustrated inFIG. 9, the distance P1from the center of the center salient pole14bof W phase and the center of the tip end part18bof the auxiliary core18is an odd multiple of one fourth of the magnetic pole pitch P2between N-N poles of the field magnet part5. If it is set to be an even multiple of one fourth of the magnetic pole pitch, the cogging force of the auxiliary core18strengthens the cogging force of the salient pole14bof W phase.

Here, in consideration of a mounting space of auxiliary cores18or actual cogging occurrence, the distance P1between the center of the salient pole14bof W phase and the center of the auxiliary core18may be slightly shifted from an odd multiple of one fourth of the magnetic pole pitch. Such a case may be included in the scope of the present invention featuring that the distance P1is substantially an odd multiple of one fourth of the magnetic pole pitch.

EXAMPLES

A linear motor is used of which the magnetic pole pitch between N-N poles of the field magnet part5is 39 mm. When this is applied to the formula shown inFIG. 9, the distance P1between the center of the salient pole14bof W phase and the center of the auxiliary core18is 39×(¼)×5=48.75 mm. In fact, the auxiliary core18is arranged at the position of 39×(¼)×4.8=46.8 mm. Then, cogging is compared between before and after mounting of the auxiliary core18.

FIGS. 10A and 10Billustrate results of cogging comparison.FIG. 10Aillustrates the cogging before mounting of the auxiliary core18andFIG. 10Billustrates the cogging after mounting of the auxiliary core18. These show that the cogging force can be reduced about 50% from 11.4 N to 5.86 N by mounting of the auxiliary core18.

The present invention is not limited to the above-described exemplary embodiment and may be embodied in various forms without departing from the scope of the present invention. For example, when three salient poles form one set, two sets of salient poles, that is six salient poles, may be provided. In such a case, there are two center salient poles and the center of the two salient poles is treated as the center of the center salient pole. When totally nine salient poles are provided, the fifth salient pole from the end is treated as the center salient pole.

In addition, the auxiliary cores do not need to be provided at the respective sides of the armature, or one auxiliary core may be provided at one side of the armature. The tip end part of the auxiliary core does not need to be thin or may have a straight shape of cross section that does not vary from the base part to the tip end part. Between the auxiliary core and the core, a non-magnetic material maybe interposed in place of the gap. The auxiliary core may be provided at the side surface side of the table, not at the lower surface side of the table.

Further, although, in the above-described exemplary embodiment, the armature as the mover is moved and the field magnet part as the stator is fixed, the field magnet part may be moved and the armature may be fixed.

The present application is based on Japanese Patent Application No. 2007-240143 filed on Sep. 14, 2007, and its contents are incorporated by reference herein.