Linear motor with reduced cogging

Provided is a linear motor capable of preventing an increase in length of the armature in the moving direction and also of reducing cogging. The linear motor has a magnetic field part having a plurality of permanent magnets arranged in a straight line in such a manner that N poles and S poles are formed alternately; and an armature having a core which has a plurality of teeth arranged opposite to the magnetic field part with a gap created therebetween and a plurality of coils wound on the teeth of the core. Among the teeth with the coils wound around, a width TW1 in a relative moving direction of each of teeth placed at both ends in the relative moving direction of the armature is smaller, from a base part to an end part thereof, than a width TW2 in the relative moving direction of each of other teeth.

TECHNICAL FIELD

The present invention relates to a linear motor which has a magnetic field part and an armature linearly moving relative to the magnetic field part. Particularly, the present invention relates to a linear motor capable of reducing cogging.

BACKGROUND ART

The linear motor has a magnetic field part having a plurality of permanent magnets and an armature arranged in an opposite manner on the magnetic field part with a gap created therebetween. In the magnetic field part, the permanent magnets are arranged in a straight line in such a manner the N poles and S poles are formed alternately. The armature has a core having a plurality of teeth opposing to the permanent magnets of the magnetic field part and a plurality of coils wound on the respective teeth. When alternate current is made to pass through the phase coils wound on the respective teeth, there occurs a moving magnetic field. This moving magnetic field and the magnetic field of the permanent magnets interact with each other, which generates a thrust so that the armature linearly moves relative to the magnetic field part.

In a linear motor that moves linearly, the length of the armature is limited in the moving direction, while the armature of a rotary motor is formed endless. Therefore, when the armature moves relative to the magnetic field part, there likely occurs cogging. Cogging is a phenomenon of magnetic forces between the core of the armature and permanent magnets pulses depending on the electrical angle.

Generally, the core is made of a magnetic material. When current does not flow into the coil, the magnetic attraction force is generated between the teeth of the core and the permanent magnets. When the armature moves relative to the magnetic field part, the teeth of the core are attracted by permanent magnet in front or retracted by permanent magnet in the rear. This is considered to cause such a cogging that the magnetic attraction force applied to the armature varies periodically in every magnetic pole pitch of the permanent magnets. When the current is passed through the coils, the cogging remains and acts as external disturbance.

In a conventional art, there are known auxiliary magnetic poles provided at both ends of the core of the armature in the moving direction in order to cancel the cogging (see the patent literature 1). On each of the auxiliary magnetic poles at both ends, no coil is wound. The distance between the auxiliary magnetic poles at both ends is set to such a distance that magnetic attraction forces generated at the respective ends cancel each other.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Examined Utility Model Application Publication No. 7-53427

SUMMARY OF INVENTION

Technical Problem

However, in the linear motor disclosed in the patent literature 1, as auxiliary magnetic poles are provided in pair at both ends of the core of the armature in the moving direction, the length of the armature in the moving direction is problematically increased.

Then, the present invention aims to provide a linear motor of new structure capable of reducing cogging.

Solution to Problem

In order to solve the above-mentioned problem, one aspect of the present invention is a linear motor comprising: a magnetic field part having a plurality of permanent magnets arranged in a straight line in such a manner that N poles and S poles are formed alternately; and an armature having a core which has a plurality of teeth arranged opposite to the magnetic field part with a gap created therebetween and a plurality of coils wound on the teeth of the core, the armature linearly moving relative to the magnetic field part, wherein among the teeth with the coils wound around, a width in a relative moving direction of each of teeth placed at both ends in the relative moving direction is smaller, from a base part to an end part thereof, than a width in the relative moving direction of each of other teeth.

Advantageous Effects of Invention

According to the present invention, as the width of each of the teeth placed at both ends of the armature in the relative moving direction is smaller, from its base part to its end part, than the width in the relative moving direction of each of the other teeth, it is possible to reduce variation in magnetic attraction forces generated in a sinusoidal wave manner in the U-phase, V-phase and W-phase teeth. Therefore, it is possible to reduce the cogging as a total sum of magnetic attraction forces generated in a sinusoidal wave manner in the U-phase, V-phase and W-phase teeth.

DESCRIPTION OF EMBODIMENTS

With reference to the accompanying drawings, an embodiment of the present invention will be described in detail below.FIGS. 1 and 2are views illustrating an overall structure of the linear motor. In the drawings, same components are denoted by same reference numerals.

On a base4elongated in a narrow shape, a magnetic field part5of the linear motor is mounted. The magnetic field part5is placed opposite to an armature10with a predetermined gap created therebetween. In this embodiment, the armature10is mounted on the under surface of a table3and moves linearly in the longitudinal direction of the base4, together with the table3.

On the base4, there is mounted a linear guide9for guiding linear movement of the table3. The table3is mounted on the upper surfaces of moving blocks7of the linear guide9. The armature10is provided between left and right linear guides9on the under surface of the table3. The armature10is mounted on the table3with use of a fastening member such as a bolt or screw.

As illustrated in the front view ofFIG. 2, the gap g is created between the armature10and the magnetic field part5. The linear guide9guides linear movement of the table3while holding the gap g constant.

The base4has a bottom wall part4aand a pair of side wall parts4bprovided at respective sides of the bottom wall part4ain the width direction. On an upper surface of the bottom wall part4a, the magnetic field part5is mounted. On upper surfaces of the side wall parts4a, raceway rails8of the linear guides9are mounted, respectively. On each of the raceway rails8, moving blocks7are mounted slidable. Between the raceway rail8and each moving block7, a plurality of balls is provided rollable. In each moving block7, a circuitry ball circulation path is formed for circulating the balls. When the moving block7moves linearly relative to the raceway rail8, the balls circulate in the circuitry ball circulation path.

The table3is made of, for example, a nonmagnetic material such as aluminum. On the table3, position detecting means12such as a linear scale is provided for detecting the position of the table3relative to the base4. A position signal detected by the position detecting means12is sent to a driver (not shown) for driving the linear motor. The driver controls current to supply to the armature10so that the table3can move in accordance with a position command from a higher controller.

FIG. 3provides detailed views of the armature10mounted on the under surface of the table. The armature10has a core14made of a magnetic material such as silicon steel or electromagnetic steel and a plurality of coils16wound on a plurality of teeth14aof the core14.

The core14has aback yoke14bformed with a square-shaped flat surface and the plural teeth14aprojecting toward the magnetic field part5from the back yoke14b. Ends of the back yoke14bin the moving direction hang over the respective end teeth14a-1in the moving direction outward in the moving direction. In the back yoke14b, screw holes14care formed for mounting the core14onto the table3.

When seen in the plan view ofFIG. 3(a), each of the teeth14ais formed into a narrow plate elongated in the width direction. When seen in the side view ofFIG. 3(b), each of the teeth14ais formed into a narrow rectangular shape elongated in the vertical direction and juts in the direction orthogonal to the back yoke14b. Side surfaces15aand15bin pair of each of the teeth14a(end surfaces in the moving direction) are formed into flat surfaces and parallel to each other. The end surface17(under surface) of each of the teeth14ais formed into a flat surface along the length in the moving direction and is orthogonal to the paired side surfaces15aand15b. The cross sectional shape of the core14along the moving direction is the same as the side surface shape of the core14and is held constant all over the width. The core14is formed by stacking thin steel plates in the width direction of the core14, each thin steel plate having a thickness of less than 1 mm and being formed by press punching into the same shape as the ide surface.

The pitch P1between teeth14a(the distance between the centers in the moving direction of adjacent teeth14a) is held equal all over the teeth14a. Among the teeth14a, the widths in the moving direction of the teeth14a-1positioned at the respective ends (end teeth14a-1) in the moving direction are indicated by TW1and equal to each other. The width of each of the end teeth14a-1positioned at both ends in the moving direction is held constant at TW1from the base part18to the end part19. As to the other teeth14a-2, their widths in the moving direction are indicated by TW2and equal to each other. The width of each of the teeth14a-2is held constant at TW2from the base part18to the end part19. The width TW1of each of the end teeth14a-1at both ends is narrower than the width TW2of each of the other teeth14a-2, from the base part18to the end part19. Specifically, the size of TW1is set to 0.7×TW2≦TW1<TW2. The projection amounts L1of the teeth14afrom the back yoke14bare set to be equal to each other. Therefore, the gap from the end part of each of the end teeth14a-1positioned at both ends to the magnetic field part5is equal to the gap from the end part of each of the other teeth14a-2to the magnetic field part5.

The number of teeth14ais set to be a multiple of 3. In this example, the number of teeth14ais 6, including two U-phase teeth, two V-phase teeth and two W-phase teeth. Teeth14aare wound by the U-phase, V-phase and W-phase coils16, respectively. In this example, each of the teeth14ais wound by one-phase coil16in a concentrated way (concentrated winding). Winding of the coils16is not limited to concentrated winding, but may be distributed winding (lap winding). The wires of the coils16of U-phase, V-phase and W-phase are all equal in wire thickness and the number of turns to each other, and they are also equal in the total size. As described above, as the width of each of the end teeth14a-1positioned at both ends in the moving direction is smaller than the width of each of the other teeth4a-2, the gap between the end teeth14a-1in the moving direction and the coils16becomes larger than the gap between the other teeth14a-2and the coils16. The coils16of U-phase, V-phase and W-phase are wound around the teeth14a, respectively, and then, the coils16are molded in resin. With this process, the coils16are fixed to the core14.

FIG. 4illustrates the magnetic field part5mounted on the upper surface of the base4. The magnetic field part5has a thin yoke20and a plurality of plate-shaped permanent magnets21aligned in a line in the moving direction of the armature on the upper surface of the yoke20. The permanent magnets21are made of rare-earth magnets such as neodymium magnets having a higher coercive force. In each of the plate-shaped permanent magnets21, one of the N pole and the S pole is formed on the front side thereof and the other is formed on the back side. The plate-shaped permanent magnets21are arranged in such a manner that the N poles and S poles are formed alternately n the longitudinal direction. The permanent magnets21are fixed to the yoke20by adhesion or the like. The permanent magnets21fixed to the yoke20is covered with a cover plate22made of a nonmagnetic material. The cover plate22is also fixed to the yoke20by adhesion or the like. The yoke20to which the permanent magnets21and the cover plate22are fixed is mounted onto the base4by a fastening member like a bolt23. The magnetic field part5is unitized and a plurality of unitized magnetic field parts5are mounted on the base4in accordance with the length of the base4. The reference numeral24denotes a bolt (fastening member) for mounting the base4onto another device.

FIG. 5is a plan view of the magnetic field part5. In this example, the planer shape of each of the permanent magnets21is a rectangular and each permanent magnet is inclined relative to the moving direction of the armature10. Paired end sides21-2of the permanent magnet21are parallel to each other and inclined by a predetermined angle relative to the line L2orthogonal to the moving direction of the armature10. Paired end sides21-1in the width direction of each permanent magnet21are parallel to each other and orthogonal to the end sides21-2. The distance P2between the center of one S pole permanent magnet21aand the center of another adjacent S pole permanent magnet21ais an S pole-to-S pole pitch and twice as long as the N pole-to-S pole pitch P3.

When the core14of the armature10moves relative to the magnetic field part5, a magnetic attraction force acts between the teeth14aof the core14and the permanent magnets21. In this magnetic attraction force, a component in the moving direction of the armature10causes cogging. A component of the force orthogonal to the moving direction of the armature10(attraction force in the vertical direction) is received by the liner guide9and does not affect the cogging. The cogging fluctuates periodically for every magnetic pole pitch P2of the magnetic field part5.

The inventors have focused attention on the relationship between cogging the width of each of the end teeth14a-1in the moving direction of the armature10. Then, they have calculated, by magnetic field analysis, a cogging force for each of various widths of the end tooth14a-1. As a result, they have found that the cogging can be reduced by making the width of each end tooth14a-1smaller, from its base part18to its end part19, than the width of each of the other teeth14a-2(see examples described later andFIGS. 9 and 13).

When the armature10is moved relative to the magnetic field part5, there occurs a magnetic attraction force in each tooth14a, the magnetic attraction force being sinusoidal for every magnetic pole pitch P2. The total sum of magnetic attraction forces generated in the teeth14abecomes cogging of the armature10. Here, description is made with the teeth divided into U-phase, V-phase and W-phase teeth14a. When it is assumed that ideal magnetic attraction forces of equal peak values and 120-degree different phases act on the U-phase, V-phase and W-phase teeth14a, respectively, the total sum of the magnetic attraction forces act on the U-phase, V-phase and W-phase teeth14abecomes zero, and there is to occur no cogging.

As the width of each of the end teeth14a-1is smaller like in this embodiment, the magnetic attraction forces on U-phase, V-phase and W-phase teeth are made closer to the ideal magnetic attraction forces of equal peak values and 120-degree different phases. With this configuration, it is expected that the cogging, which is the total sum of the magnetic attraction forces on the U-phase, V-phase and W-phase teeth14a, is reduced (see examples described later andFIGS. 11 and 15)

However, as described above, if the width of each of the end teeth14a-1placed at both ends in the moving direction of the armature10is narrowed, the induced voltage of the coils16wound around the end teeth14a-1becomes small and the thrust of the linear motor is reduced accordingly. This reduction in induced voltage can be prevented by forming the end surface of each of the end teeth14a-1into a flat surface.

As illustrated inFIG. 9, as the width between the teeth14aat both ends varies, the curve of cogging is shown as a valley, though details will be described later. That is, if the width of each tooth14ais too small, the cogging is increased contrarily. If the width of each tooth14ais further narrowed, the induced voltage of the coils16wound on the teeth14abecomes small and the thrust of the liner motor is reduced accordingly. Therefore, the width of each end tooth14a-1is preferably 70% or more of the width of each of the other teeth14a-2.

The present invention is not limited to the above-described embodiment and may be embodied in various forms without departing from the scope of the present invention.

For example, the structure of the linear motor is not limited to the above-mentioned structure in which the table is guided by the linear guides and may be modified as appropriate.

The linear movement of the armature relative to the magnetic field part is relative movement, and it may be configured that the magnetic field part moves and the armature is fixed.

As illustrated inFIG. 6, the number of teeth14amay be 3 or may be any number such as 9, 12, 15 or the like. And the three-phase coils may be replaced with two-phase coils. In such a case, the number of teeth is set to 4, 6, 8 or the like.

As illustrated inFIG. 7, one teeth unit is made of six teeth14aand such teeth units (U1and U2) are provided two or more in the longitudinal direction of the back yoke14b. In this case, in each of the teeth units U1, U2of equal pitch between teeth14a, the width of each of the end teeth14a-1only needs to be smaller than the width of each of the other teeth14a-2.

The back yoke and the teeth may not be formed into one piece but may be formed as separate members. After the coils are wound on the teeth, the teeth may be connected to the back yoke by fitting.

As the cogging can be reduced by narrowing the width of each of the end teeth, no auxiliary core is required. However, in order to further reduce the cogging, auxiliary cores with no coil wound on may be provided at both ends in the moving direction of the core.

By the magnetic field analysis, the cogging was calculated for various widths of the end tooth. As an analysis model, a core with six teeth was used as illustrated inFIG. 8. The teeth were composed of two U-phase teeth, two V-phase teeth and two W-phase teeth. The width of each end tooth was changed from 8 mm, 8.5 mm, 9.5 mm and to 10.5 mm and the width of each of the other teeth was fixed to 9.5 mm.

As shown inFIG. 9, when the width of each end tooth is narrowed to 8.5 mm, the cogging is most reduced. When the tooth width is 8 mm, the cogging is smaller than that of the width of 9.5 mm, but is larger than that of the width of 8.5 mm. When the tooth width is 9.5 mm or more, the cogging tends to be increased.

FIG. 10illustrates changes in cogging when the armature is moved by 360-degree electrical angle (one magnetic pole pitch of the magnetic field part5). The maximum values of the cogging forces of respective widths are expressed as the cogging forces inFIG. 9. When the width of each end tooth is 8.5 mm, the cogging becomes lowest throughout almost all of the electrical angles.

FIG. 11shows the total cogging of each width inFIG. 10is decomposed into U-phase, V-phase and W-phase cogging forces (magnetic attraction forces). Conversely, when the U-phase, V-phase and W-phase cogging forces (magnetic attraction forces) shown inFIG. 11are added together, the cogging as shown in the graph ofFIG. 10is obtained.FIG. 11shows four graphs corresponding to the widths of end tooth of 8 mm, 8.5 mm, 9.5 mm and 10.5 mm.

InFIG. 11, comparison is made about variations in peak values of the decomposed U-phase, V-phase and W-phase cogging forces (magnetic attraction forces) for respective tooth widths. Variation in peak value is a difference (%) between an average peak value shown by dotted line inFIG. 11and the peak value of each phase. When the tooth width is 8 mm, the V-phase variation is largest to 5.6%, and the W-phase variation is smallest to 3.2%. A difference between V and W phases is 2.48%. When the tooth width is 8.5 mm, a difference between the U phase of largest variation and W phase of smallest variation is 1.91%. When the tooth width is 9.5 mm, a difference between the V phase of largest variation and the U phase of smallest variation is 3.85%. When the tooth width is 10.5 mm, a difference between the V phase of largest variation and the U phase of smallest variation is 11.62%.

As illustrated in the graph ofFIG. 11, when the width of each end tooth is 8.5 mm, the U-phase, V-phase and W-phase cogging forces (magnetic attraction forces) can be made closer to ideal sinusoidal waves of equal peak values and 120-degree different phases. With this, it is expected that the total cogging as the total sum of U-phase, V-phase and W-phase cogging forces (magnetic attraction forces) is reduced.

The magnetic field analysis was performed on another linear motor which was different from the linear motor of the example 1. The linear motor of the example 2 was different from that of the example 1 in vertical and horizontal sizes of each plate-shaped magnet of the magnetic field part, inclined angle and shape of the core of the armature.FIG. 12illustrates the core of the armature used in the magnetic field analysis. The width of each of the central four teeth is set to 10 mm and the width of each end tooth is changed from 8.5 mm, 9 mm, 9.5 mm, 10 mm and to 11 mm.

FIG. 13illustrates the relationship between the width of the end tooth and cogging. When the tooth width is 9 mm, the cogging is lowest. When the tooth width is narrowed to 8.5 mm, the cogging becomes smaller than that of the width of 10 mm, but the cogging is larger than that of the width of 9 mm. The cogging curve is shown like a valley.

FIG. 14illustrates changes in cogging forces when the armature is moved by 360-degree electrical angle. When the width of each end tooth is 9 mm, the cogging can be reduced in a most stable manner throughout the 360-degree electrical angle.

FIG. 15illustrates comparison in variation of peak value of each of U-phase, V-phase and W-phase cogging forces (magnetic attraction forces) for respective tooth widths, the U-phase, V-phase and W-phase cogging forces being obtained by decomposing the cogging of each width inFIG. 14into U-phase, V-phase and W-phase cogging forces (magnetic attraction forces). In this example, when the tooth width is 9.5, the variation in peak value becomes smallest. When not only the peak value but also the whole of the sinusoidal wave are considered, the variation becomes smallest when the tooth width is 9 mm.

The cogging of the linear motor was measured by experiment. The armature is travelled actually, and the generated cogging was measured. In the experiment, the core illustrated inFIG. 16was used. As illustrated inFIG. 16, the width of each of the central four teeth is 9.5 mm. As to the width of each end tooth used, the widths of 9.5 mm and 8.5 mm are employed. As illustrated inFIG. 17, when the width of each end tooth is 8.5 mm, the cogging can be reduced as compared with that when the width is 9.5 mm.

A linear motor other than the linear motor of the example 3 was used and the cogging was measured by experiment. In this linear motor, the number of teeth in the core is much larger than that of the core of the example 3.FIG. 18illustrates the core used in the experiment. The width of each of central sixteen teeth is 9.5 mm. As to the width of each end tooth used, the widths of 9.5 mm and 8.5 mm are employed. As illustrated inFIG. 19, when the width of each end tooth is 8.5 mm, the cogging can be reduced as compared with that when the width is 9.5 mm.

The disclosure of Japanese Patent Application No. 2010-137400, filed on Jun. 16, 2010, including the specification, drawings, and abstract, is incorporated herein by reference in its entirety.

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