Linear stepper motor

Abstract

Linear stepper motor that includes: a field magnet (1) with north poles and south poles which are alternately magnetized in the axial direction; an armature (2) having at least two phase coils (4a, 4b) surrounding the field magnet (1) and an inner core (8) made of a magnetic material and arranged in the coils (4a, 4b), a gap being present between the field magnet (1) and the inner core (8); and a control unit for exciting the coils (4a, 4b) and for switching the coils (4a, 4b) to be excited, wherein an attraction and / or repulsion between magnetic poles of the field magnet (1) and magnetic poles generated at both ends of the excited coils (4a, 4b) in the axial direction is used to move the field magnet (1) rectilinearly relative to the armature (2) by a predetermined step distance, and the inner core (8) comprises divided inner cores (8a, 8b) whose number is equal to that of the coils (4a, 4b), and the divided inner cores (8a, 8b) are arranged in the respective coils (4a, 4b).

Description

TECHNICAL FIELD

[0001] The present invention relates to a linear stepping motor capable of moving a field magnet relative to an armature linearly by a predetermined step distance by switching an excitation current of at least two phase coils of the armature. STATE OF THE ART

[0002] A stepper motor is a motor that rotates a given angle in proportion to a given number of pulses. Since the position and speed are determined by only one command pulse, feedback-free control of the rotor's position and speed is possible.

[0003] A stepper motor drive circuit is configured to supply power from a DC power source to each coil of the stepper motor and to sequentially switch the coils to be energized each time it receives a command pulse. Each time the coils to be energized are switched, the rotor rotates a specific angle of one step. A linear stepper motor directly moves a moving device in a straight line, while in a rotary stepper motor, the rotary motion of a rotor is converted into a straight line motion via motion conversion means such as a rack and pinion mechanism.

[0004] Commonly used stepper motors include VR (variable reductance) linear motors, which do not use a permanent magnet, and HB (hybrid) linear motors, which combine a permanent magnet and an electromagnet. Generally, HB linear stepper motors, which can increase thrust, are widely used (see, for example, Patent Document 1).

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. JP S61-173660 A

[0006] JP 2005-204 449 A also discloses a linear motor. DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] HB linear stepper motors have the advantage of reducing the step distance and increasing thrust by forming a large number of comb teeth in the motor and stator. However, manufacturing is difficult because the comb teeth must be formed with a complex shape and the distance between the comb teeth of the stator and the motor must be maintained constant in the assembled state.

[0008] Then, the present invention has an object to provide a linear stepping motor and a method for manufacturing the linear stepping motor, wherein the linear stepping motor has a simple structure and can generate a high thrust. MEANS TO SOLVE THE PROBLEMS

[0009] To solve the above-mentioned problems, the invention according to claim 1 is a linear stepping motor comprising: a field magnet having north poles and south poles magnetized alternately in the axial direction; an armature having at least two phase coils surrounding the field magnet and an inner core made of a magnetic material and disposed in the coils, wherein a gap is provided between the field magnet and the inner core;and a control unit for energizing the coils and switching the coils to be energized, wherein attraction and / or repulsion between magnetic poles of the field magnet and magnetic poles generated at both ends of the energized coils in the axial direction is used to move the field magnet rectilinearly relative to the armature by a predetermined pitch, wherein the inner core comprises divided inner cores equal in number to that of the coils, and the divided inner cores are arranged in the respective coils.;

[0010] The invention according to claim 2 is characterized in that in the linear stepping motor according to claim 2, a length of each of the divided inner cores in the axial direction is equal to or slightly larger than a length of each of the coils in the axial direction.

[0011] The invention according to claim 3 is characterized in that in the linear stepping motor according to any one of claims 1 to 3, a non-magnetic material is provided at each end of each of the coils in the axial direction.

[0012] The invention according to claim 4 is characterized in that in the linear stepping motor according to any one of claims 1 to 4, the armature has a yoke made of a magnetic material covering the coils, a bushing made of a non-magnetic material provided at each end of a group of the coils to guide the rectilinear movement of the field magnet relative to the armature, and a spacer made of a non-magnetic material provided between the coils to shift phases of the coils.

[0013] The invention according to claim 5 is characterized in that in the linear stepping motor according to any one of claims 1 to 5, the field magnet comprises a unit magnet having individual magnets having a north pole and a south pole magnetized in the axial direction and arranged such that the north poles face each other and the south poles face each other, wherein paired end surfaces of each of the individual magnets where the north pole and the south pole are magnetized are parallel to each other and inclined relative to a plane perpendicular to the axial direction.

[0014] The invention according to claim 6 is characterized in that in the linear stepping motor according to claim 6, the coils are two-phase coils and the paired end faces are inclined relative to the plane perpendicular to the axial direction by an angle θ calculated by the following expression: θ=tan−1(P / 2R) where P is a magnetic pole pitch between the north pole and the south pole and R is a diameter of the single magnet.

[0015] The invention according to claim 7 is a method for manufacturing a linear stepping motor capable of moving a field magnet rectilinearly relative to an armature by a predetermined step distance using attraction and / or repulsion between magnetic poles of the field magnet and magnetic poles generated at both ends in the axial direction of each of at least two phase coils of the armature by energizing the coils and switching the coils to be energized, the method comprising: an inner core insertion step of inserting tubular split inner cores made of magnetic material into an interior space of the respective coils; a coil insertion step of inserting the coils into a tubular yoke; and a magnet insertion step of inserting the field magnet into an interior space of the inner core, the field magnet having north poles and south poles magnetized alternately in the axial direction.

[0016] The invention according to claim 8 is characterized in that in the method according to claim 8, in the coil inserting step, a bushing made of a non-magnetic material is arranged at each end of a group of the coils in the axial direction to guide a rectilinear movement of the field magnet relative to the armature, and a spacer made of a non-magnetic material is arranged between every two of the coils to shift phases of the coils. EFFECTS OF THE INVENTION

[0017] To obtain a linear stepping motor that generates high thrust, the magnetic poles on both sides of each coil must be strengthened in view of the working principles of the linear stepping motor. According to the invention as defined in claim 1, since the inner core made of magnetic material is arranged in the coils, the magnetic resistance in the coils is reduced, and the magnetic flux density at both ends of the coils is increased. Therefore, the obtained linear stepping motor can generate high thrust. In addition, since the inner core is arranged in the coils, it becomes possible to prevent the occurrence of a displacement force due to the core and, furthermore, to prevent the magnetic poles generated at both ends of the coils from being influenced by the magnetic poles arranged at both ends of adjacent coils, as is the case when toroidal cores are arranged at both ends of the coils.

[0018] According to the invention of claim 1, since the split inner cores are arranged in the respective coils, the magnetic poles generated at both ends of each coil can be prevented from being influenced by the magnetic poles generated at both ends of an adjacent coil. When a single inner core is provided for the multiple coils, it is difficult to cause strong magnetic poles to appear at both ends of each coil.

[0019] According to the invention of claim 2, since the magnetic poles are formed at both ends of the inner core, which is a machined part, it becomes possible to precisely position the magnetic poles of the coils and the magnetic poles of the field magnet. Since there is no need to control the axial length of the coils with high accuracy, winding the coils is facilitated.

[0020] According to the invention of claim 3, it becomes possible to prevent the magnetic poles generated at both ends of the coils from being influenced by the magnetic poles generated at both ends of adjacent coils.

[0021] According to the invention of claim 4, it becomes possible to use the non-magnetic materials at both ends of the coils for bushings and a spacer. Furthermore, since the coils are covered by the yoke made of magnetic material, the magnetic resistance outside the coils becomes small and the magnetic flux density at both ends of the coils is further increased.

[0022] According to the invention of claim 5, since the end faces of the individual magnets are inclined, it becomes possible to reduce the displacement force exerted between the inner core and the field magnet. Since the coils are configured to surround the field magnet, the reduction in thrust due to the inclination of the end faces of the magnets is minimal.

[0023] According to the invention of claim 6, it is possible to minimize the displacement force occurring between the inner core and the field magnet.

[0024] According to the invention of claim 7, since the inner core made of magnetic material is disposed in the coils, the magnetic resistance in the coils is reduced, and the magnetic flux density at both ends of the coils is increased. Therefore, the resulting linear stepping motor can generate high thrust. Furthermore, since the inner core is disposed in the coils, it becomes possible to prevent the occurrence of a displacement force caused by the core and, furthermore, to prevent the magnetic poles generated at both ends of the coils from being influenced by the magnetic poles generated at both ends of adjacent coils, as would be the case if toroidal cores were disposed at both ends of the coils.

[0025] According to the invention of claim 8, it becomes possible to prevent the magnetic poles generated at both ends of the coils from being influenced by the magnetic poles generated at both ends of adjacent coils. Furthermore, it is possible to use the non-magnetic materials at both ends of the coils for bushings and a spacer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a linear stepping motor according to a first embodiment of the present invention. Fig. 2 is a cross-sectional view of the linear stepper motor along the axial line. Fig. 3 is a detailed view of a rod Fig. 4 is a side view of a single magnet. Fig. 5 is a plan view of a force application device. Fig. 6 is a cross-sectional view of the force applying device along the axial direction. Fig. 7(a), Fig. 7(b) are views each illustrating a yoke ( Fig. 7(a) is a plan view and Fig. 7(b) is a side view thereof). Fig. Figure 8 is a cross-sectional view of a coil. Fig. 9 is a circuit diagram of coils. Fig. 10(a), Fig. 10(b) are views each illustrating a split inner core ( Fig. 10(a) is a plan view and Fig. 10(b) is a side view thereof. Fig. 11(a), Fig. 11(b) are views each illustrating a socket ( Fig. 11(a) is a plan view and Fig. 11(b) is a side view thereof. Fig. 12(a), Fig. 12(b) are views each illustrating a spacer ( Fig. 12(a) is a plan view and Fig. 12(b) is a side view thereof). Fig. 13 is a view illustrating an example of a coil excitation system. Fig. Figure 14 is a view illustrating the motion principles of the linear stepper motor. Fig. 15 is a graph of an analysis result of a displacement force. Fig. Figure 16 is a graph of an induced voltage induced in the coil. Fig. Figure 17 is a graph illustrating the principles for reducing a displacement force. Fig. 18(a), Fig. 18(b) are views illustrating the split inner cores with poles and the split inner cores without poles ( Fig. 18(a) is a view of the split inner cores with poles and Fig. 18(b) is a view of the split inner cores without poles). Fig. 19(a), Fig. 19(b) are graphs illustrating comparison results of back EMF constants of the coils between the case with poles and the case without poles ( Fig. 19(a) is a view for the case with Poland and Fig. 19(b) is a view for the case without poles). Fig. 20 is a cross-sectional view illustrating a linear stepping motor according to a second embodiment of the present invention. REFERENCE SYMBOL 1 rod (field magnet) 2 force application device (anchor) 4 two-phase coil 4a, 4b Coil of individual phases 5 single magnet 5a, 5b Front face of a magnet 8 inner core 8a, 8b split inner core 9 yoke 11 socket 12 spacers BEST MODE FOR CARRYING OUT THE INVENTION

[0026] Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Fig. Figure 1 illustrates a linear stepper motor according to a first embodiment of the present invention. The linear stepper motor has an elongated rod 1 and a circular force-applying device 2 surrounding the rod 1. The rod 1 serves as a field magnet of the linear stepper motor, while the force-applying device 2 serves as an armature. The rod 1 moves linearly in the axial direction relative to the force-applying device 2. Either the rod 1 or the force-applying device 2 is fixed, while the other moves.

[0027] Fig. Figure 2 is a cross-sectional view of the linear stepper motor. In the rod 1, the north pole and the south pole are alternately magnetized in the axial direction. The force applying device 2 has a two-phase coil 4 wound around the rod 1 so as to create a gap therebetween. The two-phase coil 4 has a pair of coils 4a and 4b with corresponding phases arranged in alignment in the axial direction. By energizing the coils 4a and 4b, the north pole and the south pole at both ends of the coils 4a and 4b are magnetized. The attraction and / or repulsion between the magnetic poles of the rod 1 and the magnetic poles at both ends of the coils 4a and 4b results in thrust, thereby moving the rod 1 linearly relative to the force applying device 2. Then, by switching the excitation current of the coils 4a and 4b, the rod 1 is moved linearly relative to the force applying device 2 by a predetermined step.

[0028] Fig. 3 is a detailed view of the rod 1. The rod 1 has a plurality of magnets 5 inserted into a tubular tube 3. The tubular tube 3 is made of metal such as stainless steel or resin. A single magnet 5 is a rare earth magnet such as a neodymium magnet, which has a high coercive force. The single magnet 5 has a round shape when viewed from the front and a parallelogram shape when viewed from the side. The single magnet 5 is a bonded magnet (e.g., a plastic magnet) and is manufactured by injection molding a composite material of magnetic powder and resin. In the single magnets, the north pole and the south pole are magnetized in the axial direction. That is, the north pole is magnetized in the axial direction on one end face 5a of the single magnet 5, while the south pole is magnetized on the other end face 5b.The individual magnets 5 are arranged such that the north pole faces the north pole and the south pole faces the south pole. With this arrangement, the magnets 5 form a unit magnet. In the unit magnet, the magnetic poles of the north pole and the south pole are arranged alternately in the axial direction and at a predetermined pitch.

[0029] Fig. 4 is a side view of the single magnet 5. In the single magnet 5, a pair of end surfaces 5a and 5b, on which the north pole and the south pole are magnetized, are parallel to each other, and these end surfaces 5a and 5b are inclined relative to a surface 6 perpendicular to the axial direction. In this embodiment, an inclination angle θ of the pair of end surfaces 5a and 5b is substantially equal to θ = tan -1(P / 2R). Where P is a magnetic pole pitch between the north pole and the south pole, and R is a diameter of the individual magnet. The end faces 5a and 5b of the individual magnet 5 are inclined to reduce a displacement force acting between an inner core (see Fig. 2) and the individual magnets 5. The relationship between the displacement force and the inclination angle θ of the end surfaces 5a and 5b will be described later. Since the individual magnets 5 are bonded magnets formed by injection molding, such an inclination of the end surfaces 5a and 5b can be easily achieved.

[0030] As in Fig. 3, after the plurality of magnets 5 are inserted into the pipe 3, end plugs 7 are used to close both ends of the pipe 3. An exposed surface 7a of each end plug 7 is perpendicular to the axial direction of the rod 1. An end surface 7b on the back of the end plug 7 is inclined corresponding to the end surfaces 5a of the magnet 5. The end plug 7 is fixed to the pipe 3, for example, by connecting means such as screwing or adhesive. A screw 7c is formed in the end plug 7 for attaching a moving body to be moved rectilinearly. The cross-sectional shape of the rod 1 need not be round, but may be flat elliptical or polygonal such as rectangular.

[0031] Fig. 5 is a plan view of the force application device 2, while Fig. 6 is a cross-sectional view of the force applying device 2. In a tubular yoke 9 made of a magnetic material, the two-phase coil 4 is arranged with coils in line in the axial direction. Tubular split inner cores 8a and 8b made of a magnetic material are arranged in the coils 4a and 4b. At each end of the two-phase coil 4 in the axial direction, a bushing 11 made of resin (non-magnetic material) is provided to guide the rectilinear movement of the rod 1 relative to the force applying device 2. A spacer 12 is provided between the coils as a non-magnetic member to shift the phase of each coil.

[0032] Fig. 7 illustrates the yoke. The yoke 9 is made of a magnetic material such as silicon steel and is tubular in shape. At each end of the yoke 9 in the axial direction, a claw 9a is formed, which can be deformed by folding. Such a claw 9a is provided in plural numbers in a circumferential direction. The bushings 11, the two-phase coil 4, and the spacer 12 are inserted into the yoke 9, and the claws 9a are folded over to engage with the bushings 11, so that the bushings 11 are fixed to the yoke 9. The two-phase coil 4 and the spacer 12 are fixed in position by being inserted between the bushings 11.

[0033] Fig. Figure 8 illustrates each coil 4a, 4b. The coil 4a, 4b is a wire coated with an insulator and wound helically. Lead terminals 13 are provided at each end of the coil 4a, 4b, forming a starting point and an end point of the copper wire. In this embodiment, the two-phase coil 4 consists of two coils 4a and 4b, but it may consist of four or six coils.

[0034] Fig. Figure 9 is a circuit diagram of the two-phase coil 4. The two-phase coil 4 consists of an A-phase coil 4a and a B-phase coil 4b. When the current flowing through the A-phase coil 4a is inverted, the coil is in the -A phase, and when the current flowing through the B-phase coil 4b is inverted, the coil is in the -B phase.

[0035] Fig. 10 illustrates a split inner core 8a, 8b. The split inner core 8a, 8b is made of a magnetic material such as silicon steel and has a tubular shape. The axial length of the split inner core 8a, 8b is equal to or slightly longer than the axial length of each coil 4a, 4b. The inner diameter of the split inner core 8a, 8b is larger than the outer diameter of the rod 1, creating a gap between the split inner core 8a, 8b and the rod 1.

[0036] As in Fig. As shown in Figure 2, the axial length L1 of the split inner core 8a, 8b (magnetic pole pitch between two ends of the coil 4a, 4b) is set substantially equal to (2N + 1) times the magnetic pole pitch L2 between the north pole and the south pole of the rod 1 (N: positive integer). In other words, when one end of the coil 4a, 4b is located at the north pole of the rod 1, the other end is located at the south pole of the rod 1.

[0037] Fig. 11 illustrates a bushing 11. The bushing 11 is formed as a ring. Since the rod 11 slides on the inner surface of the bushing 11, the bushing 11 is manufactured by injection molding a resin with low friction resistance. The bushing 11 acts as a seal, preventing iron powder adhering to the rod 1 from entering the force application device 2. A recess 11a is formed in the bushing 11, which engages the claw 9a.

[0038] Fig. Figure 12 illustrates a spacer 12. The spacer 12 is also formed into a ring. The spacer 12 is provided to maintain a constant distance between the coils 4a and 4b. The length L3 of the spacer 12 in the axial direction is determined such that the phase of the two-phase coil 4 is shifted by 90 electrical degrees. As shown in Fig. 2, in this embodiment, it is set to three-quarters of the magnetic pole pitch L4 between the north poles of the rod 1. To keep the spacer 12 out of contact with the rod 1 moving in the axial direction, the inner diameter of the spacer 12 is set larger than the inner diameter of the bushing 11.

[0039] The force application device 2 is manufactured through the following steps. First, the split inner cores 8a and 8b are inserted into the coils 4a and 4b, respectively. Next, the bushing 11, the coil 4a, the spacer 12, the coil 4b, and the bushing 11 are inserted into the yoke 9 in this order. Then, the claws 9a of the yoke 9 are crimped to fix the bushings 11 to the yoke 9. Through this process, the force application device 2 is assembled. Next, the rod 1 is inserted into the force application device 2. The insertion of the rod 1 is guided by the bushings 11.

[0040] Fig. 13 illustrates an example of an excitation system of the two-phase coil 4 by a control unit. In Fig. 13, the excitation system includes a phase in which current flows in only one phase. In the first step, the A-phase coil 4a is energized, and in the next step, the B-phase coil 4b is energized. In the following step, the current flows in the opposite direction through the A-phase coil 4a (-A phase), and in the next step, the current flows in the opposite direction through the B-phase coil (-B phase). The control unit repeats these steps every time it receives a command pulse. Steps 1 to 4 form a period in which the rod 1 moves the magnetic pitch between the north poles. Instead of the single-phase excitation system, a two-phase excitation system can be adopted in which current flows through the two phases of the A and B phases. Furthermore, the two-phase coil 4 can be excited by the unipolar system or the bipolar system.

[0041] Fig. Figure 14 is a view for explaining the motion principles of the linear stepping motor. The pitch between two ends 10 of the coil 4a, 4b is (2N + 1) times longer than the magnetic pole pitch between the north and south poles of the rod 1 (N: positive integer), and the two ends 10 as a pair necessarily face the north pole and the south pole of the rod 1, respectively. First, in (1), when the A-phase coil 4a is energized, the rod 1 faces the two ends 10 of the A-phase coil 4a with the north pole facing the south pole. In this state, when an external force is added to move the rod 1, a force tending to return the rod 1 to the position in (1) is exerted, whereby the rod 1 can be positioned. Then, the center of the two ends 10 of the B-phase coil 4b is positioned at the boundary between the north and south poles of the rod 1. An A-phase coil 4a is shifted by 90 electrical degrees from the B-phase coil 4b.

[0042] Then, in (2), when the current of the A-phase coil 4a is turned off and the current of the B-phase coil 4b is turned on, the rod 1 moves in the rightward direction by one-quarter of the magnetic pole pitch between the north poles (ie, by one step), is attracted by the B-phase coil 4b and stopped.

[0043] Next, in (3), a current is passed through the A-phase coil 4a in the opposite direction to (1). Rod 1 moves one step to the right, is attracted by the A-phase coil 4a, and stops.

[0044] Then, in (4), a current is passed through the B-phase coil 4b in the opposite direction to (2). The polarity of each end 10 of the B-phase coil is opposite to that in (2), and the rod 1 moves one step in the right direction and is stopped.

[0045] Then, a return to (1) occurs, repeating steps (2) to (4). At each transition to the next step from (1) to (4), rod 1 moves one step. The process described up to this point represents the motion principles of the linear stepper motor.

[0046] To obtain a linear stepper motor that generates high thrust, it is necessary to strengthen the magnetic poles of both ends 10 of the coils 4a and 4b in accordance with the operating principles of the linear stepper motor. By utilizing the split inner cores 8a and 8b of the magnetic material arranged in the coils 4a and 4b, respectively, the magnetic resistance in the coils 4a and 4b is reduced and the magnetic flux density at both ends 10 of the coils 4a and 4b is further increased. Therefore, a high-thrust linear stepper motor can be obtained. In addition, since the divided inner cores 8a and 8b are arranged in the coils 4a and 4b, it becomes possible to prevent the magnetic poles at both ends of each of the coils 4a and 4b from being influenced by the magnetic poles generated at both ends of adjacent coils, as when toroidal cores are arranged at both ends of the coils 4a and 4b, and also to prevent the occurrence of a displacement force due to the toroidal core.

[0047] The reduced size of the coil 4 does not change between the case where the inner core 8 is arranged inside the coil 4 and the case where the toroidal core is arranged at both ends of the coil. This is because a gap in the coil 4 reduced by the inner core 8 is almost equal to a gap at both ends of the coil 4 reduced by the toroidal core. Regardless of a small change in the size of the coil 4, it has been experimentally determined that the thrust of the motor with the inner core 8 arranged is three times that of the motor with the toroidal core arranged.

[0048] When the inner core 8 is inserted into the coil 4, a displacement force is generated between the inner core 8 and the rod 1. This displacement force can be suppressed by tilting (skewing) the end faces 5a and 5b of the individual magnets 5 of the rod 1.

[0049] Fig. Figure 15 illustrates an analysis result of a displacement force generated when the rod 1 is moved rectilinearly relative to the force applying device 2. When the rod 1 is moved rectilinearly, the displacement force is generated in the rod 1 even if no current is passed through the two-phase coil 4 of the force applying device 2. Fig. Figure 15 shows the offset force generated when rod 1 is moved in a straight line at a fixed speed. The horizontal axis indicates the position of rod 1, while the vertical axis indicates the offset force. Since the offset force is a force that prevents the motor thrust, the offset force must be suppressed. As shown in Fig. As shown in Figure 15, it is possible to reduce the variation of the offset force by inclining the end face of the single motor by 12 or 20 degrees rather than when the end face is not inclined (normal).

[0050] Fig. Figure 16 illustrates an induced voltage induced by the coil 4. When the rod 1 is moved linearly relative to the force applying device 2, the induced voltage is generated in the coil 4. Assuming that the motor is an electric generator, the induced voltage represents the strength of the motor. As can be seen from Fig. As can be seen from Figure 16, the thrust of the motor is hardly reduced when the end faces 5a and 5b of each magnet 5 are inclined by 12 or 20 degrees. Unlike the flat linear motor with a plate-shaped field magnet facing a coil, this linear motor is structured so that the bar-shaped field magnet is covered by the coil 4. This structure is believed to prevent the reduction of thrust.

[0051] Fig. Figure 17 is a graph illustrating the principles of reducing a displacement force. The horizontal axis indicates the position of the rod 1, while the vertical axis indicates the displacement force. The displacement force C1 generated by the split inner core 8a and the displacement force C2 generated by the split inner core 8b have peaks that are shifted from each other. Then, a total displacement force C3, which is a sum of displacement forces of the split inner cores 8a and 8b, has a total of four peaks because the displacement force C1 and the displacement force C2 cancel each other out. When the end surfaces 5a and 5b of the individual magnets 5 are tilted, the peak of the displacement force C1 generated by the split inner core 8a is shifted to the left, and the peak of the displacement force C2 generated by the split inner core 8b is shifted to the right.Since they are shifted so that the offset forces C1 and C2 balance each other, the total offset force C3 can be further reduced.

[0052] As a result of the analysis, it has been found that the displacement force can be completely minimized by inclining the end faces 5a and 5b of the individual magnets 5 so that the following expression is satisfied: θ=tan−1(P / 2R) where P is a magnetic pole pitch between north and south poles and R is a diameter of a single magnet.

[0053] Instead of the individual magnets 5, the coils 4 can be inclined, but it is difficult to incline the coil 4 during manufacture.

[0054] Here, in the case of a permanent magnet synchronous motor, the north and south poles of the field magnet are magnetized in a radial direction, and the magnetic flux density distribution of the field magnet becomes trapezoidal rather than a sine wave. When the magnetic flux density distribution deviates from a sine wave, shear ripples may occur. Therefore, to prevent the occurrence of shear ripples, the end surfaces of each magnet are tilted to approximate the magnetic flux density distribution of the field magnet to a sine wave. Meanwhile, in the case of the linear stepping motor of the present invention, the excitation current of the coils 4a and 4b is stepped, so it is not necessary to approximate the magnetic flux density distribution of the field magnet to a sine wave.The end surfaces 5a and 5b of the magnets 5 are inclined to reduce the displacement force generated by inserting the core, but not to make the magnetic flux density distribution a sine wave as in the permanent magnet synchronous motor.

[0055] The Fig. 18(a) and Fig. 18(b) illustrates the comparison between the split inner cores 8a and 8b having poles made of magnetic material provided at both ends thereof in the axial direction and the split inner cores 8a and 8b when no poles are provided thereon. Fig. 18(a) is an example with Poland and Fig. 18(b) is an example without poles. The Fig. 19(a) and Fig. 19(b) illustrates comparison results of the back EMF constants of coil 4 between the case with poles and the case without poles. Fig. 19(a) is a graph with pole and Fig. Figure 19(b) is a graph without poles. When poles are provided, the thrust per current is increased by about 4%. However, the provision of poles causes problems in reducing the number of turns of coil 4 and the influence of the magnetic poles of adjacent split inner cores 8a and 8b (the voltages of the coils of the two-phase coil 4 with a phase difference of 90 degrees approach an in-phase state). Therefore, it is preferable to provide no poles.

[0056] Fig.20 illustrates a linear stepping motor according to a second embodiment of the present invention. The structure of the force applying device 2 is the same as that in Embodiment 1 described above. In this embodiment, for example, the end faces of each individual magnet 5 held in the rod 1 are not inclined. The end faces of each individual magnet 5 need not be inclined if the offset force exerted between the split cores 8a and 8b and the rod during motor operation is not problematic.

[0057] The present invention is not limited to the above-mentioned embodiments, but can be embodied in many different forms without departing from the scope of the invention. For example, the coil can be a three-phase coil or a five-phase coil. The coil excitation system can be a microstep drive, in which a full step distance can be divided by n. Furthermore, the force application device can move instead of the rod.

[0058] The present invention is based on Japanese Patent Application No. 2007-340465, filed on December 28, 2007, the contents of which are incorporated herein by reference.

Claims

[1] Linear stepper motor comprising: a field magnet (1) with north poles and south poles which are alternately magnetized in the axial direction; an armature (2) having at least two phase coils (4a, 4b) surrounding the field magnet (1) and an inner core (8) made of a magnetic material and arranged in the coils (4a, 4b), a gap being present between the field magnet (1) and the inner core (8); and a control unit for exciting the coils (4a, 4b) and for switching the coils (4a, 4b) to be excited, wherein an attraction and / or repulsion between magnetic poles of the field magnet (1) and magnetic poles generated at both ends of the excited coils (4a, 4b) in the axial direction is used to move the field magnet (1) rectilinearly relative to the armature (2) by a predetermined step distance, and the inner core (8) comprises divided inner cores (8a, 8b) whose number is equal to that of the coils (4a, 4b), and the divided inner cores (8a, 8b) are arranged in the respective coils (4a, 4b). [2] A linear stepping motor according to claim 1, wherein a length of each of said divided inner cores (8a, 8b) in the axial direction is equal to or slightly longer than a length of each of said coils (4a, 4b) in the axial direction. [3] A linear stepping motor according to claim 1 or 2, wherein a non-magnetic material is provided at each end of each of the coils (4a, 4b) in the axial direction. [4] A linear stepping motor according to any one of claims 1 to 3, wherein the armature (2) has a yoke (9) made of a magnetic material covering the coils (4a, 4b), a bushing (11) made of a non-magnetic material provided at each end of a group of the coils (4a, 4b) to guide the rectilinear movement of the field magnet (1) relative to the armature (2), and a spacer (12) made of a non-magnetic material provided between the coils (4a, 4b) to shift phases of the coils (4a, 4b). [5] A linear stepping motor according to any one of claims 1 to 4, wherein the field magnet (1) comprises a unit magnet having individual magnets (5) having a north pole and a south pole magnetized in the axial direction and arranged such that the north poles face each other and the south poles face each other, wherein paired end surfaces (5a, 5b) of each of the individual magnets (5) where the north pole and the south pole are magnetized are parallel to each other and inclined relative to a plane perpendicular to the axial direction. [6] A linear stepping motor according to claim 5, wherein the coils (4a, 4b) are two-phase coils (4a, 4b) and the paired end surfaces (5a, 5b) are inclined relative to the plane perpendicular to the axial direction by an angle θ calculated by the following expression: θ=tan−1(P / 2R) where P is a magnetic pole pitch between the north pole and the south pole and R is a diameter of the single magnet (5). [7] A method for manufacturing a linear stepping motor capable of moving a field magnet (1) rectilinearly relative to an armature (2) by a predetermined step distance using an attraction and / or repulsion between magnetic poles of the field magnet (1) and magnetic poles generated at both ends in the axial direction of each of the at least two phase coils (4a, 4b) of the armature (2) by energizing the coils (4a, 4b) and switching the coils (4a, 4b) to be energized, the method comprising: an inner core insertion step of inserting tubular split inner cores (8a, 8b) made of magnetic material into an inner space of the respective coils (4a, 4b); a coil insertion step for inserting the coils (4a, 4b) into a tubular yoke (9); and a magnet insertion step for inserting the field magnet (1) into an interior space of the inner core (8), the field magnet (1) having north poles and south poles that are alternately magnetized in the axial direction. [8] A method according to claim 7, wherein in the coil inserting step, a bushing (11) made of a non-magnetic material is arranged at each end of a group of the coils (4a, 4b) in the axial direction to guide a rectilinear movement of the field magnet (1) relative to the armature (2), and a spacer (12) made of a non-magnetic material is arranged between every two of the coils (4a, 4b) to shift phases of the coils (4a, 4b).

Images