Abstract:
A multihead-type coreless linear motor includes a permanent field magnet having P permanent magnets that are arranged such that different magnetic poles appear alternately, and an armature having M armature coils that are closely wounded and connected in three phases. One of the armature and the permanent field magnet is configured as a moving part and the other is configured as a stator. A plurality of moving parts are arranged on the same stator and are driven separately. In this multihead-type coreless linear motor, the plurality of moving parts includes a large-thrust moving part and a small-thrust moving part that are different in relationship with the stator, the relationship being determined by the number of magnetic poles P of the permanent magnets and the number of armature coils M.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Applications No. 2009-189574 filed Aug. 18, 2009 and No. 2010-022597 filed on Feb. 3, 2010. The contents of the application are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to multihead-type coreless linear motors. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No. 2001-211630, published Aug. 3, 2001, discloses an example of a coreless linear motor that is used as a feeder mounted on a table of a glass-substrate conveying apparatus, a semiconductor manufacturing apparatus, a machine tool, or the like. This coreless linear motor is of a multihead type where a plurality of moving parts of the same size are arranged on the same stator, so that the plurality of moving parts can be driven separately. 
     In this multihead-type coreless linear motor, each moving part that constitutes an armature includes a moving part base and a plurality of armature coils that are closely wounded and connected in three phases. The stator that constitutes a field magnet is disposed opposite the armature with a magnetic gap therebetween. The stator includes field yokes and a plurality of permanent magnets that are linearly arranged along the length of the field yokes such that different magnetic poles appear alternately. With this configuration where a plurality of moving parts of the same size are arranged on the same stator, it is possible to move different works placed on the respective moving parts. 
     A configuration of moving parts for a multihead-type coreless linear motor will be described in detail here. For example, a combination of P=4 and M=3 representing a relationship between the number of magnetic poles P of a permanent field magnet and the number of armature coils M is basically regarded as one set. Then, with a configuration where a small-thrust moving part corresponds to one set and a large-thrust moving part corresponds to two sets and has a length twice that of the small-thrust moving part, the length of each moving part can be determined depending on the necessary thrust for each work. In such a multihead-type coreless linear motor where a combination of the number of magnetic poles and the number of armature coils is regarded as one set or a minimum unit, it is possible to accommodate different works by varying the number of sets for each moving part depending on the necessary thrust for the moving part. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the problems described above. An object of the present invention is to provide a multihead-type coreless linear motor that can be minimized in size, even if there is a significant difference in necessary thrust between a plurality of works to be placed on respective moving parts of the linear motor. 
     According to a first aspect of the present invention, a multihead-type coreless linear motor includes a permanent field magnet having P permanent magnets that are linearly arranged such that different magnetic poles appear alternately; and an armature disposed opposite the permanent field magnet with a magnetic gap therebetween, the armature having M armature coils that are closely wounded and connected in three phases. One of the armature and the permanent field magnet is configured as a moving part and the other is configured as a stator. A plurality of moving parts are arranged on the same stator, and are driven separately relative to the stator. The plurality of moving parts include a large-thrust moving part and a small-thrust moving part that are different in relationship with the stator, the relationship being determined by the number of magnetic poles P of the permanent magnets and the number of armature coils M. 
     In the multihead-type coreless linear motor according to the present invention, a plurality of moving parts that are different in the relationship between the number of magnetic poles and the number of coils are arranged on the same stator. Therefore, even if there is a significant difference in necessary thrust between a plurality of works, it is possible to reduce the motor size to a minimum and realize a compact apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described in further detail with reference to the accompanying drawings wherein: 
         FIG. 1A  is a side view of a multihead-type coreless linear motor according to a first embodiment of the present invention; 
         FIG. 1B  is a front view of  FIG. 1A ; 
         FIG. 2A  is a side view of a multihead-type coreless linear motor according to a second embodiment of the present invention; 
         FIG. 2B  is a front view of  FIG. 2A ; 
         FIG. 3  is an enlarged cross-sectional plan view illustrating how a stator and a small-thrust moving part of  FIG. 2A  are arranged; 
         FIG. 4A  is a side view of a multihead-type coreless linear motor according to a third embodiment of the present invention; 
         FIG. 4B  is a front view of  FIG. 4A ; 
         FIG. 5  is an enlarged cross-sectional plan view illustrating how a stator and a small-thrust moving part of  FIG. 4A  are arranged; 
         FIG. 6A  is a side view of a multihead-type coreless linear motor according to a fourth embodiment of the present invention; 
         FIG. 6B  is a front view of  FIG. 6A ; 
         FIG. 7  is an enlarged cross-sectional plan view illustrating how a stator and a small-thrust moving part of  FIG. 6A  are arranged; 
         FIG. 8A  is a side view of a multihead-type coreless linear motor according to a fifth embodiment of the present invention; 
         FIG. 8B  is a front view of  FIG. 8A ; 
         FIG. 9A  is a side view of a multihead-type coreless linear motor according to a sixth embodiment of the present invention; 
         FIG. 9B  is a front view of  FIG. 9A ; 
         FIG. 10A  is a side view of a multihead-type coreless linear motor according to a seventh embodiment of the present invention; 
         FIG. 10B  is a front view of  FIG. 10A ; 
         FIG. 11A  is a side view of a multihead-type coreless linear motor according to an eighth embodiment of the present invention; 
         FIG. 11B  is a front view of  FIG. 11A ; and 
         FIG. 12  shows a comparison table that compares, for each embodiment, motor characteristics and the levels of necessary thrust calculated on the basis of the motor characteristics, between a large-thrust moving part and a small-thrust moving part on which works are to be placed. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the drawings. Note that identical components are given the same reference numerals and their redundant description will be omitted. Linear motors illustrated in the drawings are configured such that a moving part is an armature and a stator is a field magnet. 
     First Embodiment 
       FIG. 1A  is a side view of a multihead-type coreless linear motor according to a first embodiment of the present invention.  FIG. 1B  is a front view of  FIG. 1A . 
     The linear motor illustrated in  FIG. 1A  includes a large-thrust moving part  11   a , a small-thrust moving part  12   b , a large-thrust coil  31   a , and a small-thrust coil  32   b.    
     The first embodiment is different from the related art in that a plurality of moving parts  1  (i.e., the large-thrust moving part  11   a  and the small-thrust moving part  12   b ) that are different in relationship determined by the number of magnetic poles P of a permanent field magnet constituting a stator and the number of armature coils M constituting a moving part are arranged on the same stator  4 . 
     Specifically, in  FIG. 1A , the large-thrust moving part  11   a  corresponds to P:M=4:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   a  is 4τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. The small-thrust moving part  12   b  corresponds to P:M=2:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the small-thrust moving part  12   b  is 2τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   a  and the small-thrust moving part  12   b  having different sizes in the first embodiment. 
       FIG. 12  shows a comparison table that compares, for each embodiment, motor characteristics and the levels of necessary thrust calculated on the basis of the motor characteristics, between a large-thrust moving part and a small-thrust moving part on which works are to be placed. In the table of  FIG. 12 , motor characteristics of a large-thrust moving part are taken as 100% in each embodiment. Also, in each embodiment, coils for the large-thrust moving part and the small-thrust moving part have the same conductor diameter, and if the number of coils for each phase is more than one, these coils are connected in parallel. In the table of  FIG. 12 , a large-thrust moving part is indicated by A, a small-thrust moving part is indicated by B, and a length of a moving part is indicated by L. The motor characteristics include a winding factor indicated by Cw, the number of winding turns indicated by Nw, an inductive voltage indicated by V, a winding resistance indicated by Rw, and a motor constant indicated by Cm. Additionally, a maximum thrust is indicated by P. 
     Generally, when the size of a work (or load) to be placed on each moving part is different depending on the application of the linear motor, the level of necessary thrust required for each moving part on which the work is to be placed is also different. 
     Specifically, in the first embodiment, when there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   a  and small-thrust moving part  12   b  (see  FIG. 1A ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for the large-thrust moving part  11   a  is set to P:M=4:3 and that for the small-thrust moving part  12   b  is set to P:M=2:3, the large-thrust moving part  11   a  and the small-thrust moving part  12   b  being arranged on the same stator  4 . At the same time, the length of the large-thrust moving part  11   a  and that of the small-thrust moving part  12   b  are set to 4τp×N (where N is an integer greater than or equal to 1) and 2τp, respectively. The two moving parts, the large-thrust moving part  11   a  and the small-thrust moving part  12   b  indicated by A and B, respectively, in  FIG. 12 , are thus different in winding factor (Cw) (the ratio of Cw for A to that for B is 100 to 67) and the number of winding turns (Nw) (the ratio of Nw for A to that for B is 100 to 48) of the armature coils. Therefore, the two moving parts are different in inductive voltage constant (V) (the ratio of thrust constant for A to that for B is 100 to 32). Then, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio of Rw for A to that for B is 100 to 44). Moreover, when the two moving parts are different in inductive voltage constant and winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 48). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 23). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   a  and the small-thrust moving part  12   b  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the first embodiment where a large-thrust moving part and a small-thrust moving part that are different in the relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     Second Embodiment 
       FIG. 2A  is a side view of a multihead-type coreless linear motor according to a second embodiment of the present invention.  FIG. 2B  is a front view of  FIG. 2A .  FIG. 3  is an enlarged cross-sectional plan view illustrating how a stator and a small-thrust moving part of  FIG. 2A  are arranged. 
     The linear motor illustrated in  FIG. 2A  includes the large-thrust moving part  11   a , a small-thrust moving part  12   c , the large-thrust coil  31   a , and a small-thrust coil  32   c . Referring to  FIG. 3 , the small-thrust coil  32   c  includes a U-phase coil  32   u , a V-phase coil  32   v , and a W-phase coil  32   w′.    
     In the second embodiment, as illustrated in  FIG. 2A , the large-thrust moving part  11   a  corresponds to P:M=4:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   a  is 4τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. As illustrated in  FIG. 2A  and  FIG. 3 , the small-thrust moving part  12   c  corresponds to P:M=2:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The W-phase coil  32   w ′ is displaced by an electrical angle of 180° to reverse the winding direction of the W-phase coil  32   w ′, so that the small-thrust moving part  12   c  has a length of (4/3)τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   a  and the small-thrust moving part  12   c  having different sizes in the second embodiment. 
     When there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   a  and small-thrust moving part  12   c  (see  FIG. 2A  and  FIG. 3 ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for the large-thrust moving part  11   a  is set to P:M=4:3 and that for the small-thrust moving part  12   c  is set to P:M=2:3, the large-thrust moving part  11   a  and the small-thrust moving part  12   c  being arranged on the same stator  4 . At the same time, the W-phase coil  32   w ′ for the small-thrust moving part  12   c  is displaced by an electrical angle of 180°, and the lengths of the large-thrust moving part  11   a  and the small-thrust moving part  12   c  are set to 4τp×N (where N is an integer greater than or equal to 1) and (4/3)τp, respectively. The two moving parts, the large-thrust moving part  11   a  and the small-thrust moving part  12   c  indicated by A and B, respectively, in  FIG. 12 , are thus different in winding factor (Cw) (the ratio of Cw for A to that for B is 100 to 67) and the number of winding turns (Nw) (the ratio of Nw for A to that for B is 100 to 48) of the armature coils. Therefore, the two moving parts are different in inductive voltage constant (V) (the ratio of thrust constant for A to that for B is 100 to 32). Then, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio of Rw for A to that for B is 100 to 88). Moreover, when the two moving parts are different in inductive voltage constant and winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 34). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 12). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   a  and the small-thrust moving part  12   c  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the second embodiment where a large-thrust moving part and a small-thrust moving part that are different in the relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     Third Embodiment 
       FIG. 4A  is a side view of a multihead-type coreless linear motor according to a third embodiment of the present invention.  FIG. 4B  is a front view of  FIG. 4A .  FIG. 5  is an enlarged cross-sectional plan view illustrating how a stator and a small-thrust moving part of  FIG. 4A  are arranged. 
     The linear motor illustrated in  FIG. 4A  includes the large-thrust moving part  11   a , a small-thrust moving part  12   d , the large-thrust coil  31   a , and a small-thrust coil  32   d . Referring to  FIG. 5 , the small-thrust coil  32   d  includes the U-phase coil  32   u , the V-phase coil  32   v , and a W-phase coil  32   w.    
     In the third embodiment, as illustrated in  FIG. 4A  and  FIG. 5 , the large-thrust moving part  11   a  and the small-thrust moving part  12   d  both correspond to P:M=4:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   a  is 4τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. As for the small-thrust moving part  12   d , the V-phase coil  32   v  is displaced by an electrical angle of 360°, so that the small-thrust moving part  12   d  has a length of (8/3)τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   a  and the small-thrust moving part  12   d  having different sizes in the third embodiment. 
     When there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   a  and small-thrust moving part  12   d  (see  FIG. 4A  and  FIG. 5 ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for both the large-thrust moving part  11   a  and the small-thrust moving part  12   d  arranged on the same stator  4  is set to P:M=4:3. At the same time, the V-phase coil  32   v  for the small-thrust moving part  12   d  is displaced by an electrical angle of 360°, and the lengths of the large-thrust moving part  11   a  and the small-thrust moving part  12   d  are set to 4τp×N (where N is an integer greater than or equal to 1) and (8/3)τp, respectively. Since the two moving parts, the large-thrust moving part  11   a  and the small-thrust moving part  12   d  indicated by A and B, respectively, in  FIG. 12 , are the same in winding factor (Cw) and the number of winding turns (Nw) of the armature coils, there is no difference in inductive voltage constant (V) between the two moving parts. However, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio of Rw for A to that for B is 100 to 200). Moreover, when the two moving parts are different in winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 71). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 50). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   a  and the small-thrust moving part  12   d  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the third embodiment where a large-thrust moving part and a small-thrust moving part that correspond to the same relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     Fourth Embodiment 
       FIG. 6A  is a side view of a multihead-type coreless linear motor according to a fourth embodiment of the present invention.  FIG. 6B  is a front view of  FIG. 6A .  FIG. 7  is an enlarged cross-sectional plan view illustrating how a stator and a small-thrust moving part of  FIG. 6A  are arranged. 
     The linear motor illustrated in  FIG. 6A  includes the large-thrust moving part  11   a , a small-thrust moving part  12   e , the large-thrust coil  31   a , and a small-thrust coil  32   e . Referring to  FIG. 7 , the small-thrust coil  32   e  includes the U-phase coil  32   u , the V-phase coil  32   v , and the W-phase coil  32   w.    
     In the fourth embodiment, as illustrated in  FIG. 6A , the large-thrust moving part  11   a  corresponds to P:M=4:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   a  is 4τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. As illustrated in  FIG. 6A  and  FIG. 7 , the small-thrust moving part  12   e  corresponds to P:M=5:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The W-phase coil  32   w  is displaced by an electrical angle of 360°, so that the small-thrust moving part  12   e  has a length of (10/3)τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   a  and the small-thrust moving part  12   e  having different sizes in the fourth embodiment. 
     When there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   a  and small-thrust moving part  12   e  (see  FIG. 6A  and  FIG. 7 ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for the large-thrust moving part  11   a  is set to P:M=4:3 and that for the small-thrust moving part  12   e  is set to P:M=5:3, the large-thrust moving part  11   a  and the small-thrust moving part  12   e  being arranged on the same stator  4 . At the same time, the W-phase coil  32   w  for the small-thrust moving part  12   e  is displaced by an electrical angle of 360°, and the lengths of the large-thrust moving part  11   a  and the small-thrust moving part  12   e  are set to 4τp×N (where N is an integer greater than or equal to 1) and (10/3)τp, respectively. The two moving parts, the large-thrust moving part  11   a  and the small-thrust moving part  12   e  indicated by A and B, respectively, in  FIG. 12 , are thus different in winding factor (Cw) (the ratio of Cw for A to that for B is 100 to 98) and the number of winding turns (Nw) (the ratio of Nw for A to that for B is 100 to 127) of the armature coils. Therefore, the two moving parts are different in inductive voltage constant (V) (the ratio of thrust constant for A to that for B is 100 to 124). Then, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio of Rw for A to that for B is 100 to 263). Moreover, when the two moving parts are different in inductive voltage constant and winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 77). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 59). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   a  and the small-thrust moving part  12   e  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the fourth embodiment where a large-thrust moving part and a small-thrust moving part that are different in the relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     Fifth Embodiment 
       FIG. 8A  is a side view of a multihead-type coreless linear motor according to a fifth embodiment of the present invention.  FIG. 8B  is a front view of  FIG. 8A . 
     The linear motor illustrated in  FIG. 8A  includes a large-thrust moving part  11   b , the small-thrust moving part  12   b , a large-thrust coil  31   b , and the small-thrust coil  32   b.    
     In the fifth embodiment, as illustrated in  FIG. 8A , the large-thrust moving part  11   b  corresponds to P:M=5:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   b  is 5τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. The small-thrust moving part  12   b  corresponds to P:M=2:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the small-thrust moving part  12   b  is 2τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   b  and the small-thrust moving part  12   b  having different sizes in the fifth embodiment. 
     When there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   b  and small-thrust moving part  12   b  (see  FIG. 8A ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for the large-thrust moving part  11   b  is set to P:M=5:3 and that for the small-thrust moving part  12   b  is set to P:M=2:3, the large-thrust moving part  11   b  and the small-thrust moving part  12   b  being arranged on the same stator  4 . At the same time, the lengths of the large-thrust moving part  11   b  and the small-thrust moving part  12   b  are set to 5τp×N (where N is an integer greater than or equal to 1) and 2τp, respectively. The two moving parts, the large-thrust moving part  11   b  and the small-thrust moving part  12   b  indicated by A and B, respectively, in  FIG. 12 , are thus different in winding factor (Cw) (the ratio Cw for A to that for B is 100 to 68) and the number of winding turns (Nw) (the ratio of Nw for A to that for B is 100 to 38) of the armature coils. Therefore, the two moving parts are different in inductive voltage constant (V) (the ratio of thrust constant for A to that for B is 100 to 26). Then, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio of Rw for A to that for B is 100 to 34). Moreover, when the two moving parts are different in inductive voltage constant and winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 44). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 20). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   b  and the small-thrust moving part  12   b  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the fifth embodiment where a large-thrust moving part and a small-thrust moving part that are different in the relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     Sixth Embodiment 
       FIG. 9A  is a side view of a multihead-type coreless linear motor according to a sixth embodiment of the present invention.  FIG. 9B  is a front view of  FIG. 9A . Note that how a stator and a small-thrust moving part of  FIG. 9A  are arranged is illustrated in  FIG. 3 . 
     The linear motor illustrated in  FIG. 9A  includes the large-thrust moving part  11   b , the small-thrust moving part  12   c , the large-thrust coil  31   b , and the small-thrust coil  32   c.    
     In the sixth embodiment, as illustrated in  FIG. 9A , the large-thrust moving part  11   b  corresponds to P:M=5:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   b  is 5τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. As illustrated in  FIG. 9A  and  FIG. 3 , the small-thrust moving part  12   c  corresponds to P:M=2:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The W-phase coil  32   w ′ is displaced by an electrical angle of 180° to reverse the winding direction of the W-phase coil  32   w ′, so that the small-thrust moving part  12   c  has a length of (4/3)τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   b  and the small-thrust moving part  12   c  having different sizes in the sixth embodiment. 
     When there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   b  and small-thrust moving part  12   c  (see  FIG. 9A  and  FIG. 3 ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for the large-thrust moving part  11   b  is set to P:M=5:3 and that for the small-thrust moving part  12   c  is set to P:M=2:3, the large-thrust moving part  11   b  and the small-thrust moving part  12   c  being arranged on the same stator  4 . At the same time, the W-phase coil  32   w ′ for the small-thrust moving part  12   c  is displaced by an electrical angle of 180°, and the lengths of the large-thrust moving part  11   b  and the small-thrust moving part  12   c  are set to 5τp×N (where N is an integer greater than or equal to 1) and (4/3)τp, respectively. The two moving parts, the large-thrust moving part  11   b  and the small-thrust moving part  12   c  indicated by A and B, respectively, in  FIG. 12 , are thus different in winding factor (Cw) (the ratio of Cw for A to that for B is 100 to 68) and the number of winding turns (Nw) (the ratio of Nw for A to that for B is 100 to 38) of the armature coils. Therefore, the two moving parts are different in inductive voltage constant (V) (the ratio of thrust constant for A to that for B is 100 to 26). Then, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio Rw for A to that for B is 100 to 67). Moreover, when the two moving parts are different in inductive voltage constant and winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 31). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 10). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   b  and the small-thrust moving part  12   c  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the sixth embodiment where a large-thrust moving part and a small-thrust moving part that are different in the relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     Seventh Embodiment 
       FIG. 10A  is a side view of a multihead-type coreless linear motor according to a seventh embodiment of the present invention.  FIG. 10B  is a front view of  FIG. 10A . Note that how a stator and a small-thrust moving part of  FIG. 10A  are arranged is illustrated in  FIG. 5 . 
     The linear motor illustrated in  FIG. 10A  includes the large-thrust moving part  11   b , the small-thrust moving part  12   d , the large-thrust coil  31   b , and the small-thrust coil  32   d.    
     In the seventh embodiment, as illustrated in  FIG. 10A , the large-thrust moving part  11   b  corresponds to P:M=5:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   b  is 5τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. As illustrated in  FIG. 10A  and  FIG. 5 , the small-thrust moving part  12   d  corresponds to P:M=4:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The V-phase coil  32   v  is displaced by an electrical angle of 360°, so that the small-thrust moving part  12   d  has a length of (8/3)τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   b  and the small-thrust moving part  12   d  having different sizes in the seventh embodiment. 
     When there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   b  and small-thrust moving part  12   d  (see  FIG. 10A  and  FIG. 5 ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for the large-thrust moving part  11   b  is set to P:M=5:3 and that for the small-thrust moving part  12   d  is set to P:M=2:3, the large-thrust moving part  11   b  and the small-thrust moving part  12   d  being arranged on the same stator  4 . At the same time, the V-phase coil  32   v  for the small-thrust moving part  12   d  is displaced by an electrical angle of 360°, and the lengths of the large-thrust moving part  11   b  and the small-thrust moving part  12   d  are set to 5τp×N (where N is an integer greater than or equal to 1) and (8/3)τp, respectively. The two moving parts, the large-thrust moving part  11   b  and the small-thrust moving part  12   d  indicated by A and B, respectively, in  FIG. 12 , are thus different in winding factor (Cw) (the ratio of Cw for A to that for B is 100 to 102) and the number of winding turns (Nw) (the ratio of Nw for A to that for B is 100 to 79) of the armature coils. Therefore, the two moving parts are different in inductive voltage constant (V) (the ratio of thrust constant for A to that for B is 100 to 80). Then, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio of Rw for A to that for B is 100 to 152). Moreover, when the two moving parts are different in inductive voltage constant and winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 65). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 43). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   b  and the small-thrust moving part  12   d  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the seventh embodiment where a large-thrust moving part and a small-thrust moving part that are different in the relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     Eighth Embodiment 
       FIG. 11A  is a side view of a multihead-type coreless linear motor according to an eighth embodiment of the present invention.  FIG. 11B  is a front view of  FIG. 11A . Note that how a stator and a small-thrust moving part of  FIG. 11A  are arranged is illustrated in  FIG. 7 . 
     The linear motor illustrated in  FIG. 11A  includes the large-thrust moving part  11   b , the small-thrust moving part  12   e , the large-thrust coil  31   b , and the small-thrust coil  32   e.    
     In the eighth embodiment, as illustrated in  FIG. 11A  and  FIG. 7 , the large-thrust moving part  11   b  and the small-thrust moving part  12   e  both correspond to P:M=5:3 representing a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M. The length of the large-thrust moving part  11   b  is 5τp×N, where τp is a pole pitch and N is an integer greater than or equal to 1. As for the small-thrust moving part  12   e , the W-phase coil  32   w  is displaced by an electrical angle of 360°, so that the small-thrust moving part  12   e  has a length of (10/3)τp. 
     With reference to  FIG. 12 , a description will be given of a concept of necessary thrust for the large-thrust moving part  11   b  and the small-thrust moving part  12   e  having different sizes in the eighth embodiment. 
     When there is a significant difference between the levels of necessary thrust for works to be placed on the respective large-thrust moving part  11   b  and small-thrust moving part  12   e  (see  FIG. 11A  and  FIG. 7 ), a relationship between the number of magnetic poles P of the permanent field magnet and the number of armature coils M for both the large-thrust moving part  11   b  and the small-thrust moving part  12   e  arranged on the same stator  4  is set to P:M=5:3. At the same time, the W-phase coil  32   w  for the small-thrust moving part  12   e  is displaced by an electrical angle of 360°, and the lengths of the large-thrust moving part  11   b  and the small-thrust moving part  12   e  are set to 5τp×N (where N is an integer greater than or equal to 1) and (10/3)τp, respectively. Since the two moving parts, the large-thrust moving part  11   b  and the small-thrust moving part  12   e  indicated by A and B, respectively, in  FIG. 12 , are the same in winding factor (Cw) and the number of winding turns (Nw) of the armature coils, there is no difference in inductive voltage constant (V) between the two moving parts. However, since winding spaces of the armature coils for the two moving parts are different, the two moving parts are different in winding resistance (Rw) (the ratio of Rw for A to that for B is 100 to 200). Moreover, when the two moving parts are different in winding resistance, the two moving parts are also different in motor constant (Cm) (the ratio of Cm for A to that for B is 100 to 71). By calculating the ratio of the levels of thrust for the two moving parts from the duplicate ratio of the motor constants for the two moving parts, the levels of necessary thrust (P) for the two moving parts can be obtained (the ratio of P for A to that for B is 100 to 50). Thus, it is possible to design the linear motor such that the large-thrust moving part  11   b  and the small-thrust moving part  12   e  can be adjusted to lengths (or sizes) that are most appropriate for the sizes of works (or loads) to be placed on the respective moving parts (i.e., most appropriate for the difference between the levels of necessary thrust required for the respective moving parts). 
     As described above, in the eighth embodiment where a large-thrust moving part and a small-thrust moving part that correspond to the same relationship between the number of magnetic poles and the number of coils are arranged on the same stator, even if there is a significant difference between the levels of necessary thrust for works to be placed on the respective moving parts, it is possible to reduce the motor size to a minimum. 
     An armature and a field magnet are configured as a moving part and a stator, respectively, in the embodiments described above. However, an armature and a field magnet may be configured as a stator and a moving part, respectively.