Abstract:
An ironless AC linear motor for generating a thrust such that a carrier can move relative to a stator along a movement axis, includes parallel rows of permanent magnets arranged along the movement axis with a given magnet pitch to form a magnetic gap, a row of ironless coils having through holes and a manifold connected to the cooling tubes for distributing the coolant having a length equal to the magnet pitch in the direction of the movement axis.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a linear motor and, more specifically, the present invention relates to an ironless AC linear motor having cooling tubes for cooling ironless coils.  
       BACKGROUND OF THE INVENTION  
       [0002]     An ironless linear motor is preferable for machining tools and semiconductor manufacturing devices which need precise positioning since it has no cogging caused by an iron core. Japanese Patent Laid-Open No. 10-23735 discloses an ironless AC multi-phase linear motor having cooling tubes for cooling ironless coils. A carrier of this linear motor includes permanent magnets which are provided on a magnet plate so as to form a magnetic gap, and a stator includes U-phase, V-phase and W-phase ironless coils arranged in a row in the movement direction. Two copper round pipes as cooling tubes pass through holes of the ironless coils. The two round pipes are remote from each other sandwiching the magnetic gap such that a magnetic flux generated by permanent magnets does not penetrate the two round pipes. Therefore, eddy current, which is generated when an electrically conductive body crosses the magnetic field, is prevented and viscous resistance is not generated.  
         [0003]     Japanese patent Laid-Open No. 10-23735 further discloses a single, flat, cooling tube keeping in touch with all inner surfaces of ironless coils. This flat cooling tube has a higher cooling efficiency than the two round pipes. The Japanese Patent Laid-Open No. 10-23735 further discloses a single flat multi-bore cooling tube formed by adhering eight cooling pipes insulated from each other. The flat multi-bore cooling tube increases cooling efficiency and decreases eddy current.  
         [0004]     The above-noted technology and other related technology does not have a high cooling efficiency which can reduce cogging and keep a constant thrust in a simple way and does not have a high cooling efficiency which can reduce viscous resistance.  
       SUMMARY OF THE INVENTION  
       [0005]     An object of the present invention is to provide an ironless AC linear motor having a high cooling efficiency which can reduce cogging and keep a constant thrust in a simple way.  
         [0006]     A further object of the present invention is to provide an ironless AC linear motor having a high cooling efficiency which can reduce viscous resistance.  
         [0007]     According to the present invention, an ironless AC linear motor for generating a thrust such that a carrier can move relative to a stator along a movement axis (X). The ironless AC linear motor includes parallel rows of permanent magnets arranged along the movement axis with a given magnet pitch (P) to form a magnetic gap ( 10 ) therebetween, a row of ironless coils having through holes, a plurality of coolant tubes arranged in the magnetic gap and passing through the through holes of the ironless coils, and a manifold ( 6 ,  7 ) to which one ends of the coolant tubes are connected to the manifold having the length (B) equal to the magnet pitch in the direction of the movement axis.  
         [0008]     According to the present invention, the manifold and plurality of cooling tubes are made preferably from a nonmagnetic austenitic stainless steel. Because the austenitic stainless steel has a high electrical resistance, eddy current is reduced. Further, it is preferable that the plurality of coolant tubes are spaced from each other with a small space to reduce eddy current. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a side cross-sectional drawing of an ironless AC linear motor in accordance with the present invention;  
         [0010]      FIG. 2  is a plan view of the ironless AC linear motor viewed along line C-C in  FIG. 1 ;  
         [0011]      FIG. 3  is a front elevation of the ironless AC linear motor of  FIG. 1 ;  
         [0012]      FIG. 4  is a side view of the frame assembly of the ironless AC linear motor of  FIG. 1 ;  
         [0013]      FIG. 5  is a plan view of the frame assembly viewed along line E-E in  FIG. 4 ;  
         [0014]      FIG. 6  is a front view of the frame assembly of  FIG. 4 ;  
         [0015]      FIG. 7  is a cross sectional view of the cooling tubes viewed along line F-F in  FIG. 4 ; and  
         [0016]      FIG. 8  is a graph showing relationship between the length of the manifold and cogging. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     As shown in  FIG. 1 , the ironless AC multi-phase linear motor generates a thrust causing a linear motion between a primary and a secondary of the motor along a horizontal movement axis X. At the secondary for the linear motor comprises a pair of spaced side yokes  8  and  9 , a center yoke  11 , and parallel rows of permanent magnets  81 ,  82 , . . .  8   n  and  91 ,  92 , . . .  9   n . The pair of side yokes  8  and  9  are arranged parallel to each other, and are connected by the center yoke  11 . The parallel rows of permanent magnets  81 ,  82 , . . .  8   n  and  91 ,  92 , . . .  9   n  are mounted on the side of yokes  8  and  9 , respectively, to form a magnetic gap  10  therebetween. Successive magnets of each of the rows are of alternate polarity. As shown in  FIG. 1 , the permanent magnets  81 ,  82 , . . .  8   n  and  91 ,  92 , . . .  9   n  are arranged with a given magnet pitch P along the axis X.  
         [0018]     The primary for the linear motor comprises a frame assembly and a plurality of flat ironless coils  1 . As shown in  FIGS. 4, 5  and  6 , the frame assembly is constituted by a base plate  4 , a pair of manifolds  6  and  7 , and a plurality of cooling tubes  51 ,  52 , . . .  5   n . The base plate  4  is spaced apart from the magnetic gap  10  and extends in parallel to the axis X. The pair of manifolds  6  and  7  are attached on respective ends of the base plate  4  and extend down toward the magnetic gap  10 .  
         [0019]     As shown in  FIG. 1 , as the manifolds  6  and  7  have a length B equal to the magnet pitch P in the direction of the axis X, magnetic repulsion and attraction generated between the permanent magnets and the manifolds  6  and  7  are balanced. As a result, as shown in the graph of  FIG. 8 , cogging is drastically decreased and positioning settling time is shortened.  
         [0020]     The manifolds  6  and  7  have respective openings  61  and  71  as an inlet or outlet for coolant. Each of the cooling tubes  51 ,  52 , . . .  5   n  extend in parallel to the axis X and arranged in a row in the perpendicular direction. One end of the cooling tubes is connected is connected to the manifold  6  by brazing or welding and the other end thereof is connected to the manifold  7  in the same manner. The manifolds  6  and  7  have respective coolant passages  62  and  72 . Coolant is distributed to the cooling tubes  51 ,  52 , . . .  5   n  from one of the coolant passages  62  and  72 , and collected at the other of the coolant passages. Each cooling tube  51 ,  52 , . . .  5   n  and the greater part of the manifolds  6  and  7  are positioned in the magnetic gap  10 .  
         [0021]     The manifolds  6  and  7  are preferably made of stainless steel. Because stainless steel has a high electrical resistance, eddy current is reduced. It is further preferable that the manifolds  6  and  7  are made of an austenitic stainless steel as defined in Japanese Industrial Standard SUS316L or SUS316LN. Though the austenitic stainless steel has weight nearly three times greater than that of an aluminum alloy at the maximum, it is nonmagnetic. Therefore, cogging due to magnetic attraction generated between the permanent magnets and the manifolds  6  and  7  is decreased. Further, the austenitic stainless steel is preferable because of its high rigidity and high corrosive characteristics which makes it resistant against a variety of coolants. Austenitic stainless steel is defined in Japanese Industrial Standard SUS316L is comprised of 18% of Cr, 14% of Ni, 2.5% of Mo, low-carbon and the balance being Fe, and has an electrical resistance (Ω·cm) more than ten times higher than that of an aluminum alloy.  
         [0022]     The cooling tubes  51 ,  52 , . . .  5   n  are also preferably made of an austenitic stainless steel instead of an electric conducting material such as an aluminum alloy, a copper or a copper alloy. As shown in  FIG. 7 , the adjoining cooling tubes are spaced apart from each other with a small space d to prevent flow of eddy current. As such, there is a reduction of eddy current and the viscous resistance of the linear motor is low.  
         [0023]     In the illustrated embodiment, there are nine flat, three-phase ironless coils  1  of substantially rectangular shape which have U-phase, V-phase and W-phase and are arranged in that order repeatedly from left in  FIG. 1  along the movement axis X. The cooling tubes  51 ,  52 , . . .  5   n  pass through all through holes  2  of the coils  1 . Each ironless coil overlaps with one another, are touching the cooling tubes  51 ,  52 , . . .  5   n  and are covered by resin  3 . The ironless coils  1  were made by winding an enamel coated copper wire of 0.44 mm diameter onto a preform frame of 17×50 mm section in 76 turns. The preform frame was pulled out and a resultant space corresponds to the through hole  2 . AS shown in  FIG. 1 , lead wires  12  of coils  1  are pulled out through the base plate  4 . The ironless coils  1  are covered with a mold and epoxy resin is filled into the mold. Thus, the ironless coils  1  are fixed to the manifolds  6  and  7  and the cooling tubes  51 ,  52 , . . .  5   n  by the hardened resin. In  FIGS. 1 and 3 , a resin block is formed by the mold.  
         [0024]     The embodiment was selected for the purpose of explaining the essence and practical application of the invention. In light of the above description, various improvements are possible. For example, the cooling tubes  51 ,  52 , . . .  5   n  in the embodiment are arranged perpendicularly in a row, but a plurality of rows of cooling tubes could be arranged perpendicularly. The scope of the invention is defined by the appended claims.