Abstract:
It is an object of the invention to provide an improved technique for controlling the output characteristic of a motor. Representative power tool includes a tool bit and a motor. The motor drives the tool bit and includes an armature, a coordinator, a plurality of segments and a plurality of armature windings. The armature has a plurality of slots. The communicator rotates together with the armature. Plurality of segments is provided on the communicator. Respective armature windings are connected at the both ends of the segments. Each of the armature windings is defined by coils that are wound between respective pairs of the slots of the armature. Each of the armature windings is formed by at least two coils connected in series. At least one of the coils defining the armature winding has a different number of turns of a wire wound between the associated slots from the other coils in the same armature winding. On the same time, the total number of turns of the coils of each of the armature windings is the same. Thus, as a result, the magnetic field around each of the armature windings can be kept substantially the same while providing fine adjustment of the numbers of turns in the armature windings.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power tool with a driving motor and more particularly, to a technique for controlling the output characteristic of the motor by adjusting the number of turns of a wire of a coil forming an armature winding. 
   2. Description of the Related Art 
   Generally, an armature of a DC motor has a plurality of slots. Coils are formed by winding a wire a number of turns between the slots. The coils are connected to associated segments of a communicator and define armature windings. The driving current of the motor is supplied to the coil of the armature windings via the segments and brushes which are in sliding contact with the segments. Upon such supply of current, a magnetic field is generated around the coils and interacts with the field generated by a stator which is fixedly disposed around the armature. 
   In the known DC motor, the same number of brushes as the number of poles of the stator is provided. However, the resistance loss caused by friction between the commutator and the brushes during the rotation of the armature may increase with increase in the number of brushes. Further, the number of parts increases as the number of brushes increases. In this connection, Japanese non-examined laid-open Patent Publication No. 2-184246 discloses a motor which can be driven while having a four-pole stator and two brushes by short-circuiting diametrically opposed segments of a commutator such that the coils connected to the opposed segments are short-circuited, 
   All of the coils that form armature windings consist of the same number of turns of a wire wound between slots, which number is selected according to a desired output characteristic of a motor, i.e. a desired torque or rotation speed. If a different number of turns is selected for each coil, the magnetic field generated around each of the coils when the driving current passes through the coils will wary in strength, because each of the coils that form armature windings is connected to associated segments. As a result, the commutator is deteriorated and suffers degradation in performance 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the invention to provide an improved technique for controlling the output characteristic of a motor by adjusting the number of turns of a wire of a coil that defines the armature winding. 
   This object is achieved by providing a power tool bit and a motor. The tool bit performs a predetermined operation to a work piece. The motor drives the tool bit and includes an armature, a commutator, a plurality of segments and a plurality of armature windings. The armature has a plurality of slots. The commutator rotates together with the armature. Plurality of segments is provided on the commutator. Respective armature windings are connected at the both ends to the segments. Each of the armature windings is defined by coils that are wound between respective pairs of the slots of the armature. Each of the armature windings is formed by at least two coils connected in series. At least one of the coils defining the armature windings has a different number of turns of a wire wound between the associated slots from the other coils in the same armature winding. On the same time, the total number of turns of the coils of each of the armature windings is the same. 
   According to the invention, because of the total number of turns of coils defining each of the armature windings is the same, the magnetic field around each of the armature windings can be kept substantially the same and, as a result, the commutator is not easily deteriorated and can readily keep high commutating performance. On the other hand, at the same time, fine adjustment of the numbers of turns in the armature windings can be easily made by optimizing turns of respective coils in each armature winding without changing the strength the magnetic field around the armature winding. 
   Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an entire impact driver  100  as an example of a power tool of the present invention. 
       FIG. 2  is a sectional view showing a driving motor  121  of the impact driver  100 . 
       FIG. 3  shows an example of an armature winding that is formed by coils wound between the slots of an armature  133 . 
       FIG. 4  is a sectional view of the armature  133  with armature windings  20 ,  21  wound between the slots. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide and manufacture improved power tools and method for using such power tools and devices utilized therein. Representative examples of the present invention, which examples utilized many of these additional features and methods steps in conjunction, will now be described in detail with reference to the drawings. The detailed description is merely intended to teach a person skilled in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed within the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe some representative examples of the invention, which detailed description will now be given with reference to the accompanying drawings. 
   A representative embodiment of the present invention will now be described with reference to  FIGS. 1 to 4 . 
   An electric (battery-powered) impact driver  100  will be described as a representative example of the “power tool” of the present invention. The impact driver  100  has a driver motor provided as a four-pole two-brush DC motor. Within the motor, ten segments are provided on a commutator and ten slots are formed in a armature. Diametrically opposed segments are short-circuited by means of a wire of coils that are wound between slots. 
     FIG. 1  is a side view, partly in section, schematically showing the entire impact driver  100 .  FIG. 2  is a sectional view showing the structure of the driving motor of the impact driver  100 .  FIG. 3  shows an example of the armature winding that is formed by coils wound between slots of the armature of the motor shown in  FIG. 2 .  FIG. 4  is a sectional view of the armature with the armature windings wound between the slots. 
   As shown in  FIG. 1 , the impact driver  100  according to the representative embodiment includes a body  101  and a driver bit  109 . The driver bit  109  is detachably coupled to the tip end region of the body  101  and adapted to tighten various types of screws. The driver bit  109  is a feature that corresponds to the “tool bit” according to the present invention. The body  101  includes a motor housing  103 , a gear housing  105  and a handgrip  107 . The motor housing  103  houses a driving motor  121 . A trigger  125  is mounted on the handgrip  107 , and depressing the trigger  125  turns on a power switch of the driving motor  121 . 
   The gear housing  105  houses a speed reducing mechanism  111 , a spindle  112 , a hammer  114  and an anvil  115 . The speed reducing mechanism  111  mainly includes a planetary gear and appropriately reduces the speed of rotation of an output shaft  122  of the driving motor  121 . The spindle  112  is rotated by the speed reducing mechanism  111 . The rotation of the spindle  112  causes the anvil  115  to rotate. The hammer  114  can move with respect to the spindle  112  in its longitudinal direction and is urged toward the anvil  115  by a compression spring  116 . An end of the anvil  115  protrudes from the end of the gear housing  105 , and the driver bit  109  is detachably coupled to the protruded end of the anvil  115 . 
   When the driving motor  121  is driven for a screw tightening operation and the screw tightening torque of the driver bit  109  is low, the spindle  112  and the hammer  114  rotate together. Under such low-load conditions, the hammer  114  is held in engagement with an anvil  115  by the biasing force of the compression spring  116 . Thus, the anvil  115  also rotates together with the hammer  114 , so that the driver bit  109  performs a screw-tightening operation. 
   When the tightening torque is increased to a predetermined high level, the hammer  114  moves away from the anvil  115  against the biasing force of the compression spring  116 . Thereafter, the hammer  114  engages with the anvil  115  as carrying impulsive rotating torque by the biasing force of the compression spring  116 . Thus, high tightening torque is produced on the driver bit  109  via the anvil  115 . The operating principle of the impact driver  100  itself is known and thus will not be described in detail. 
   The construction of the driving motor  121  will now be described in brief with reference to  FIG. 2 . The driving motor  121  in this embodiment is a four-pole DC motor powered by the battery  127 . The driving motor  121  includes an output shaft  122 , an armature  133 , a stator  135 , a commutator  137  and two brushes  145   a ,  145   b  (see  FIGS. 3 and 4 ). The armature  133  rotates together with the output shaft  122 , and coils that form armature windings are wound on the armature  133 . The stator  135  is secured to the motor housing  103  and generates a magnetic field around the armature  133 . The commutator  137  is fitted onto the output shaft  122  near its end (which is remote from the speed reducing mechanism  111 ). The two brushes  145   a ,  145   b  supply driving current to the armature windings on the armature  133  in sliding contact with a plurality of segments provided on the outside surface of the commutator  137 . 
   One end (the rear end, or the left end as viewed in  FIG. 2 ) of the output shaft  122  is rotatably supported on the motor housing  103  via a bearing  123 . The other end (on the side of the speed reducing mechanism, or the right side as viewed in  FIG. 2 ) of the output shaft  122  is rotatably supported on the gear housing  105  via a bearing  124 . The output shaft  122 , the armature  133  and the commutator  137  form a rotor. 
   When the power to the driving motor  121  having the above construction is turned on, driving current is supplied to the armature windings of the armature  133  within the magnetic field of the stator  135 , and the armature  133  and the output shaft  122  are caused to rotated together. At this time, the commutator  137  and the brushes  145   a ,  145   b  appropriately change the direction of current that passes through the armature windings such that the armature  133  and the output shaft  122  can continuously rotate in a predetermined direction. 
   The construction of the driving motor  121  will now be described in more detail with reference to  FIGS. 3 and 4 . 
   First, the armature  133 , the stator  135  and the commutator  137  will be explained with reference to  FIG. 4 . A ring magnet is provided on the inside surface of the stator  135  which faces the armature  133 . The ring magnet is polarized such that it is divided into four regions and each pair of the two opposed regions have the same polarity (the north pole or the south pole) on its inside surface. The armature  133  has ten radially extending teeth  30 - 39  and is thus shaped liked a gear in section. Ten slots are formed between the adjacent teeth  30 - 39  and a wire of coils that form an armature winding is wound between the slots. 
   The commutator  137  is fitted onto the end of the output shaft  122  that is inserted through the center of the armature  133 . Ten segments  40 - 49  are formed on the outside surface of the commutator  137  and the brushes  145   a ,  145   b  come in sliding contact with the segments. Adjacent segments are insulated from each other. Further, each of the segments is connected to the associated armature winding or other segment by wiring (see  FIG. 3 ). 
   The brushes  145   a ,  145   b  are spaced 90° apart from each other in the direction of sliding contact with the commutator  137 . 
   Next, the construction of an armature winding that is formed by a coil wound between the slots will be explained in detail with reference to  FIG. 3 , taking armature windings  20 ,  21  as an example. Arrows shown in the drawing indicate the direction of winding the coil wire (and not the direction of flow of the driving current). 
   A wire is connected to the segment  40  is inserted through a slot between the teeth  37  and  38  and passed over the teeth  37 ,  36 ,  35  and then inserted through a slot between the teeth  35  and  34 . Thus, the wire is wound one turn around the teeth  37 ,  36 ,  35  between these slots. A coil A 1  is formed by thus winding the wire seven turns between these slots. Thereafter, the wire is inserted through a slot between the teeth  30  and  39 . Thus, the wire is wound one turn around the teeth  32 ,  31 ,  30  between these slots. A coil B 1  is formed by thus winding the wire eight turns between these slots. Then the wire is connected to the segment  46 . In this manner, the coils A 1  and B 1  are connected in series and form the armature winding  20 . 
   The segment  46  is connected to the segment  41  (as shown by arrow c) and short-circuited. The wire connected to the segment  46  is insert through a slot between the teeth  38  and  39  and passed over the teeth  38 ,  37 ,  36  and then inserted through a slot between the teeth  36  and  35 . Thus, the wire is wound one turn around the teeth  38 ,  37 ,  36  between these slots. A coil A 2  is formed by thus winding the wire eight turns between these slots. Thereafter, the wire is inserted through a slot between the teeth  33  and  34  (as shown by arrow d) and passed over the teeth  33 ,  32 ,  31  and then inserted through a slot between the teeth  31  and  30 . Thus, the wire is wound one turn around the teeth  33 ,  32 ,  31  between these slots. A coil B 2  is formed by thus winding the wire seven turns between these slots. Then the wire is connected to the segment  47 . In this manner, the coils A 2  and B 2  are connected in series and from the armature winding  21 . 
   In a similar manner, armature windings  22 ,  23  and  24  are formed by coil A 3  of seven turns and a coil B 3  of eight turns, by coil A 4  of eight turns and a coil B 4  of seven turns, and by a coil A 5  of seven turns and a coil B 5  of eight turns, respectively, which are connected in series. 
   Specifically, as shown in  FIG. 4  the coil A 1  (of seven turns) of the armature winding  20  is installed in the slot between the teeth  37  and  38  and the slot between the teeth  34  and  35 ; the coil A 2  (of eight turns) of the armature winding  21  is installed in the slot between the teeth  38  and  39  and the slot between the teeth  35  and  36 ; the coil A 3  (of seven turns) of the armature winding  22  is installed in the slot between the teeth  39  and  30  and the slot between the teeth  36  and  37 ; the coil A 4  (of eight turns) of the armature winding  23  is installed in the slot between the teeth  30  and  31  and the slot between the teeth  37  and  38 ; and the coil A 5  (of seven turns) of the armature winding  24  is installed in the slot between the teeth  31  and  32  and the slot between the teeth  38  and  39 . 
   Further, the coil B 1  (of eight turns) of the armature winding  20  is installed in the slot between the teeth  32  and  33  and the slot between the teeth  39  and  30 ; the coil B 2  (of seven turns) of the armature winding  21  is installed in the slot between the teeth  33  and  34  and the slot between the teeth  30  and  31 ; the coil B 3  (of eight turns) of the armature winding  22  is installed in the slot between the teeth  34  and  35  and the slot between the teeth  31  and  32 ; and the coil B 4  (of seven turns) of the armature winding  23  is installed in the slot between the teeth  35  and  36  and the slot between the teeth  32  and  33 ; and the coil B 5  (of eight turns) of the armature winding  24  is installed in the slot between the teeth  36  and  37  and the slot between the teeth  33  and  34 . The armature windings  22  to  24  are not illustrated in  FIG. 4 , and only the reference marks of the coils are indicated together with the number of turns. 
   Therefore, as for the coils A (coils A 1  to A 5 ) which form the respective armature windings, the coils A 1  to A 5  of which numbers of turns are 7, 8, 7, 8, 7, respectively, are arranged in this order in the circumferential direction (in the direction of reverse rotation shown in  FIGS. 3 and 4 , in this embodiment).As for the coils B (coils B 1  to B 5 ) which also form the respective armature windings, the coils A 1  to A 5  of which numbers of turns are 8, 7, 8, 7, 8, respectively, are arranged in this order in the circumferential direction (in the direction of the reverse rotation shown in  FIGS. 3 and 4 , in this embodiment). Therefore, when the armature windings  20  to  24  are thus formed by winding the wire, as shown in  FIG. 4 , a wire of fifteen turns (15=7+8) is installed within each slot. 
   The “coils A 1  to A 5 ” and “coil B 1  to B 5 ” in this embodiment correspond to the “coils” in this invention. The coils A (coils A 1  to A 5 ) and coils B (coils B 1  to B 5 ) in this embodiment correspond to the “first coil” and the “second coil”, respectively, in this invention. 
   As for the armature windings  20 ,  21 , as shown in  FIG. 3 , the driving current to the driving motor flows from the brush  145   a  connected to the positive electrode of the power to the brush  145   b  connected to the negative electrode of the power, via the segment  40 , the coil A 1  of the armature winding  20 , the coil B 1  of the armature winding  20 , the segment  46 , the segment  41 , the coil A 2  of the armature winding  21 , coil B 2  of the armature winding  21 , the segment  47  and the segment  42 , in this order. 
   By the passage of current through the armature windings, a magnetic field is generated around the coils A 1 , A 2 , B 1 , B 2  and interacts with the field generated by the magnetic ring of the stator  135 . As a result, the armature  133  is caused to rotate clockwise as viewed in  FIG. 4  (leftward as viewed in  FIG. 3 ). 
   Actually, the armature windings  20  to  24  are provided between the segments. When the armature  133  rotates, the segments of the commutator  137  keep moving into sliding contact with the brushes  145   a ,  145   b  in ascending order of reference numerals of the segments shown in  FIGS. 3 and 4  ( 40 - 41 - . . . - 49 - 40  . . . ). Therefore, the armature  133  keeps rotating clockwise as viewed in  FIG. 4  while the commutator  137  and brushes  145   a ,  145   b  commutate the driving current passing through the armature windings. 
   In the case of reverse rotation, the brush  145   b  is connected to the positive electrode of the power and the brush  145   a  is connected to the negative electrode power. Thus, the armature  133  keeps rotating counterclockwise as viewed in  FIG. 4 . 
   With the impact driver  100  according to this embodiment, the total number of turns of each of the armature windings  20  to  24  between the associated segments, or the total number of turns of the coil A and the coil B (e.g. the coil A 1  and the coil B 1 , or the coil A 2  and the coil B 2 ) is the same (fifteen in this embodiment) as that of the other armature windings. However, in the same armature winding, one coil can have a different number of turns from the others. In this embodiment, each of the armature windings has two coils, the coil A and the coil B, one of which consists of seven turns while the other of eight turns. 
   Thus, the armature windings connected between the segments adjacent to the commutator consist of the same number of turns. Therefore, the magnetic field generated around each of the armature windings when the driving current passes through the armature windings do not easily vary in strength. As a result, in the driving motor of the power tool of this embodiment, the commutator is not easily deteriorated and can readily keep high commutating performance. Further, fine adjustment of the numbers of turns of the armature windings can be easily made. For example, if all the coils consist of eight turns, which means that the total number of turns of the armature windings is sixteen, the torque output characteristic may be too high. However, if all the coils consist of seven turns, which means that the total number of turns of the armature windings is fourteen, the torque output characteristic may be too low. In such case, fine adjustment can be made such that the total numbers of turns of the armature windings is fifteen like in this embodiment. Thus, the output characteristic of the motor of the power tool can be easily adjusted to any desired level. 
   Further, according to the impact driver  100 , the number of the wires of the coils installed in each of the slots of the armature  133  is the same (fifteen in this embodiment) as that of the wires in the other slots. Therefore, imbalance in the number of turns of the wire is not substantially increased in the circumferential direction of the armature  133  on which the armature windings  20  to  24  are wound. As a result, application of excessive load to the output shaft  122  of the motor  121  can be prevented. Thus, stable operation of the motor can be realized. 
   The present invention is not limited to the constructions as described above, but rather, may be added to, changed, replaced with alternatives or otherwise modified. 
   In the above embodiment, the driving motor is describe as a four-pole two-brush DC motor, but the numbers of poles and brushes are not limited to this. For example, the motor may be of six-pole two-brush type. In such a case, each of the armature windings consists of three coils connected in series, and the numbers of turns of the three coils may be selected, for example, to be (7, 7, 8), (8, 7, 7), (7, 8, 7), (7, 7, 8) for the respective armature windings. Further, while defining each armature winding with a plurality of coils connected in series, the number of turns of each coil within each armature winding may be randomized as long as the total number of turns of the coils of each of the armature windings is the same with the other armature windings. 
   Further, in this embodiment, the wire of the coils is installed by distributed winding. However, the same effect as this embodiment can also be obtained by concentrated winding. 
   DESCRIPTION OF NUMERALS 
   
       
         20 ,  21 ,  22 ,  23 ,  24  armature winding 
         100  impact driver (power tool) 
         101  body 
         103  motor housing 
         105  gear housing 
         107  handgrip 
         109  driver bit (tool bit) 
         111  speed reducing mechanism 
         112  spindle 
         113  ball 
         114  hammer 
         115  anvil 
         116  compression spring 
         121  driving motor 
         122  output shaft 
         123 ,  124  bearing 
         125  trigger 
         127  battery 
         133  armature 
         135  stator 
         137  commutator 
         145   a ,  145   b  brush 
       A 1 , A 2 , A 3 , A 4 , A 5  coil 
       B 1 , B 2 , B 3 , B 4 , B 5  coil