Abstract:
A switch for controlling a rotational speed of a motor includes an operation member, a first circuit, and a second circuit. The first circuit includes a brush coupled to the operation member having a contact and also includes a variable resistive plate having a resistance that changes in response to a contact position of the contact point of the brush, so that the first circuit outputs a control signal to the motor according to the contact position of the contact point. The second circuit connects the brush and the resistive plate without through the contact point when the brush is positioned at a given position relative to the resistive plate.

Description:
This application claims priority to Japanese patent application serial number 2009-204734, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable speed switch for adjusting a rotational speed of an electric power tool, which includes a resistive plate and a brush that slides in a given direction with respect to the resistive plate by a pull operation of a trigger, and in which resistance of the resistive plate varies according to the slide of the brush with respect to the resistive plate. 
     2. Description of the Related Art 
     An example of this kind of variable speed switches is shown in  FIG. 7(A) . 
     The variable speed switch  100  includes a substrate  101 , a brush  107  that slides in an arrowed direction (horizontal direction) with respect to the substrate  101 , and a trigger (not shown) to which the brush  107  is attached, A conductor part  103  and a printed circuit resistor  105  are formed on the surface of the substrate  101  extending horizontally. A far left side and a far right side of the printed circuit resistor  105  are connected to terminals T 0  and T 1 , respectively, and the conductor part  103  is connected to a terminal T 2 . Further, the far left part and the far right part of the brush  107  can be slidably contacted to the conductor part  103  and the printed circuit resistor  105 , respectively. 
     When the trigger is pulled in the variable speed switch  100 , the brush  107  slides from the far left side in a rightward direction according to pulling amount of the trigger and the resistance between the terminals T 1  and T 2  (the resistance of the variable speed switch  100 ) decreases gradually.  FIG. 7(A)  shows a state in which the trigger is pulled to the maximum position.  FIG. 7(B)  shows an equivalent circuit of the variable speed switch  100 . 
       FIG. 8  is a schematic showing an electric circuit of the electric power tool equipped with the above-described variable speed switch  100 . A reference voltage from a controller  102  is applied between the terminals T 0  and T 1  of the printed circuit resistor  105  in the variable speed switch  100 . Consequently, a voltage signal outputted from the terminal T 2  drops as the resistance of the variable speed switch  100  decreases. A controller  102  of the electric power tool controls a switching element FET such that a current flowing through a motor M increases according to a drop of the voltage signal outputted from the variable speed switch  100 . Thus, a rotational speed of the electric power tool increases by the pull operation of the trigger in the variable speed switch  100 . 
     However, because the above-described variable speed switch  100  is constructed such that resistance varies by sliding the brush  107  with respect to the substrate  101 , a contact between the brush  107  and the substrate  101  becomes unstable when the brush  107  etc. becomes worn with time, and the resistance varies unstably when the trigger is pulled as illustrated in  FIG. 7(C) . Especially, when the resistance fluctuates unstably at the time of the trigger being pulled to the maximum position (refer to the arrow R of FIG.  7 (C)), the maximum output (the maximum rotational speed) of the electric power tool fluctuates (refer to the arrow N of FIG.  7 (D)), which may deteriorate work efficiency. 
     To improve this defect, a configuration of electric circuit is used in which a mechanical contact  103  is provided in parallel with the switching element FET, and the mechanical contact  103  is switched ON when the trigger is pulled to the maximum position. Consequently, the maximum output of the electric power tool may not drop and deterioration in the work efficiency may be reduced. However, in this modification, even if the controller  102  controls motor current by sensing a signal showing a drop of a battery voltage  104 , the controller  102  cannot prevent over-discharge of the battery  104 . This is because motor current flows regardless of a discharge control signal from the controller  102  when the trigger is pulled to the maximum position. 
     In a variable speed switch described in Japanese Laid-Open Patent Publication No. 7-220563, a rotational speed of the electric tool can be adjusted according to a pressure force applied to a pressure sensor attached to the trigger, which may solve the above described problem. 
     However, a configuration in which the pressure sensor is attached to the trigger of the variable speed switch may cause a large cost increase. 
     Thus, there is a need in the art to obtain a rotational stability at low cost and to prevent over-discharge of the battery when an electric power tool is operated at a full speed, without modifying a configuration for sliding a brush with respect to the resistive plate. 
     SUMMARY OF THE INVENTION 
     A switch for controlling a rotational speed of a motor includes an operation member, a first circuit, and a second circuit. The first circuit includes a brush coupled to the operation member having a contact and also includes a variable resistive plate having a resistance that changes in response to a contact position of the contact point of the brush, so that the first circuit outputs a control signal to the motor according to the contact position of the contact point. The second circuit connects the brush and the resistive plate without through the contact point when the brush is positioned at a given position relative to the resistive plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematically lateral view of the electric power tool equipped with a variable speed switch according to an example of the present invention. 
         FIG. 2  (A) is a schematic view showing an electrical circuit of the electric power tool. 
         FIG. 2  (B) is a schematic view showing a variable speed switch in which a trigger returns to the original position. 
         FIG. 2  (C) is a schematic view showing a variable speed switch in which a trigger is pulled to the maximum position. 
         FIG. 3  (A) is a schematic view showing the relationship between pulling amount of the trigger and the resistance of the variable speed switch. 
         FIG. 3  (B) is a schematic view showing the relationship between pulling amount of the trigger and rotational speed of the electric power tool. 
         FIG. 4  (A) is a schematic view showing an electrical circuit of the power tool equipped with a variable speed switch according to a modified example. 
         FIG. 4  (B) is a schematic view showing the relationship between pulling amount of the trigger and the resistance of the variable speed switch. 
         FIG. 4  (C) is a schematic view showing the relationship between pulling amount of the trigger and rotational speed of the electric power tool. 
         FIG. 5  is a schematic view showing an electrical circuit of the power tool equipped with a variable speed switch according to a modified example. 
         FIG. 6  is a schematic view showing an electrical circuit of the power tool equipped with a variable speed switch according to a modified example. 
         FIG. 7  (A) is a schematic view showing a known variable speed switch in which the trigger is pulled to the maximum position. 
         FIG. 7  (B) is a schematic view showing an equivalent circuit of the variable speed switch. 
         FIG. 7  (C) is a schematic view showing the relationship between pulling amount of the trigger and the resistance of the variable speed switch. 
         FIG. 7  (D) is a schematic view showing the relationship between pulling amount of the trigger and rotational speed of the electric power tool. 
         FIG. 8  is a schematic view showing an electrical circuit of a known electric power tool. 
         FIG. 9  is a schematic view showing an electrical circuit of a known electric power tool. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved variable speed switch. Representative examples of the present teaching, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill 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 in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful examples of the present teachings. 
     One construction for a switch for adjusting a rotational speed of an electric power tool can include a resistive plate and a brush for sliding in a given direction with respect to the resistive plate by a pull operation of a trigger. The switch is a variable speed switch in which the resistance varies by sliding the brush with respect to the resistive plate. The resistance varies according to a sliding position of the brush with respect to the resistive plate. And the switch can include a main switch-circuit and a subsidiary switch-circuit for outputting a signal. The main switch-circuit is constructed such that a signal voltage is outputted from a terminal of the brush in a state of a reference voltage being applied between a terminal of a brush-sliding-start side in the resistive plate and a terminal of a brush-sliding-end side in the resistive plate, and the subsidiary switch-circuit is constructed such that the terminal of the brush-sliding-end side is electrically connected to the terminal of the brush when the trigger is pulled to the given position. 
     According to this construction, when the trigger is pulled to the given position, the terminal of the brush-sliding-end side in the resistive plate is electrically connected to the terminal of the brush via the subsidiary switch-circuit. 
     For example, when the trigger is pulled to the given position and then the terminal of the brush-sliding-end side in the resistive plate and the terminal of the brush are connected via the subsidiary switch-circuit, a signal voltage of the terminal of the brush becomes equal to the terminal voltage at the terminal of the brush-sliding-end side in the resistive plate. Further, when the terminal of the brush-sliding-end side in the resistive plate and the terminal of the brush are connected by a constant resistance via the subsidiary switch-circuit, a signal voltage at the terminal of the brush is proportional to a terminal voltage at the terminal of the brush-sliding-end side in the resistive plate. 
     Consequently, even if the resistance of the main switch-circuit fluctuates unstably because of wear of the brush or the resistive plate over time, a signal voltage at the terminal of the brush becomes constant when the trigger is pulled to the given position. Thus, when the trigger is pulled to the given position, a rotational speed of the electric power tool cannot be unstable and the working activity cannot be deteriorated by using The electric power tool in this condition. 
     Most cases, the electric power tool may be used in a state in which the trigger is pulled to the maximum position (a setting position where the maximum rotational speed can be provided). Therefore, there arises little problem if a rotational speed of the electric power tool becomes unstable during the process of the trigger being pulled to the maximum position, and thus, in many cases, a state in which the trigger is pulled to the maximum position is set as a given position. 
     A state in which the trigger is pulled to the maximum position is a state in which a signal voltage equals to a voltage produced when the trigger is pulled to the limit position and includes a state in which the trigger is pulled to the vicinity of the limit position. 
     In this way, because a configuration that includes the resistive plate and the brush can be used with little modification, a rotational stability of the electric power tool can be obtained at a low cost. Further, since there is no need to provide a mechanical contact parallel to a switching element in the electrical circuit, over-discharge of the battery can be prevented. 
     According to another construction, the subsidiary switch-circuit can include a mechanical contact that connects the terminal of the brush-sliding-end side in the resistive plate and the terminal of the brush. 
     According to another construction, the subsidiary switch-circuit can include a brush that can slide together with the trigger in a given direction and a conductor to which the brush is connected when the trigger is pulled to the given position, and the mechanical contact is formed between the brush and the conductor. 
     Thus, it is ensured that when the trigger is pulled to the given position, the mechanical contact in the subsidiary switch-circuit can be switched ON. 
     According to another construction, the subsidiary switch-circuit can include a semiconductor switch and is configured such that the semiconductor switch becomes conductive when the trigger is pulled to the given position. 
     Thus, durability of the subsidiary switch-circuit can be improved. 
     According to the above, a rotational stability when the electric power tool rotates at the full speed can be obtained at the low cost. Further, over-discharge of the battery can be prevented. 
     A variable speed switch according to an example, which is a trigger-type operational switch used in an impact driver (hereinafter termed an electric power tool), will be described below with reference to  FIG. 1  to  FIG. 6 . 
     The electric power tool will be briefly explained below before the variable speed switch is explained. 
     An electric power tool  10  according to an example includes a tubular housing main body  12  and a grip part  15  protruding from a lateral part of the housing main body  12  (a lower part in  FIG. 1 ) as shown in  FIG. 1 . 
     A motor  18  is accommodated at a rear part of the housing main body  12 , and a driving mechanism  19  for increasing a rotational power of the motor  18  and for transmitting the power to an end tool  11  is accommodated in front of the motor  18 . Further, a circuit board  17  is mounted at the rear side of the motor  18 , and a switching element FET and a controller  32  for controlling the switching FET are mounted on the circuit substrate  17  (refer to  FIG. 2(A) ). 
     The grip part  15  includes a holding part  15   h  that can be held by a user when he or she operates the electric power tool  10  and also includes a lower part  15   p  that is located below the holding part  15   h  (an end side). A trigger-type variable speed switch  20 , which is operated when a user pulls the trigger, is provided at an end part of the holding part  15   h . Further, a coupling mechanism (not shown) that couples the grip part  15  to a battery pack  16  is provided at the lower part  15   p  of the grip part  15 . 
     A variable speed switch  20  is for increasing a rotational speed of the electric power tool  10  (a motor  18 ) according to pulling amount of a trigger  21  and includes a main switch-circuit  22  and a subsidiary switch-circuit  27  for outputting a signal which are connected in parallel, as shown in  FIGS. 2(A) to 2(C) . 
     The main switch-circuit  22  includes a substrate  23  and a brush  24  that slides in the arrowed direction (a horizontal direction) with respect to the substrate  23  and moves together with the trigger  21 . On a surface of the substrate  23 , a conductor part  23   c  and a printed circuit resistor  23   r  are formed extending in the sliding direction of the brush  24  (a horizontal direction). One end (a left end) of the printed circuit resistor  23   r  is connected to a terminal T 0 , another end (a right end) is connected to a terminal T 1 , and the conductor part  23   c  is connected to a terminal T 2 . Further, a left-end sliding part  24   e  of the brush  24  can be contacted to the conductor part  23   c  and a right-end sliding part  24   f  of the brush  24  can be contacted to the printed circuit resistor  23   r . Thus, when the trigger  21  of the variable speed switch  20  is pulled, the brush  24  slides in the right direction from a left end position shown in  FIG. 2(B)  according to pulling amount of the trigger  21  and the resistance of the main switch-circuit  22  decreases gradually. Finally, when the trigger  21  is pulled to the maximum position and then the brush  24  reaches the right end position as shown in  FIG. 2(C) , the resistance of the main switch-circuit  22  becomes zero. 
     Namely, the substrate  23  that includes both the conductor part  23   c  and the printed circuit resistor  23   r  corresponds to a resistive plate of the present invention. And the terminal T 0  that is one end (a left end) of the printed circuit resistor  23   r  corresponds to a terminal of a brush-sliding-start side in the resistive plate of the present invention, and the terminal T 1  that is another end (a right end) of the printed circuit resistor  23   r  corresponds to a terminal of a brush-sliding-end side in the resistive plate of the present invention. Further, the terminal T 2  connected to the brush  24  via the conductor part  23   c  corresponds to a terminal of the brush of the present invention. 
     The subsidiary switch-circuit  27  includes a mechanical contact  27   s . One end side of the mechanical contact  27   s  is connected to the terminal T 1  and the other end side is connected to the terminal T 2 . The mechanical switch  27   s  is constructed such that it is switched ON when the trigger  21  is pulled to the maximum position (refer to  FIG. 2(C) ) and is switched OFF otherwise. Thus, during the process of the trigger  21  being pulled from the minimum position to nearly the maximum position, the mechanical contact  27   s  of the subsidiary switch-circuit  27  is switched OFF and the resistance of the main switch-circuit  22  emerges between the terminals T 1  and T 2 . When the trigger  21  is pulled to the maximum position, the mechanical contact  27   s  of the subsidiary switch-circuit  27  is switched ON and the resistance between the terminals T 1  and T 2  becomes zero. Namely, the terminal Ti of the brush-sliding-end side in the printed circuit resistor  23   r  and the terminal T 2  of the brush  24  in the main switch-circuit  22  are short-circuited. 
     As shown in  FIG. 2(A) , an electric circuit  30  of an electric power tool  10  includes a switching element FET for controlling a current supplied to a motor  18  and a controller  32  for controlling the switching FET according to a signal from a variable switch  20 . The controller  32  is constructed such that it can apply a reference voltage between the terminals T 0  and T 1  of the printed circuit resistor  23   r  in the variable speed switch  20  (the main switch-circuit  22 ). Therefore, as the resistance between the terminal T 1  of the main switch-circuit  22  and the terminal T 2  (the terminal T 2  of the brush  24 ) decreases, a voltage signal outputted from the terminal T 2  of the brush  24  drops. The controller  32  controls the switching element FET so that a current supplied to the motor  18  increases according to a drop of the voltage signal of the variable speed switch  20  (the voltage signal of the terminal T 2 ). As described above, the variable speed switch  20  (the main switch-circuit  22 ) is constructed such that the resistance decreases according to pulling amount of the trigger  21 , and thus pulling the trigger  21  can increase a rotational speed of the electric power tool  10  via the controller  32 . 
     In the above-described variable speed switch  20 , when the trigger  21  is pulled to the maximum position, the terminal T 1  of the brush-sliding-end side in the printed circuit resistor  23   r  of the main switch-circuit  22  and the terminal T 2  of the brush  24  are electrically connected via the subsidiary switch-circuit  27 . Namely, the terminal T 1  of the brush-sliding-end side in the printed circuit resistor  23   r  of the main switch-circuit  22  and the terminal T 2  of the brush  24  are short-circuited. Consequently, a signal voltage of the terminal T 2  of the brush  24  becomes equal to a terminal voltage of the terminal T 1  of the brush-sliding-end side in the printed circuit resistor  23   r  of the main switch-circuit  22 . Therefore, even if the resistance of the main switch-circuit  22  (a signal voltage of the terminal T 2 ) fluctuates unstably as shown in  FIG. 3(A)  because the brush  24  or the printed circuit resistor  23   r  wears over time, a signal voltage of the terminal T 2  of the brush  24  becomes constant when the trigger  21  is pulled to a given position (refer to the R part of  FIG. 3(A) ). Thus, as shown in the arrowed N of  FIG. 3(B) , a rotational speed (the maximum speed) of the electric power tool cannot be unstable when the trigger  21  is pulled to the maximum position, and working activity can be prevented from deteriorating by using the electric power tool in this condition. Most cases, the electric power tool can be used in a state in which the trigger  21  is pulled to the maximum position, and thus there arises little problem even if a rotational speed becomes unstable during the process of the trigger  21  being moved to the given position. 
     As described above, a rotational stability of the electric power tool  10  can be obtained at the low cost, because the substrate  23  and the brush  24  which are included in the main switch-circuit  22  can be used with little modification. 
     The above construction may not be limited by the above-described example and various changes may be made without departing from the scope of the invention. For example, the above example shows that the electric power tool  10  includes the variable speed switch  20  that is constructed such that the resistance decreases according to pulling amount of the trigger  21 . However, as shown in  FIG. 4(B) , the above construction can be applied to electric power tools in which the variable speed switch  20 , which is constructed such that the resistance (a signal voltage) increases according to pulling amount of the trigger  21 , is used. 
     In this case, as shown in  FIG. 4(A) , the variable speed switch  20  is constructed such that the terminal T 0  and the terminal T 1  are connected to a negative side and a positive side of the controller  32 , respectively, and the reference voltage is applied between the terminals T 0  and T 1 . Consequently, the voltage signal outputted from the terminal T 2  of the brush  24  increases according to pulling amount of the trigger  21 . The controller  32  controls the switching FET such that a current supplied to the motor  18  increases according to an increase of the voltage signal of the variable speed switch  20  (the voltage signal of the terminal T 2 ). 
     The subsidiary switch-circuit  27  includes the mechanical contact  27   s , and one end of the mechanical contact  27   s  is connected to the terminal T 1  and the other end is connected to the terminal T 2 . 
     Consequently, even if the resistance of the main switch-circuit  22  fluctuates unstably owing to wear of the brush  24  or the printed circuit resistor  23   r  over time, the mechanical contact  27   s  of the subsidiary switch-circuit  27  is switched ON when the trigger  21  is pulled to the maximum position and the signal voltage of the terminal T 2  of the brush  24  becomes constant when the trigger is pulled to the given position of the trigger  21 . Therefore, as shown in the arrow N of  FIG. 4(C) , a rotational speed of the electric power tool at the time of the trigger  21  being pulled to the maximum position becomes stable and the working activity cannot be deteriorated. 
     Instead of the mechanical contact  27   s  of the subsidiary switch-circuit  27  in  FIG. 4(A) , it is possible to construct such that a brush  27   b  that moves together with the trigger  21  is provided and one end of the brush  27   b  is connected to the terminal T 4  when the trigger  21  is pulled to the maximum position, as shown in  FIG. 5 . 
     Further, as shown in  FIG. 6 , it is possible to construct such that the subsidiary switch-circuit  27  includes the contact  27   s  and a semiconductor switch  27   e.    
     Further, the example shows that the subsidiary switch-circuit  27  is switched ON when the trigger  21  is pulled to the maximum position, but it is possible to construct such that the subsidiary switch-circuit  27  is switched ON when the trigger  21  is pulled to the given position. 
     Further, the example shows that the subsidiary switch-circuit  27  is included inside the variable speed switch  20 , but it is possible to construct such that the subsidiary switch-circuit  27  is provided outside the variable speed switch  20 . It is also possible to include the subsidiary switch-circuit  27  and the controller  32  inside the variable speed switch  20 .