Patent Publication Number: US-2022216746-A1

Title: Electric tool

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to electric tools, and more specifically relates to an electric tool including an electric motor. 
     BACKGROUND ART 
     An electric tool configured to control the rotation number of an electric motor has been known (e.g., Patent Literature 1). The electric tool described in Patent Literature 1 includes a brushless DC motor (an electric motor), a battery voltage detector, a rotational position detector, and a control unit. The battery voltage detector is configured to detect the voltage of a battery used to drive the brushless DC motor. The rotational position detector is configured to detect the rotational position of the brushless DC motor. The control unit is configured to control a drive output to the brushless DC motor based on a signal from the rotational position detector. The control unit is configured to, when controlling the drive output to the brushless DC motor, control a conduction angle or an advance angle to the brushless DC motor such that the rotation number or the energizing current of the brushless DC motor is a target value corresponding to the battery voltage detected by the battery voltage detector. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2014-144496 A 
     SUMMARY OF INVENTION 
     It is an object of the present disclosure to provide an electric tool configured to increase the rotation number of an electric motor. 
     An electric tool according to one aspect of the present disclosure includes an electric motor and a control unit. The electric motor includes a permanent magnet and a coil. The control unit is configured to perform control on operation of the electric motor. The control performed by the control unit includes field weakening control by which the control unit causes a flux-weakening current to flow through the coil. The flux-weakening current is a current that generates, in the coil, a magnetic flux that weakens a magnetic flux of the permanent magnet. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an electric tool according to an embodiment; 
         FIG. 2  is a diagram schematically illustrating the electric tool; 
         FIG. 3  is a flowchart illustrating an operation example of the electric tool; and 
         FIG. 4  is a graph illustrating the operation example of the electric tool. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of an electric tool  1  will now be described in detail with reference to the accompanying drawings. Note that the embodiment to be described below is only an exemplary one of various embodiments of the present disclosure and should not be construed as limiting. Rather, the exemplary embodiment may be readily modified in various manners depending on a design choice or any other factor without departing from the scope of the present disclosure. Also,  FIG. 2  to be referred to in the following description of the embodiment is a schematic representation. That is to say, the ratio of the dimensions (including thicknesses) of respective constituent elements illustrated in  FIG. 2  does not always reflect their actual dimensional ratio. 
     (1) Overview 
     An electric tool  1  according to an exemplary embodiment may be used as an impact screwdriver, a drill screwdriver, or an impact wrench. The electric tool  1  includes an AC motor  15  (an electric motor) and a control unit  4  as shown in  FIG. 1 . The AC motor  15  may be a brushless motor, for example. In particular, the AC motor  15  according to this embodiment is a synchronous motor and is more specifically a Permanent Magnet Synchronous Motor (PMSM). The control unit  4  performs control on the operation of the AC motor  15 . 
     The AC motor  15  includes a rotor  13  having a permanent magnet  131  and a stator  14  having a coil  141 . The rotor  13  includes an output shaft  16 . Electromagnetic interaction between the coil  141  and the permanent magnet  131  rotates the rotor  13  with respect to the stator  14 . The control unit  4  performs vector control for controlling a flux-weakening current (d-axis current) supplied to the AC motor  15  and a torque current (q-axis current) supplied to the AC motor  15  independently of each other. The control performed by the control unit  4  includes field weakening control by the vector control. In the field weakening control, the control unit  4  causes the flux-weakening current (d-axis current) to flow through the coil  141  of the AC motor  15 . The flux-weakening current generates, in the coil  141 , a magnetic flux that weakens the magnetic flux of the permanent magnet  131  (weakening flux). In other words, the flux-weakening current generates, in the coil  141 , a magnetic flux, of which the direction is opposite from the direction of the magnetic flux of the permanent magnet  131 . This increases the rotation number of the AC motor  15  (the rotation number of the output shaft  16 ). 
     The control performed by the control unit  4  further includes regular control. The control unit  4  does not cause the flux-weakening current to flow through the coil  141  in the regular control. That is, a current that flows through the coil  141  in the regular control is only the torque current (q-axis current). When the torque current of the AC motor  15  is relatively large (e.g., the magnitude of the torque current exceeds a predetermined value), the control unit  4  changes its control from the field weakening control to the regular control. Thus, when the AC motor  15  needs a relatively large torque, the regular control provides the relatively large torque. 
     (2) Electric Tool 
     The electric tool  1  includes the AC motor  15 , a power supply  32 , a driving force transmission mechanism  18 , an impact mechanism  17 , a socket  23 , a trigger volume  29 , the control unit  4 , a torque measuring unit  26 , a bit rotation measuring unit  25 , and a motor rotation measuring unit  27  as shown in  FIG. 2 . In addition, the electric tool  1  further includes a tip tool. 
     The impact mechanism  17  has an output shaft  21 . The output shaft  21  is a member to rotate with driving force transmitted from the AC motor  15 . The socket  23  is a member, which is fixed to the output shaft  21  and to which the tip tool is attached removably. The electric tool  1  is a tool for driving the tip tool with the driving force supplied from the AC motor  15 . The tip tool (hereinafter also referred to as a “bit”) may be a screwdriver or a drill, for example. A tip tool is selected from various types of tip tools according to the intended use and attached to the socket  23  to have some type of machining work done. Optionally, the tip tool may be directly attached to the output shaft  21 . 
     The AC motor  15  is a drive source for driving the tip tool. The AC motor  15  includes the output shaft  16  for outputting rotational driving force. The power supply  32  is an AC power supply for supplying a current for driving the AC motor  15 . The power supply  32  includes a single or a plurality of secondary batteries. The driving force transmission mechanism  18  regulates the rotational driving force of the AC motor  15  and outputs a desired torque. The driving force transmission mechanism  18  includes a drive shaft  22  as its output member. 
     The drive shaft  22  of the driving force transmission mechanism  18  is connected to the impact mechanism  17 . The impact mechanism  17  transforms the rotational driving force supplied from the AC motor  15  via the driving force transmission mechanism  18  into a pulsed torque, thereby generating impacting force. The impact mechanism  17  includes a hammer  19 , an anvil  20 , the output shaft  21 , and a spring  24 . The hammer  19  is attached to the drive shaft  22  of the driving force transmission mechanism  18  via a cam mechanism. The anvil  20  is coupled to the hammer  19  and rotates along with the hammer  19 . The spring  24  biases the hammer  19  toward the anvil  20 . The anvil  20  is formed integrally with the output shaft  21 . Alternatively, the anvil  20  may be formed separately from the output shaft  21  and fixed to the output shaft  21 . 
     Unless a load (torque), of which the magnitude is greater than or equal to a predetermined value, is applied to the output shaft  21 , the drive shaft  22  and the hammer  19  which are coupled together via the cam mechanism turn along with each other, and in addition, the hammer  19  and the anvil  20  turn along with each other. Thus, the output shaft  21  formed integrally with the anvil  20  turns accordingly. On the other hand, if a load, of which the magnitude is greater than or equal to the predetermined value, is applied to the output shaft  21 , then the hammer  19  moves backward (i.e., moves away from the anvil  20 ) against the spring  24  while being regulated by the cam mechanism. At a point in time when the hammer  19  is decoupled from the anvil  20 , the hammer  19  starts moving forward while turning, thus applying impacting force to the anvil  20  in the rotational direction and thereby turning the output shaft  21 . 
     The trigger volume  29  is an operating member for accepting an operating command for controlling the rotation of the AC motor  15 . The ON/OFF states of the AC motor  15  may be switched by pulling the trigger volume  29 . In addition, the rotational velocity of the output shaft  21 , i.e., the rotational velocity of the AC motor  15 , is adjustable by the manipulative variable indicating how deep the trigger volume  29  has been pulled. Specifically, the greater the manipulative variable is, the higher the rotational velocity of the AC motor  15  becomes. The control unit  4  starts or stops turning the AC motor  15  and controls the rotational velocity of the AC motor  15  according to the manipulative variable indicating how deep the trigger volume  29  has been pulled. In this electric tool  1 , the tip tool is attached to the socket  23 . Controlling the rotational velocity of the AC motor  15  by operating the trigger volume  29  allows the rotational velocity of the tip tool to be controlled. 
     The electric tool  1  according to this embodiment includes the socket  23 , thus making the tip tool replaceable depending on the intended use. However, the tip tool does not have to be replaceable. Alternatively, the electric tool  1  may also be designed to allow the use of only a particular type of tip tool. 
     The torque measuring unit  26  measures the operating torque of the AC motor  15 . The torque measuring unit  26  may be a magnetostriction strain sensor which may detect torsion strain, for example. The magnetostriction strain sensor makes a coil, provided in a non-rotating portion of the AC motor  15 , detect a variation in permeability corresponding to the strain caused by the application of a torque to the output shaft  16  of the AC motor  15  and outputs a voltage signal, of which the magnitude is proportional to the magnitude of the strain. 
     The bit rotation measuring unit  25  measures the rotational angle of the output shaft  21 . In this case, the rotational angle of the output shaft  21  is equal to the rotational angle of the tip tool (bit). As the bit rotation measuring unit  25 , a photoelectric encoder or a magnetic encoder may be adopted, for example. 
     The motor rotation measuring unit  27  measures the rotational angle of the AC motor  15 . As the motor rotation measuring unit  27 , a photoelectric encoder or a magnetic encoder may be adopted, for example. 
     (3) Control Unit 
     The control unit  4  includes a computer system including one or more processors and a memory. At least some of the functions of the control unit  4  are performed by making the processor of the computer system execute a program stored in the memory of the computer system. The program may be stored in the memory. The program may also be downloaded via a telecommunications network such as the Internet or distributed after having been stored in a non-transitory storage medium such as a memory card. 
     The control performed by the control unit  4  includes the field weakening control and the regular control. In the field weakening control, the control unit  4  causes the flux-weakening current to flow from an inverter circuit section  51  through the coil  141  of the AC motor  15 . The control unit  4  does not cause the flux-weakening current to flow from the inverter circuit section  51  through the coil  141  in the regular control. When a switching condition described later are satisfied, the control performed by the control unit  4  is the field weakening control. The regular control is control performed such that a command value (target value) cid 1  of the flux-weakening current is set to 0 and the flux-weakening current converges toward the command value cid 1 . The field weakening control is control performed such that the command value cid 1  of the flux-weakening current is set to be greater than 0 and the flux-weakening current converges toward the command value cid 1 . When the command value cid 1  of the flux-weakening current is greater than 0, the flux-weakening current flows through the AC motor  15 , thereby generating the weakening flux. 
     As shown in  FIG. 1 , the control unit  4  includes a command value generating unit  41 , a velocity control unit  42 , a current control unit  43 , a first coordinate transformer  44 , a second coordinate transformer  45 , a flux control unit  46 , an estimation unit  47 , and a step-out detection unit  48 . In addition, the electric tool  1  further includes the inverter circuit section  51  and a plurality of (e.g., two in the example illustrated in  FIG. 1 ) current sensors  61  and  62 . The control unit  4  is used along with the inverter circuit section  51  and performs feedback control to control the operation of the AC motor  15 . 
     Each of the plurality of current sensors  61  and  62  includes, for example, a Hall element current sensor or a shunt resistor element. The plurality of current sensors  61  and  62  measure an electric current supplied from the power supply  32  to the AC motor  15  via the inverter circuit section  51 . In this embodiment, three-phase currents (namely, a U-phase current, a V-phase current, and a W-phase current) are supplied to the AC motor  15 . The plurality of current sensors  61  and  62  measure currents in at least two phases. In  FIG. 1 , the current sensor  61  measures the U-phase current and outputs a current measured value i u   1 , and the current sensor  62  measures the V-phase current and outputs a current measured value i v   1 . 
     The estimation unit  47  performs time differentiation on the rotational angle θ 1 , measured by the motor rotation measuring unit  27 , of the AC motor  15  to calculate an angular velocity ω 1  of the AC motor  15  (i.e., the angular velocity of the output shaft  16 ). 
     The second coordinate transformer  45  performs, based on the rotational angle θ 1 , measured by the motor rotation measuring unit  27 , of the AC motor  15 , coordinate transformation on the current measured values i u   1  and i v   1  measured by the plurality of current sensors  61  and  62 , thereby calculating current measured values id 1  and iq 1 . That is to say, the second coordinate transformer  45  transforms the current measured values i u   1  and i v   1 , corresponding to currents in two phases out of the currents in three phases, into a current measured value id 1  corresponding to a magnetic field component (d-axis current) and a current measured value iq 1  corresponding to a torque component (q-axis current). 
     The command value generating unit  41  generates a command value cω 1  of the angular velocity of the AC motor  15 . The command value generating unit  41  generates the command value cω 1  according to, for example, a magnitude corresponding to the manipulative variable indicating how deep the trigger volume  29  (see  FIG. 2 ) has been pulled. That is to say, as the manipulative variable increases, the command value generating unit  41  increases the command value cω 1  of the angular velocity accordingly. 
     The velocity control unit  42  generates a command value ciq 1  based on the difference between the command value cω 1  generated by the command value generating unit  41  and the angular velocity col calculated by the estimation unit  47 . The command value ciq 1  is a command value specifying the magnitude of the torque current (q-axis current) of the AC motor  15 . The velocity control unit  42  determines the command value ciq 1  to reduce the difference between the command value cω 1  and the angular velocity col. 
     The flux control unit  46  generates a command value cid 1  based on the angular velocity col calculated by the estimation unit  47 , a command value cvq 1  (which will be described later) generated by the current control unit  43 , and the current measured value iq 1  (q-axis current). The command value cid 1  is a command value that specifies the magnitude of the flux-weakening current (a current component that generates a magnetic flux in a d-axis direction) of the AC motor  15 . When the control performed by the control unit  4  is the regular control, the command value cid 1  generated by the flux control unit  46  is a command value for adjusting the flux-weakening current to 0. When the control performed by the control unit  4  is the field weakening control, the flux control unit  46  determines the command value cid 1  by a determination process which will be described later. 
     The current control unit  43  generates a command value cvd 1  based on the difference between the command value cid 1  generated by the flux control unit  46  and the current measured value id 1  calculated by the second coordinate transformer  45 . The command value cvd 1  is a command value that specifies the magnitude of a d-axis voltage of the AC motor  15 . The current control unit  43  determines the command value cvd 1  to reduce the difference between the command value cid 1  and the current measured value id 1 . 
     In addition, the current control unit  43  also generates the command value cvq 1  based on the difference between the command value ciq 1  generated by the velocity control unit  42  and the current measured value iq 1  calculated by the second coordinate transformer  45 . The command value cvq 1  is a command value that specifies the magnitude of a q-axis voltage of the AC motor  15 . The current control unit  43  generates the command value cvq 1  to reduce the difference between the command value ciq 1  and the current measured value iq 1 . 
     The first coordinate transformer  44  performs coordinate transformation on the command values cvd 1  and cvq 1  based on the rotational angle θ 1 , measured by the motor rotation measuring unit  27 , of the AC motor  15  to calculate command values cv u   1 , cv v   1 , and cv w   1 . Specifically, the first coordinate transformer  44  transforms the command value cvd 1  for a magnetic field component (d-axis voltage) and the command value cvq 1  for a torque component (q-axis voltage) into command values cv u   1 , cv v   1 , and cv w   1  corresponding voltages in three phases. Specifically, the command value cv u   1  corresponds to a U-phase voltage, the command value cv v   1  corresponds to a V-phase voltage, and the command value cv w   1  corresponds to a W-phase voltage. 
     The inverter circuit section  51  supplies voltages in three phases, corresponding to the command values cv u   1 , cv v   1 , and cv w   1 , respectively, to the AC motor  15 . The control unit  4  controls the power to be supplied to the AC motor  15  by performing Pulse Width Modulation (PWM) control on the inverter circuit section  51 . 
     The AC motor  15  is driven with the power (voltages in three phases) supplied from the inverter circuit section  51 , thus generating rotational driving force. 
     As a result, the control unit  4  controls the flux-weakening current such that the flux-weakening current flowing through the coil  141  of the AC motor  15  has a magnitude corresponding to the command value cid 1  generated by the flux control unit  46 . Moreover, the control unit  4  controls the angular velocity of the AC motor  15  such that the angular velocity of the AC motor  15  is an angular velocity corresponding to the command value cω 1  generated by the command value generating unit  41 . 
     The step-out detection unit  48  detects a step-out (loss of synchronism) of the AC motor  15  based on the current measured values id 1  and iq 1  acquired from the second coordinate transformer  45  and the command values cvd 1  and cvq 1  acquired from the current control unit  43 . On detecting the step-out, the step-out detection unit  48  transmits a stop signal cs 1  to the inverter circuit section  51 , thus stopping the supply of power from the inverter circuit section  51  to the AC motor  15 . 
     (4) Details of Field Weakening Control 
     Next, the determination process of the command value cid 1  by the flux control unit  46  will be described. In this embodiment, the d-axis current when the weakening flux is generated is referred to as a negative current. 
     As the command value cid 1  increases, the d-axis current adjusted in accordance with the command value cid 1  increases. The d-axis current transitions basically at 0 amperes or less (see  FIG. 4 ) except for the start of the electric tool  1 , and when the d-axis current has a negative value, the weakening flux is generated. When the d-axis current is a negative value, and as the absolute value of the d-axis current increases, the weakening flux increases. The flux control unit  46  determines the command value cid 1  of the d-axis current at predetermined time intervals (e.g., every several tens of microseconds) by the determination process shown in  FIG. 3 . 
     When the switching condition, which is predetermined, is satisfied, the control unit  4  causes the flux-weakening current to flow through the coil  141  of the AC motor  15 . That is, when the switching condition is satisfied, the control performed by the control unit  4  is the field weakening control. The switching condition includes a condition that the AC motor  15  is operating within a high-velocity range. While the AC motor  15  is operating within the high-velocity range, schematically, the rotation number of the AC motor  15  is relatively high. In the present embodiment, “while the AC motor  15  is operating within the high-velocity range” is defined by that the rotation number of the AC motor  15  is greater than or equal to a predetermined rotation number R 1  (see  FIG. 4 ) and the duty of the PWM control performed by the control unit  4  with respect to the inverter circuit section  51  is greater than or equal to a predetermined value. That is, the high-velocity range is an operation range within which the rotation number of the AC motor  15  is greater than or equal to the predetermined rotation number R 1 . Moreover, the high-velocity range is an operation range within which the duty (the degree of modulation) of the PWM control is greater than or equal to the predetermined value (hereinafter referred to as a “duty threshold”). The duty of the PWM control is a value obtained by dividing an ON time period in one period of the PWM signal by the length of the one period. The rotation number of the AC motor  15  is substantially proportional to the duty. The duty threshold is, for example, about 0.9 or 0.95. 
     Moreover, the switching condition includes the condition that the value of the torque current (the value of the q-axis current) flowing through the coil  141  of the AC motor  15  is less than or equal to a predetermined current value J 1  (see  FIG. 4 ). In the present embodiment, the control unit  4  uses the current measured value iq 1  as the value of the torque current to determine whether or not the switching condition is satisfied. Note that the control unit  4  may use, as the value of the torque current, the command value ciq 1  of the value of the torque current. 
     As described below, the switching condition is, as it turns out, the condition that the angular velocity col is greater than or equal to a reference value, the current measured value iq 1  is less than or equal to the predetermined current value J 1 , and the command value cvq 1  is greater than or equal to a reference voltage. In this case, the reference voltage corresponds to a value obtained by converting the duty threshold into a voltage. 
     The control unit  4  uses, as a value corresponding to the rotation number of the AC motor  15 , the angular velocity ω 1  calculated by the estimation unit  47 . That is, as illustrated in  FIG. 3 , the control unit  4  compares the angular velocity ω 1  with the reference value (step ST 1 ) to determine whether or not the rotation number of the AC motor  15  is greater than or equal to the predetermined rotation number R 1 . The reference value is a value obtained by converting the predetermined rotation number R 1  into an angular velocity. The reference value is stored in advance in, for example, memory of a microcontroller included in the control unit  4 . If the angular velocity ω 1  is less than the reference value (step ST 1 : YES), the control unit  4  continues comparing the angular velocity ω 1  with the reference value. In contrast, if the angular velocity ω 1  is greater than or equal to the reference value (step ST 1 : NO), the control unit  4  removes a pulsation component of the current measured value iq 1  of the q-axis current (step ST 2 ). Specifically, the control unit  4  removes the pulsation component of the current measured value iq 1  by a low pass filter having a cutoff frequency of several tens of hertz (e.g., 20 Hz). 
     The control unit  4  then compares the current measured value iq 1  with the predetermined current value J 1  (see  FIG. 4 ) (step ST 3 ). If the current measured value iq 1  is greater than the predetermined current value J 1  (step ST 3 : YES), the flux control unit  46  increases the command value cid 1  specifying the d-axis current by a predetermined amount Δi (step ST 4 ). That is, the flux control unit  46  generates the command value cid 1  which specifies a d-axis current greater than the command value cid 1  at a time point before the current measured value iq 1  is compared with the predetermined current value J 1 . The predetermined amount Δi is a predetermined fixed value and is stored in advance in, for example, the memory of the microcontroller included in the control unit  4 . 
     The flux control unit  46  thereafter performs a limiting process of the command value cid 1  (step ST 5 ). The limiting process in this case is specifically a process in which if the command value cid 1  is less than a predetermined lower limit value J 2  (see  FIG. 4 ), the command value cid 1  is changed to the predetermined lower limit value J 2 , and if the command value cid 1  is greater than the predetermined upper limit value, the command value cid 1  is changed to the predetermined upper limit value. The predetermined upper limit value is 0 amperes here. The flux control unit  46  outputs the command value cid 1  after subjected to the limiting process. 
     Moreover, if the current measured value iq 1  is less than or equal to the predetermined current value J 1  (step ST 3 : NO), the flux control unit  46  compares the command value cvq 1  specifying the q-axis voltage with the reference voltage (step ST 6 ). The reference voltage is stored in advance in, for example, the memory of the microcontroller included in the control unit  4 . If the command value cvq 1  is greater than or equal to the reference voltage (step ST 6 : NO), the flux control unit  46  decreases the command value cid 1  specifying the d-axis current by the predetermined amount Δi (step ST 7 ) and performs the limiting process (step ST 5 ) to output the command value cid 1 . When no flux-weakening current flows through the coil  141  of the AC motor  15 , decreasing the command value cid 1  in step ST 7  causes a flux-weakening current to be started to flow though the coil  141 . That is, the control performed by the control unit  4  is switched from the regular control to the field weakening control. After all, the switching condition for starting the field weakening control is the condition that the angular velocity ω 1  is greater than or equal to the reference value (step ST 1 : NO), the current measured value iq 1  is less than or equal to the predetermined current value J 1  (step ST 3 : NO), and the command value cvq 1  is greater than or equal to the reference voltage (step ST 6 : NO). 
     As already described, the reference voltage corresponds to a value obtained by converting the duty threshold of the duty of the PWM control into a voltage. That is, the flux control unit  46  determines whether or not the duty of the PWM control is greater than or equal to the duty threshold by comparing the command value cvq 1  with the reference voltage. If the command value cvq 1  is greater than or equal to the reference voltage, the duty of the PWM control may be regarded to be greater than or equal to the duty threshold. 
     In step ST 6 , if the command value cvq 1  is less than the reference voltage (step ST 6 : YES), the flux control unit  46  increases the count number by one (step ST 8 ) and compares the count number with the predetermined value (here, 100) (step ST 9 ). The count number as used herein is a count number of the number times of the determination result that the command value cvq 1  is less than the reference voltage is obtained. If the count number is less than or equal to 100 (step ST 9 : NO), the flux control unit  46  neither increases nor decreases the command value cid 1 , but the flux control unit  46  performs the limiting process (step ST 5 ), thereby outputting the command value cid 1 . In contrast, if the count number is greater than the  100  (step ST 9 : YES), the count number is initialized to 0 (step ST 10 ), and the flux control unit  46  increases the command value cid 1  by the predetermined amount Δi (step ST 11 ). Thereafter, the limiting process is performed (step ST 5 ), and the command value cid 1  is output. 
     In the following description, contents in steps ST 3  to ST 11  are summarized. If the first condition that the current measured value iq 1  of the q-axis current is greater than the predetermined current value J 1  is satisfied, the command value cid 1  of the d-axis current is increased from a negative value toward 0 (step ST 4 ). This decreases the weakening flux. In contrast, if the second condition that the current measured value iq 1  of the q-axis current is less than or equal to the predetermined current value J 1  and the command value cvq 1  of the q-axis voltage is greater than or equal to the reference voltage is satisfied, the command value cid 1  of the d-axis current is decreased from 0 or a negative value (step ST 7 ). This increases the weakening flux. Meanwhile, if the third condition that the current measured value iq 1  of the q-axis current is less than or equal to the predetermined current value J 1  and the command value cvq 1  of the q-axis voltage is less than the reference voltage is satisfied, the command value cid 1  of the d-axis current is increased from a negative value toward 0 each time the third condition is satisfied 100 times (step ST 11 ). This decreases the weakening flux. If the count number of times that the third condition is satisfied is less than or equal to 100, the command value cid 1  of the d-axis current is maintained. 
     Thus, the control unit  4  has a function of changing the magnitude of the flux-weakening current in the field weakening control. If the second condition (a predetermined increase condition) is satisfied in the field weakening control, the control unit  4  performs gradual-increase control of increasing the absolute value of the flux-weakening current over time in step ST 7 . In contrast, If the third condition (a predetermined decrease condition) is satisfied in the field weakening control, the control unit  4  performs gradual-decrease control of decreasing the absolute value of the flux-weakening current over time in steps ST 8  to ST 11 . As used herein, “over time” includes an aspect which is not that the flux-weakening current, which is a control object, is changed in one step, and the flux-weakening current stabilizes at a current value after the change but that the flux-weakening current is changed in a plurality of steps, and the value of the flux-weakening current then stabilizes. In the present embodiment, in step ST 4 , ST 7 , or ST 11 , the command value cid 1  of the d-axis current changes by the predetermined amount Δi, and the step ST 4 , ST 7 , or ST 11  is repeated a plurality of number of times, thereby eventually stabilizing the flux-weakening current. The absolute value of the flux-weakening current gradually changes over time, thereby gradually changing the rotation number of the AC motor  15 . This lessens the possibility that a worker using the electric tool  1  feels strangeness even when the rotation number is automatically changed by the control performed by the control unit  4 . 
     Here, if the command value cvq 1  is less than the reference voltage (if the second condition is satisfied) in step ST 6 , the control unit  4  performs control of increasing (gradual-increase control of) the absolute value of the flux-weakening current (d-axis current) over time in step ST 7 . In contrast, if the command value cvq 1  is greater than or equal to the reference voltage (the second condition is no longer satisfied and the third condition is satisfied) in step ST 6 , the control unit  4  performs control of decreasing (gradual-decrease control of) the absolute value of the flux-weakening current over time in steps ST 8  to ST 11 . As described above, if the count number of times that the third condition is satisfied is less than or equal to 100, the command value cid 1  of the d-axis current is maintained. Thus, the rate of change of the command value cid 1  in steps ST 8  to ST 11  is less than the rate of change of the command value cid 1  in steps ST 4  and ST 7 . That is, the increase rate of the command value cid 1  by the gradual-decrease control in the case of the third condition being continuously satisfied is less than the increase rate of the command value cid 1  in the case of the first condition being continuously satisfied and than the decrease rate of the command value cid 1  by the gradual-increase control in the case of the second condition being continuously satisfied. In sum, the rate of change of the flux-weakening current in the case of the gradual-decrease control is less than the rate of change of the flux-weakening current in the case of the gradual-increase control. Thus, for example, when the length of a time period during which the command value cvq 1  of the q-axis voltage is less than the reference voltage is substantially equal to the length of a time period during which the command value cvq 1  of the q-axis voltage is greater than or equal to the reference voltage, the flux-weakening current decreases as viewed in a time scale greater than or equal to a certain time scale. Thus, pulsation (hunting) of the flux-weakening current can be suppressed. 
       FIG. 4  shows an example of the transitions of respective parameters of the electric tool  1  over time when the AC motor  15  is controlled based on the process shown in  FIG. 3 . In  FIG. 4 , “battery current” refers to an output current of the power supply  32  of the embodiment, “battery voltage” refers to an output voltage of the power supply  32  of the embodiment, and “iq 1 ” refers to the current measured value iq 1  in the electric tool  1  of the embodiment. Moreover, in  FIG. 4 , “id 1 ” refers to the current measured value id 1  in the electric tool  1  of the embodiment, and “r 1 ” refers to the rotation number of the AC motor  15  of the electric tool  1  of the embodiment. Moreover, in  FIG. 4 , “r 2 ” refers to the rotation number of an AC motor of an electric tool according to a comparative example of the electric tool  1  of the embodiment. The electric tool according to the comparative example always maintains a state where no weakening flux flows through the AC motor. That is, in the electric tool according to the comparative example, the control performed by the control unit is always the regular control. 
     In this case, the electric tool  1  is assumed to be used as an impact screwdriver or a drill screwdriver for tightening a screw. That is, to the socket  23  (see  FIG. 2 ) of the electric tool  1 , a screwdriver is attached. A worker inserts the screwdriver into a screw at a time point before a time point T 0 . At the time point T 0 , a worker gives an operation of pulling the trigger volume  29  of the electric tool  1 , and thereby, the q-axis current (torque current) starts flowing through the AC motor  15 , so that the AC motor  15  starts rotating. Thereafter, the rotation number r 1  gradually increases in accordance with the manipulative variable indicating how deep the trigger volume  29  has been pulled. Here, the manipulative variable indicating how deep the trigger volume  29  has been pulled is maximum. Thus, the rotation number r 1  increases to an upper limit within an adjustable range. The electric tool of the comparative example is also assumed to be used as an impact screwdriver or a drill screwdriver in a similar manner to the electric tool  1  of the embodiment. Also in the electric tool of the comparative example, the rotation number r 2  gradually increases in accordance with the manipulative variable indicating ow deep the trigger volume has been pulled and increases to an upper limit within an adjustable range. 
     At a time point T 1 , the rotation number r 1  reaches the predetermined rotation number R 1 . Thereafter, at a time point T 2 , the switching condition described above is satisfied. Thus, in the electric tool  1  of the embodiment, the control performed by the control unit  4  is switched from the regular control to the field weakening control, so that the d-axis current (flux-weakening current) starts flowing. That is, at and after the time point T 2 , the current measured value id 1  of the d-axis current gradually decreases from 0. 
     As the screw is screwed into a member, the q-axis current (torque current) required to further tighten the screw may increase. As the q-axis current increases, loss in the interior resistance of the power supply  32  (battery) increases, and therefore, the battery voltage decreases. Thus, a reduction in the rotation number r 1  due to the decrease in the battery voltage can be compensated by causing the d-axis current (flux-weakening current) to flow. That is, as compared to the case where the d-axis current is not caused to flow, the reduction in the rotation number r 1  can be suppressed. In other words, the rotation number r 1  of the AC motor  15  according to the embodiment is, while the d-axis current flows, higher than the rotation number r 2  according to the comparative example. 
     In the electric tool  1  of the embodiment, at a time point T 3 , the magnitude of the current measured value id 1  of the d-axis current is a value close to the predetermined lower limit value J 2 . By the limiting process (see step ST 5 ), the current measured value id 1  of the d-axis current transitions within a range not less than the predetermined lower limit value J 2 . 
     At and after a time point T 4 , the current measured value iq 1  of the q-axis current is greater than the predetermined current value J 1  (step ST 3 : YES). Thus, the current measured value id 1  of the d-axis current increases from a negative value toward 0. Moreover, around the time point T 4 , the screw is seated on the member. The predetermined current value J 1  is set to a value less than the current measured value iq 1  of the q-axis current (torque current) when the screw is seated on the member. That is, when the screw is seated on the member and the torque is relatively large, the current measured value iq 1  of the q-axis current exceeds the predetermined current value J 1 , and in response to this, the control unit  4  thus decreases the absolute value of the d-axis current. In other words, the control unit  4  decreases the flux-weakening current. As a result, the weakening flux decreases. The decrease of the weakening flux enables the AC motor  15  to be driven by increased torque. 
     Around a time point T 5 , the magnitude of the flux-weakening current is 0. Moreover, at a time point T 6 , a worker sets the manipulative variable, indicating how deep the trigger volume  29  has been pulled, to 0, so that the AC motor  15  stops. 
     According to the embodiment described above, causing the flux-weakening current to flow through the coil  141  of the AC motor  15  can increase the rotation number of the AC motor  15  as compared to the case where the flux-weakening current is not caused to flow. This can shorten a time required for work such as tightening a screw by the electric tool  1 . In addition, the possibility that an increased rotation number may distort the shape of a hole formed in a member as a target of boring by the electric tool  1  used as a drill can be lessened. 
     Moreover, the performance characteristic of the electric tool  1  may vary due to variations of an induced electromotive voltage resulting from a production error of the AC motor  15  or variations of the voltage (the battery voltage) of the power supply  32 . Adjusting the magnitude of the flux-weakening current can correct the variations of the performance characteristics of the electric tool  1 . 
     Moreover, in the electric tool  1  of the embodiment, the weakening flux decreases when the current measured value iq 1  of the q-axis (torque component) is greater than the predetermined current value J 1 , and the weakening flux eventually reaches 0. Thus, when the torque of the AC motor  15  is relatively large, larger torque can be output. 
     That is, in a low-velocity range within which the torque of the AC motor  15  is relatively large and the rotation number of the AC motor  15  is relatively small, decreasing the weakening flux to 0 or to a relatively small value, the magnitude of the torque can be secured. In contrast, within a high-velocity range within which the rotation number of the AC motor  15  is relatively large (larger than at least the predetermined rotation number R 1  (see  FIG. 4 )), causing the flux-weakening current to flow can further increase the rotation number. That is, causing the flux-weakening current to flow enables the AC motor  15  to rotate at a further higher rotation number than the upper limit of the rotation number in the case of no flux-weakening current being caused to flow. 
     (5) Variations of Embodiment 
     Variations of the embodiment will be enumerated below. The variations described below may be accordingly combined with each other. 
     The electric tool  1  may include an operating member configured to receive, for example, an operation of setting parameters relating to operation of the electric tool  1 . The operating member may include, for example, a touch panel display configured to receive an operation input and to display information relating to the operation. 
     The field weakening control and the regular control performed by the control unit  4  may be manually changeable by an operation given to an operating member different from the trigger volume  29 . For example, the control performed by the control unit  4  may be changeable by an operation given to the touch panel display. 
     In the AC motor  15 , the rotor  13  may include the coil  141 , and the permanent magnet  131  may include the stator  14 . 
     The electric tool  1  is not limited to the impact screwdriver, the drill screwdriver, or the impact wrench. Alternatively, the electric tool  1  may be a screwdriver or a wrench having no impact mechanism  17 . Alternatively, the electric tool  1  may be a fraise, a grinder, a cleaner, or an electric tool of a kind other than these tools. 
     Here, “the AC motor  15  operates within the high-velocity range” may be defined by that the rotation number of the AC motor  15  is greater than or equal to the predetermined rotation number R 1  or by that the duty of the PWM control is greater than or equal to the duty threshold. 
     The flux control unit  46  may control such that the increase rate of the command value cid 1  of the d-axis current when the first condition is satisfied is greater than the decrease rate of the command value cid 1  of the d-axis current when the second condition is satisfied. This lessens the possibility that the d-axis current pulsates. Such a configuration can be embodied by, for example, increasing the command value cid 1  of the d-axis current each time the first condition is satisfied one time and decreasing the command value cid 1  of the d-axis current each time the second condition is satisfied a plurality of times. 
     The condition for switching the control performed by the control unit  4  to the field weakening control may be one of the following plurality of conditions or a condition obtained by accordingly combining two or more of the following plurality of conditions with each other. The plurality of conditions are, for example, a condition relating to the rotation number of the AC motor  15 , a condition relating to the duty of the PWM control, a condition relating to the q-axis current of the AC motor  15 , and a condition relating to the q-axis voltage of the AC motor  15 . 
     (6) Summary 
     The embodiment and the like described above discloses the following aspects. 
     An electric tool  1  according to a first aspect includes an AC motor  15  (an electric motor) and a control unit  4 . The AC motor  15  includes a permanent magnet  131  and a coil  141 . The control unit  4  is configured to perform control on the operation of the AC motor  15 . The control performed by the control unit  4  includes field weakening control by which the control unit  4  causes a flux-weakening current to flow through the coil  141 . The flux-weakening current is a current that generates, in the coil  141 , a magnetic flux that weakens a magnetic flux of the permanent magnet  131 . 
     With this configuration, the control unit  4  performs the field weakening control of the AC motor  15  (electric motor), thereby increasing the rotation number of the AC motor  15  as compared to the case where the field weakening control is not performed. 
     In an electric tool  1  of a second aspect referring to the first aspect, the control performed by the control unit  4  includes the field weakening control and the regular control. The control unit  4  is configured not to cause the flux-weakening current to flow through the coil  141  in the regular control. The control performed by the control unit  4  is the field weakening control when a switching condition which is predetermined is satisfied. 
     With this configuration, in the regular control, no flux-weakening current flows through the coil  141 , and therefore, the torque of the AC motor  15  (electric motor) is greater than in the case of the control performed by the control unit  4  being the field weakening control. 
     In an electric tool  1  of a third aspect referring to the second aspect, the switching condition includes a condition that the AC motor  15  (electric motor) is operating within a high-velocity range. 
     With this configuration, the control performed by the control unit  4  is automatically switched in accordance with the rotation number of the AC motor  15  (electric motor). 
     In an electric tool  1  of a fourth aspect referring to the third aspect, the high-velocity range is an operation range within which a rotation number of the AC motor  15  (electric motor) is greater than or equal to a predetermined rotation number. 
     With this configuration, the rotation number of the AC motor  15  is further increased when the rotation number of the AC motor  15  (electric motor) is relatively high. 
     In an electric tool  1  of a fifth aspect referring to the third or fourth aspect, the control unit  4  is configured to control electric power to be supplied to the AC motor  15  (electric motor) by PWM control. The high-velocity range is an operation range within which a duty of the PWM control is greater than or equal to a predetermined value. 
     With this configuration, when the duty of the PWM control is relatively large, and thus, the rotation number of the AC motor  15  (electric motor) is relatively high, the rotation number of the AC motor  15  is further increased. 
     In an electric tool  1  of a sixth aspect referring to any one of the second to fifth aspects, the switching condition includes a condition that a value of a torque current flowing through the coil  141  is less than or equal to a predetermined current value J 1 . 
     With this configuration, when the value of the torque current is relatively small, and thus, the rotation number of the AC motor  15  is relatively high, the rotation number of the AC motor  15  (electric motor) is further increased. 
     In an electric tool  1  of a seventh aspect referring to any one of the first to sixth aspects, the control unit  4  has a function of changing a magnitude of the flux-weakening current in the field weakening control. More specifically, the control unit  4  is configured to, in the field weakening control, change a magnitude of the flux-weakening current when the condition (the increase condition or the decrease condition) is satisfied. 
     This configuration enables the rotation number of the AC motor  15  (electric motor) to be more finely controlled as compared to the case where the magnitude of the flux-weakening current is constant. 
     In an electric tool  1  of an eighth aspect referring to the seventh aspect, the control unit  4  is configured to, when a predetermined increase condition is satisfied, perform gradual-increase control of increasing an absolute value of the flux-weakening current over time in the field weakening control. 
     With this configuration, the change in the rotation number of the AC motor  15  (electric motor) is moderate as compared to the case where the flux-weakening current is changed in, for example, a binary manner, and therefore, a worker easily works by using the electric tool  1 . 
     In an electric tool  1  of a ninth aspect referring to the seventh or eighth aspect, the control unit  4  is configured to, when a predetermined decrease condition is satisfied, perform gradual-decrease control of decreasing an absolute value of the flux-weakening current over time in the field weakening control. 
     With this configuration, the change in the rotation number of the AC motor  15  (electric motor) is moderate as compared to the case where the flux-weakening current is changed in, for example, a binary manner, and therefore, a worker easily works by using the electric tool  1 . 
     In an electric tool  1  of a tenth aspect referring to the eighth aspect, the control unit  4  is configured to, when a predetermined decrease condition is satisfied, perform gradual-decrease control of decreasing the absolute value of the flux-weakening current over time in the field weakening control. The predetermined decrease condition is different from the predetermined increase condition. A rate of change of the flux-weakening current in the gradual-decrease control is less than a rate of change of the flux-weakening current in the gradual-increase control. 
     This configuration lessens the possibility that the flux-weakening current pulsates along with the pulsation of parameters relating to the predetermined increase condition and the predetermined decrease condition. 
     The configurations other than the configuration of the first aspect are not essential configurations of the electric tool  1  and may accordingly be omitted. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Electric Tool 
               4  Control Unit 
               15  AC Motor (Electric Motor) 
               131  Permanent Magnet 
               141  Coil 
             J 1  Predetermined Current Value