Patent Publication Number: US-10322498-B2

Title: Electric power tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Patent Application No. 2014-213970 filed Oct. 20, 2014 and No. 2015-169406 filed Aug. 28, 2015 in the Japan Patent Office, and the entire disclosures thereof are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an electric power tool configured to generate a hitting force in a rotation direction of a tool element and transmit the hitting force to the tool element. 
     The electric power tool disclosed in Unexamined Japanese Patent Application Publication No. 2010-207951 is provided with a hitting mechanism comprising a hammer that rotates in response to a rotational force of a motor and an anvil that rotates in response to a rotational force of the hammer. The hitting mechanism is configured such that, when a torque of a predetermined value or more is externally applied to the anvil, the hammer is disengaged from the anvil to run idle, and then hits the anvil in the rotation direction. 
     According to this electric power tool, when the motor is rotated in a forward direction to fix a target object such as a screw and a nut to a plate or a bolt, it is possible to tighten the target object firmly due to the hitting of the anvil by the hammer. When loosening the tightening of the target object, the motor is rotated in a reverse direction to the direction at the time of the tightening so as to let the hammer hit the anvil. Thereby, it is possible to easily loosen the tightening of the target object. 
     SUMMARY 
     In the electric power tool described above, for example, when a user operates an operation unit such as a trigger switch with a maximum operation amount, the motor is driven with a maximum driving power of 100% driving duty thereby to rotate the motor (and a tool bit) at high speed. 
     However, in this way, if the motor is driven at high speed immediately after started to be driven, the tool bit such as a driver bit may be easily disengaged from a target object such as a screw. Tightening by the hitting mechanism may not be satisfactorily carried out. 
     For example, in a case of fixing a long screw to a workpiece by the electric power tool described above, if hitting is started in a state that the long screw is not sufficiently inserted to the workpiece, the tool bit may be disengaged from a screw head by rapid change of load and deflection of the long screw at the time when hitting is started. The workpiece, tool bit, screw head and the like may sometimes be damaged. 
     In one aspect of the present disclosure, it is desirable to be able to provide an electric power tool that can inhibit a tool element from being disengaged from a target object after hitting occurs. 
     The electric power tool in one aspect of the present disclosure comprises a motor, a hitting mechanism, a hitting detector, and a control unit. The motor generates a rotational force for rotating a tool element attached to the electric power tool. The hitting mechanism, when a torque of a predetermined value or more is applied to the tool element, generates a hitting force in a rotation direction of the tool element, and transmits the hitting force to the tool element. 
     The hitting detector detects generation of the hitting force by the hitting mechanism. The control unit controls driving of the motor such that the motor is placed into a target rotation state. The control unit further sets a control amount of the motor such that the motor rotates at a lower speed than a speed in the target rotation state during an initial driving period. When the initial driving period elapses, the control unit switches the control amount to a final control amount corresponding to the target rotation state. 
     The initial driving period is a period from when generation of the hitting force by the hitting detector is detected until a predetermined first time period elapses, after the motor is started driven. 
     According to this electric power tool, even if a rotation state requiring a maximum driving power of the motor is set as the target rotation state, increase in rotation speed of the motor is inhibited until hitting by the hitting mechanism is initiated and a predetermined period elapses, after the motor is started driven. 
     Thus, during the initial driving period, the hitting force by the hitting mechanism is reduced. It is possible to inhibit the tool element from being disengaged from a target object such as a long screw when hitting by the hitting mechanism is initiated in a state that the object is not sufficiently inserted to a workpiece. 
     In addition, if the initial driving period is set such that hitting by the hitting mechanism is performed during that period, it is possible to firmly fix an object such as a long screw to the workpiece by the hitting performed during the initial driving period. 
     When the initial driving period elapses, the control amount of the motor is switched to the final control amount and the rotation of the motor is increased. Thus, the hitting force by the hitting mechanism becomes stronger, and it is possible to inhibit time required for tightening an object such as a long screw to the workpiece from being prolonged. 
     The control unit may be configured to determine that the initial driving period has elapsed when generation of the hitting force is detected a predetermined number of times by the hitting detector, after the motor is started driven. In addition, the control unit may be configured to determine that the initial driving period has elapsed when a predetermined time elapses since generation of the hitting force has been detected by the hitting detector, after the motor is started driven. 
     In other words, the initial driving period until switching of the control amount to the final control amount after the motor is started driven may be set by the number of times of hitting by the hitting mechanism, or may be set by the time elapsed after the hitting is initiated by the hitting mechanism. 
     The control unit may be configured to drive the motor in so-called open loop control in which the driving power when the motor is driven (for example, conduction current to the motor, driving duty ratio at the time of PWM control, and the like) is simply controlled. 
     In addition, the electric power tool may comprise a speed detector configured to detect a rotation speed of the motor. In this case, the control unit may be configured to drive the motor in rotation control (so-called feedback control) in which the control amount is set such that the rotation speed detected by the speed detector becomes equal to a target rotation speed. 
     In this case, switching of the control amount of the motor along with the lapse of the initial driving period may be implemented by switching the target rotation speed in the feedback control to be higher than the speed during the initial driving period. 
     That is, when controlling the driving of the motor in the feedback control, the control amount for controlling the driving power of the motor is set in accordance with a deviation between the rotation speed of the motor and the target rotation speed. 
     Therefore, if the target rotation speed during the initial driving period is set to be low speed and the target rotation speed after the lapse of the initial driving period is switched to be higher, the above effect can be exhibited even if the control unit is already configured to drive the motor in the feedback control. 
     The electric power tool may comprise an operation unit that is configured to select an operation mode from among a plurality of operation modes different in the target rotation state, that is, for example, high speed and low speed, or high speed, medium speed and low speed, etc. 
     In this case, the control unit may be configured to set the control amount such that the rotation speed of the motor becomes high speed or low speed, or high speed, medium speed or low speed, depending on the operation mode selected via the operation unit. 
     In addition, the control unit may be configured to set the control amount of the motor such that the motor rotates at a lower speed than the speed in the target rotation state during the initial driving period, and to switch the control amount of the motor to the final control amount corresponding to the target rotation state after the lapse of the initial driving period, for each operation mode. 
     In this case, if the operation mode selected via the operation unit is a low speed mode in which the motor rotates at a lower speed than a rotation speed for the operation mode not selected, the control unit may be configured to start driving the motor with the final control amount corresponding to the target rotation state for the low speed mode. 
     That is, the control unit may be configured so as not to switch the control amount after the motor is started driven in the low speed mode, and to start driving of the motor with the final control amount corresponding to the target rotation state (in this case, low speed). 
     In this way, the control amount during the initial driving period is set to be the control amount for driving the motor at a lower speed than the speed in the target rotation state, in the low speed mode. Thus, it is possible to inhibit the rotation speed of the motor before occurrence of hitting from becoming too low, and to inhibit time required for tightening of the object from being prolonged. 
     The control unit may be configured to gradually change the control amount up to the final control amount so that the rotation speed of the motor is gradually increased upon switching the control amount of the motor to the final control amount after the lapse of the initial driving period. 
     In this case, since the rotation speed of the motor is gradually increased, it is possible to inhibit the tool element from being disengaged from the object by the switching of the control amount. 
     When the operation unit for setting the operation mode is provided, the control unit may gradually change the control amount up to the final control amount at a different rate of change for each of the plurality of operation modes, so that the rotation speed of the motor is gradually increased upon switching the control amount of the motor to the final control amount. 
     In this case, when the operation mode is a low speed mode in which the motor is rotated at low speed, the control unit may increase a rate of change of the control amount upon gradually changing the control amount up to the final control amount, as compared with a case in which the operation mode is a high speed mode in which the motor rotates at high speed. 
     In this way, when the selected operation mode is the low speed mode, the rotation state of the motor after the lapse of the initial driving period can be more quickly shifted to the target rotation state. It is possible to inhibit time required for tightening the object in the low speed mode from being prolonged. 
     Conversely, when the selected operation mode is the low speed mode, the control unit may reduce a rate of change of the control amount upon gradually changing the control amount up to the final control amount, as compared with a case in which the selected operation mode is the high speed mode. 
     In this way, when the selected operation mode is the low speed mode, the rotation state of the motor after the lapse of the initial driving period can be slowly shifted to the target rotation state. It is possible to inhibit the tool element from being disengaged from the object by switching of the control amount in the low speed mode. 
     Upon gradually changing the control amount of the motor up to the final control amount after the lapse of the initial driving period, the control unit may vary the control amount at a constant rate of change (in other words, in stages at a rate of a predetermined amount in a predetermined time). 
     In this case, the control unit may reduce the rate of change of the control amount after the lapse of the initial driving period, as compared to an initial rate of change that is a rate of change of the control amount at the time of increasing the rotation speed of the motor when the motor is started driven. 
     In this way, the rate of change of the control amount after the lapse of the initial driving period becomes less than the rate of change of the control amount when the motor is started driven so that switching of the control amount to the final control amount can be slowly carried out. Therefore, even in this way, it is possible to inhibit the tool element from being disengaged from the object due to switching of the control amount after the lapse of the initial driving period. 
     Also, upon gradually changing the control amount of the motor up to the final control amount after the lapse of the initial driving period, the control unit may vary the control amount so that a rate of change of the control amount is gradually increased in accordance with the elapsed time. 
     In this way, after the lapse of the initial driving period, the rotation speed of the motor is increased in an arched shape. Immediately after the lapse of the initial driving period, it is possible to inhibit the increase in the rotation speed of the motor and inhibit the tool element from being disengaged from the object. 
     Moreover, since the rotation speed of the motor increases in an arched shape with time elapsed from the initial driving period, it is possible to inhibit time required for the tightening of the object from being prolonged. 
     Also, in a case where the rate of change of the control amount is varied to gradually increase after the lapse of the initial driving period as such, the control unit may reduce the rate of change of the control amount as compared to the initial rate of change in a predetermined second period after the lapse of the initial driving period. 
     In this case, the control unit may increase the rate of change of the control amount with time as compared to the initial rate of change, after the lapse of the second period. As a result, it is possible to inhibit time required for the tightening of the object from being prolonged, while inhibiting the tool element from being disengaged from the object after the lapse of the initial driving period. 
     The control unit may control the rotation state of the motor after the lapse of the initial driving period to the target rotation state by increasing a duty ratio of a PWM signal used to drive and control the motor as the control amount of the motor. 
     In addition, the control unit may control the rotation state of the motor after the lapse of the initial driving period to the target rotation state by increasing a conduction angle at the time when the motor is driven as the control amount of the motor. 
     In this case, the control unit may, for example, fix the duty ratio of the PWM signal used in driving the motor to 100% to control timings of starting and ending conduction to a motor coil. 
     As a result, it is not necessary for the control unit to switch a switching element for conduction control at high speed. As compared to a case where the control unit is adapted to control the duty ratio of the PWM signal, it is possible to reduce heat generation of the switching element. 
     The hitting mechanism may be configured in any way so as to generate a hitting force in a rotation direction of a tool element and transmit the hitting force to the tool element when a torque of a predetermined value or more is applied to the tool element. 
     For example, the hitting mechanism may comprise a hammer configured to rotate by a rotational force generated by the motor, an anvil configured to rotate in response to the rotation of the hammer, and an attachment portion for attaching the tool element to the anvil. 
     In this case, the hitting mechanism may be configured such that, when a torque of a predetermined value or more is applied to the anvil, the hammer is disengaged from the anvil to run idle and hits the anvil in the rotation direction of the tool element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Hereinafter, with reference to the accompanying drawings, a description of the present disclosure will be given by way of example in which: 
         FIG. 1  is a vertical cross-sectional view of a rechargeable impact driver of an embodiment; 
         FIG. 2  is a block diagram showing an electrical configuration of a motor driving unit mounted on the rechargeable impact driver; 
         FIG. 3  is a flowchart illustrating a control process executed by a control circuit; 
         FIG. 4  is a flowchart illustrating a number-of-hits process executed in S 140  of  FIG. 3 ; 
         FIG. 5  is a flowchart illustrating a motor control process executed in S 150  in  FIG. 3 ; 
         FIG. 6  is a flowchart illustrating a motor driving process executed in S 350  in  FIG. 5 ; 
         FIG. 7  is a time chart showing a control target set by motor control of the embodiment and changes in rotation speed and current of a motor; 
         FIG. 8  is a flowchart illustrating a motor driving process of a first variation; 
         FIG. 9  is a time chart showing a control target and changes in rotation speed and current of a motor in the first variation; 
         FIG. 10  is a time chart showing a control target and changes in rotation speed and current of a motor in a second variation; 
         FIG. 11  is a flowchart illustrating a motor driving process of the second variation; 
         FIGS. 12A and 12B  are time charts for explaining an example of a setting procedure of a control target by motor control of a fourth variation; 
         FIG. 13  is a time chart for explaining another example of the setting procedure of the control target by the motor control of the fourth variation; 
         FIG. 14  is a time chart showing a control target and changes in rotation speed and current of a motor in a sixth variation; 
         FIG. 15  is a flowchart illustrating a motor driving process of the sixth variation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In this exemplary embodiment, description will be given of a case where the present disclosure is applied to a rechargeable impact driver (hereinafter referred to as “driver”)  1  which is one example of an electric power tool. 
     As shown in  FIG. 1 , the driver  1  comprises a tool body  10 , and a battery pack  30  that supplies electric power to the tool body  10 . 
     The tool body  10  comprises a housing  2  and a grip portion  3 . The housing  2  accommodates a motor  4  and a hitting mechanism  6  to be described later. The grip portion  3  is formed so as to protrude from a lower portion of the housing  2  (lower side of  FIG. 1 ). 
     Inside the housing  2 , the motor  4  is accommodated in its rear portion (left side of  FIG. 1 ), and a bell-shaped hammer case  5  is assembled to the front of the motor  4  (right side of  FIG. 1 ). The hitting mechanism  6  is accommodated in the hammer case  5 . 
     That is, inside the hammer case  5 , a spindle  7  with a hollow portion formed at its rear end is coaxially accommodated, and a ball bearing  8  provided at a rear end of the hammer case  5  pivotally supports a rear end outer periphery of the spindle  7 . 
     At a front portion of the ball bearing  8  in the spindle  7 , a planetary gear mechanism  9  comprising two planetary gears which are pivotally supported in point symmetry with respect to a rotation axis meshes with an internal gear  11  formed in a rear end inner peripheral surface of the hammer case  5 . 
     The planetary gear mechanism  9  is configured to mesh with a pinion  13  formed at a tip portion of an output shaft  12  of the motor  4 . 
     The hitting mechanism  6  comprises the spindle  7 , a hammer  14  fitted over the spindle  7 , an anvil  15  pivotally supported in front of the hammer  14 , and a coil spring  16  that urges the hammer  14  forward. 
     That is, the hammer  14  is coupled with the spindle  7  in a manner integrally rotatable and axially movable, and is urged forward (anvil  15  side) by the coil spring  16 . 
     A tip portion of the spindle  7  is pivotally supported by being loosely and coaxially inserted to a rear end of the anvil  15 . 
     The anvil  15  receives a rotational force and a hitting force by the hammer  14  thereby to axially rotate. The anvil  15  is supported in a manner freely rotatable about its axis and axially non-displaceable by a bearing  20  provided at a tip end of the housing  2 . 
     At a tip end of the anvil  15 , a chuck sleeve  19  is provided for mounting various tool bits (not shown) such as a driver bit, a socket bit and so on. 
     All of the output shaft  12  of the motor  4 , the spindle  7 , the hammer  14 , the anvil  15  and the chuck sleeve  19  are coaxially arranged. 
     In the front end surface of the hammer  14 , two hitting protrusions  17 ,  17  for providing a hitting force to the anvil  15  are circumferentially protruded at an interval of 180°. 
     The anvil  15  has at its rear end two hitting arms  18 ,  18  formed at an interval of 180°. The two hitting arms  18 ,  18  are configured such that each of the hitting protrusions  17 ,  17  of the hammer  14  can abut on the two hitting arms  18 ,  18 . 
     When the hammer  14  is urged and held to the front end side of the spindle  7  by an urging force of the coil spring  16 , each of the hitting protrusions  17 ,  17  of the hammer  14  is brought into contact with one of the hitting arms  18 ,  18  of the anvil  15 . 
     In this state, as the spindle  7  is rotated via the planetary gear mechanism  9  by a rotational force of the motor  4 , the hammer  14  is rotated together with the spindle  7 , and the rotational force of the hammer  14  is transmitted to the anvil  15  through the hitting protrusions  17 ,  17  and the hitting arms  18 ,  18 . 
     Thereby, a driver bit or the like attached to the tip end of the anvil  15  is rotated, and tightening of screw is enabled. 
     When a predetermined value or more of torque is applied from outside of the anvil  15  to the anvil  15  due to tightening of the screw to a predetermined position, the rotational force (torque) of the hammer  14  to the anvil  15  also becomes equal to or more than a predetermined value. 
     As a result, the hammer  14  is displaced rearward against the urging force of the coil spring  16 . Each of the hitting protrusions  17 ,  17  of the hammer  14  climbs over one of the hitting arms  18 ,  18  of the anvil  15 . In other words, each of the hitting protrusions  17 ,  17  of the hammer  14  is once deviated from one of the hitting arms  18 ,  18  of the anvil  15 , and runs idle. 
     In this way, when the hitting protrusions  17 ,  17  of the hammer  14  climb over the hitting arms  18 ,  18  of the anvil  15 , the hammer  14  is displaced forward again by the urging force of the coil spring  16  while rotating together with the spindle  7 . Then, each of the hitting protrusions  17 ,  17  of the hammer  14  hits one of the hitting arms  18 ,  18  of the anvil  15  in the rotation direction. 
     Accordingly, in the driver  1  of the present embodiment, each time a torque of a predetermined value or more is applied to the anvil  15 , hitting by the hammer  14  of the anvil  15  is repeated. As the hitting force of the hammer  14  is applied intermittently to the anvil  15 , it is possible to tighten a screw with high torque. 
     The grip portion  3  is a portion where an operator grips when using the driver  1 . A trigger switch  21  is provided above the grip portion  3 . 
     The trigger switch  21  comprises a trigger  21   a  to be pulled by the operator and a switch body  21   b . The switch body  21   b  is configured such that the switch body  21   b  is turned on and off by the pulling operation of the trigger  21   a  and a resistance value changes in accordance with an operation amount (amount of pull) of the trigger  21   a.    
     Also, on top of the trigger switch  21  (lower end side of the housing  2 ), a forward-reverse changeover switch  22  for switching a rotating direction of the motor  4  between a forward direction (in this embodiment, clockwise direction in a state of viewing front from the rear end side of the tool) and a reverse direction (rotation direction reverse to the forward direction) is provided. 
     Also, at a bottom front of the housing  2 , an illumination LED  23  is provided for illuminating the front of the driver  1  when the trigger  21   a  is pulled. 
     In addition, at a lower front portion of the grip portion  3 , a display panel  24  is provided for displaying an operation mode (high speed, low speed) of the driver  1 , remaining energy of a battery  29  and the like. 
     In the vicinity of the display panel  24 , a speed selector switch  26  is provided for setting the operation mode of the driver  1  to either of the low speed mode or the high speed mode (see  FIG. 2 ). 
     At a lower end of the grip portion  3 , a battery pack  30  that accommodates the battery  29  is detachably attached. The battery pack  30  is attached by being slid from the front to rear side with respect to the lower end of the grip portion  3 , at the time of the attachment. 
     The battery  29  accommodated in the battery pack  30  is a repeatedly rechargeable secondary battery such as a lithium ion battery in this embodiment. 
     The motor  4  is a three-phase brushless motor with an armature winding of each of U, V and W phases, in this embodiment. The motor  4  is provided with a rotation sensor  50  for detecting a rotational position (angle) of the motor  4  (see  FIG. 2 ). 
     The rotation sensor  50 , for example, comprises three Hall elements arranged corresponding to the respective phases of the motor  4 , and comprises a Hall IC or the like that is configured to generate a rotation detection signal for each predetermined rotational angle of the motor  4 . 
     Inside the grip portion  3 , a motor driving unit  40  is provided that receives power supply from the battery pack  30  and drives and controls the motor  4  (see  FIG. 2 ). 
     The motor driving unit  40 , as shown in  FIG. 2 , comprises a drive circuit  42 , a gate circuit  44 , a control circuit  46  and a regulator  48 . 
     The drive circuit  42  receives power supply from the battery  29  to flow a current to each phase winding of the motor  4 . In this embodiment, the drive circuit  42  comprises a three-phase full-bridge circuit having six switching elements Q 1  to Q 6 . Each of the switching elements Q 1  to Q 6  is a MOSFET, in this embodiment. 
     In the driving circuit  42 , the three switching elements Q 1  to Q 3  are provided between each of U, V, and W terminals of the motor  4  and a power supply line coupled to a positive electrode of the battery  29 , as so-called high-side switches. 
     The other three switching elements Q 4  to Q 6  are provided between each of the U, V, and W terminals of the motor  4  and a ground line coupled to a negative electrode of the battery  29 , as so-called low-side switches. 
     The gate circuit  44  turns on and off each of the switching elements Q 1  to Q 6  in the drive circuit  42  in accordance with a control signal output from the control circuit  46 , thereby to flow a current to each of the phase windings of the motor  4  to rotate the motor  4 . 
     The control circuit  46  is a microcomputer comprising a CPU, a ROM, a RAM, etc., in this embodiment. The trigger switch  21  (switch body  21   b  in particular), the forward-reverse changeover switch  22 , the illumination LED  23 , the display panel  24  and the speed selector switch  26  described above are coupled to the control circuit  46 . 
     Further, in the motor driving unit  40 , a current detection circuit  54  for detecting a current flowing to the motor  4  is provided in a current path extending from the drive circuit  42  to the negative electrode of the battery  29 . The current detection circuit  54 , for example, comprises a resistor for current detection and an input circuit for inputting to the control circuit  46  a voltage between both ends of the resistor as a current detection signal. 
     To the control circuit  46 , the current detection signal from the current detection circuit  54 , and the rotation detection signal from the rotation sensor  50  provided in the motor  4  are also input. 
     The control circuit  46 , when the trigger switch  21  is operated, obtains the rotational position and rotation speed of the motor  4  based on the rotation detection signal from the rotation sensor  50 , and drives the motor  4  in a predetermined direction of rotation in accordance with the rotation direction setting signal from the forward-reverse changeover switch  22 . 
     In addition, the control circuit  46 , when the motor  4  is driven, sets a speed command value of the motor  4  in accordance with the operation amount (amount of pull) of the trigger switch  21  and the operation mode (high speed or low speed) set via the speed selector  26 . 
     The control circuit  46  controls the rotation speed of the motor  4  by setting a driving duty ratio of each of the switching elements Q 1  to Q 6  included in the driving circuit  42  in accordance with the speed command value and outputting to the gate circuit  44  a control signal (PWM signal) in accordance with the driving duty ratio. 
     The control circuit  46 , separately from the drive control for driving the motors  4 , also performs control of turning on the illumination LED  23 , etc. when the motor is driven. 
     The regulator  48  receives power supply from the battery  29  to generate a constant power supply voltage Vcc (for example, direct current (DC) 5V) required to operate the control circuit  46 . The control circuit  46  operates when the power supply voltage Vcc is supplied from the regulator  48 . 
     Now, a control process to be executed by the control circuit  46  will be described. 
     As shown in  FIG. 3 , the control circuit  46  repeatedly executes a series of processes S 120  to S 160  (S represents a step) at the predetermined control cycle (time base). 
     That is, the control circuit  46  waits for a predetermined control period to elapse by determining whether the time base has elapsed in S 110 . When it is determined in S 110  that the time base has elapsed, the process moves to S 120 . 
     In S 120 , by checking signal inputs from the trigger switch  21 , the forward-reverse changeover switch  22  and the speed selector switch  26 , a switch operation detection process for detecting the operation of the respective switches is performed. 
     In subsequent S 130 , an A/D conversion process is performed in which an A/D conversion is applied to the operation amount (amount of pull) of the trigger switch  21 , the detection signals from the current detection circuit  54  and the rotation sensor  50 , the battery voltage supplied from the battery pack  30 , and the like. 
     In subsequent S 140 , a number of hits process is executed in which hitting by the hitting mechanism  6  and the number of times of occurrence (number of hits) are detected from changes in the rotation speed of the motor  4  obtained by the detection signal from the rotation sensor  50 . 
     In subsequent S 150 , a motor control process is executed in which the motor  4  is driven and controlled based on the operation amount of the trigger switch  21 , the operation mode set via the speed selector  26 , and the result of detection of hitting by the number of hits process. 
     Finally, in S 160 , in accordance with a command (operation setting) from a user, a display process is performed which includes a display of the remaining energy of the battery  29  on the display panel  24 , lighting of the illumination LED  23 , etc. The process moves to S 110 . 
     Now, the number of hits process to be executed in S 140  will be described. 
     As shown in  FIG. 4 , when the number of hits process is started, it is determined first in S 210  whether the trigger switch  21  is operated by the user, and is ON. If the trigger switch is ON, the process proceeds to S 220 . It is determined whether to drive the motor  4  based on the operation amount of the trigger switch  21 , etc. 
     If it is determined not to drive the motor  4  in S 220 , or if it is determined in S 210  that the trigger switch  21  is OFF, the process proceeds to S 295 . In S 295 , a time counter, a number of hits flag, and a number of hits determination flag to be described later, are all cleared. The number of hits process is terminated. 
     On the other hand, if it is determined to drive the motor  4  in S 220 , the process proceeds to S 230 . It is determined whether the number of hits flag is set. In S 230 , if it is determined that the number of hits flag is not set (that is, if it is cleared), the process proceeds to S 240 . A hitting determination process is executed. 
     In the hitting determination process, hitting by the hitting mechanism  6  is detected from changes in the rotation speed of the motor  4 , and the number of times of detection (that is, number of hits) is counted. 
     Note that detection of hitting in the hitting determination process can be implemented by detecting changes in the current detected by the current detection circuit  54 . Detection of hitting is also implemented by detecting vibration generated by hitting with an acceleration sensor or the like. 
     When the hitting determination process of S 240  is complete, the process proceeds to S 250 . It is determined whether the number of hits counted in the hitting determination process is equal to or more than a specified number of times. If the number of hits is equal to or more than the specified number of times, the number of hits flag is set in S 260 , and the number of hits process is terminated. Unless the number of hits is equal to or more than the specified number of times, the number of hits process is terminated. 
     Next, in S 230 , when it is determined that the number of hits flag is set, the process proceeds to S 270 . A time counter for counting drive time of the motor  4  from when the number of hits flag is set in S 260  is incremented (+1). The process moves to S 280 . 
     In S 280 , based on the count value of the time counter, it is determined whether time elapsed from when the number of hits reaches the specified number of times is equal to or more than a specified time set in advance. If the elapsed time is equal to or more than the specified time, the process proceeds to S 290  to set the number of hits determination flag. 
     If it is determined in S 290  to set number of hits determination flag, or it is determined in S 280  that the elapsed time has not reached the specified time, the number of hits process is terminated. 
     Next, the motor control process executed in S 150  of  FIG. 3  will be described. 
     As shown in  FIG. 5 , when the motor control process is started, it is determined first in S 310  whether the trigger switch  21  is operated by the user and is ON. If the trigger switch  21  is ON, the process proceeds to S 320 . It is determined whether to drive the motor  4  based on the operation amount of the trigger switch  21 . 
     If it is determined not to drive the motor  4  in S 320 , or if it is determined in S 310  that the trigger switch  21  is OFF, the process proceeds to S 400 . A motor stopping process for stopping the motor  4  is executed. 
     In the motor stopping process, the motor  4  is stopped either by generating a braking force in the motor  4  via the drive circuit  42 , or simply cutting off the power supply to place the motor  4  into a free-run state. 
     In subsequent S 410 , a high speed switching flag and a driving duty ratio that is an output value (in other words, control amount of the motor  4 ) for driving the motor  4  via the drive circuit  42  to be described later are both cleared. The motor control process is terminated. 
     Next, in S 320 , if it is determined to drive the motor  4 , the process moves to S 330 . It is determined whether the operation mode set through the speed selector  26  is a high speed mode. 
     In S 330 , if it is determined that the operation mode of the driver  1  is not a high speed mode (that is, low speed mode), the process proceeds to S 340 . A low speed setting process is executed in which a low speed target value for driving the motor  4  in the low speed mode is set. The process proceeds to S 350 . 
     Note that the low speed target value is an output value (driving duty ratio) required to control the rotation speed at no-load of the motor  4  to a set rotation speed N 1  (see  FIG. 7 ) in accordance with the operation amount of the trigger switch  21 . 
     In S 340 , using a map or an arithmetic expression for the low speed mode having the operation amount of the trigger switch  21  as a parameter, the low speed target value (low speed target Duty shown in  FIG. 7 ) is set. 
     Next, in S 330 , if it is determined that the operation mode of the driver  1  is the high speed mode, the process proceeds to S 360 . It is determined whether the high speed switching flag is set. 
     In S 360 , if it is determined that the high speed switching flag is not set, the process proceeds to S 370 . It is determined whether the number of hits determination flag is set. 
     In S 370 , if it is determined that the number of hits determination flag is not set, the low speed setting process is executed in S 340  and the process proceeds to S 350 . 
     On the other hand, if it is determined in S 360  that the high speed switching flag is set, or, if it is determined in S 370  that the number of hits determination flag is set, the process proceeds to S 380 . A high speed setting process is executed in which a high speed target value for driving the motor  4  in the high speed mode is set. 
     In subsequent S 390 , the high speed switching flag is set, and the process proceeds to S 350 . 
     Note that the high speed target value set in S 380  is an output value (driving duty ratio) required to control the rotation speed at no-load of the motor  4  to a setting rotation speed N 2  (see  FIG. 7 ) in accordance with the operation amount of the trigger switch  21 . 
     In S 380 , using a map or arithmetic expression for the high speed mode having the operation amount of the trigger switch  21  as a parameter, the high speed target value (high speed target Duty shown in  FIG. 7 ) is set. 
     Next, in S 350 , based on the low speed target value set in S 340  or the high speed target value set in S 380 , the output value that is the control amount for actually controlling the motor  4  is set. Then, the motor driving process is executed in which a control signal is generated based on the set output value and is output to the gate circuit  44 . After the execution of the motor driving process of S 350 , the motor control process is terminated. 
     Next, the motor driving process will be described along the flowchart shown in  FIG. 6 . 
     As shown in  FIG. 6 , in the motor control process, it is determined first in S 510  whether the high speed switching flag is set. 
     If the high speed switching flag is not set, a stage time counter to be described later is cleared in S 520 , and the output value (driving duty ratio) to be used for the drive control of the motor  4  is updated by adding a predetermined addition value A to the current output value in S 530 . The process proceeds to S 540 . 
     In S 540 , it is determined whether the output value exceeds the low speed target value. If the output value exceeds the low speed target value, the output value is set to be the low speed target value in S 550 , and the motor driving process is terminated. If the output value does not exceed the low speed target value, the motor driving process is terminated. 
     Note that the output value is used to generate a control signal (PWM signal) for applying PWM control to the power supplied to the motor  4  via the gate circuit  44  in a control signal output process separately executed in the control circuit  46 . 
     The output value, since it is the driving duty ratio and is “zero” at the time when the motor is stopped, gradually increases from the initial value “zero” by the addition value A to reach the low speed target value (low speed target Duty shown in  FIG. 7 ), due to repetitive execution of the process of S 530  after the motor  4  is started driven. 
     Next, in S 510 , if it is determined that the high speed switching flag is set, that is, when the operation mode is the high speed mode, the number of hits by the hitting mechanism  6  reaches the specified number, the specified time has elapsed and the high speed target value has been set in S 380 , the process moves to the S 560 . 
     In S 560 , it is determined whether the output value is equal to the high speed target value. If the output value is equal to the high speed target value, the motor driving process is terminated. If the output value is not equal to the high speed target value, the process proceeds to S 570 . 
     In S 570 , the stage time counter is incremented (+1), and the process moves to S 580 . In S 580 , it is determined whether a stage time counted by the stage time counter is equal to a predetermined set time. Unless the stage time is equal to the set time, the motor driving process is terminated. If the stage time is equal to the set time, the process moves to S 590 . 
     In S 590 , the stage time counter is cleared, and the output value (driving duty ratio) is updated by adding a predetermined addition value B to the current output value. Note that the addition value B is set to be less than the addition value A. 
     In subsequent S 600 , it is determined whether the output value has exceeded the high speed target value. If the output value has exceeded the high speed target value, the output value is set to be the high speed target value in S 610 , and the motor drive process is terminated. If the output value has not exceeded the high speed target value, the motor driving process is terminated. 
     As described above, in the driver  1  of the present embodiment, when the trigger switch  21  is operated and the motor  4  is driven, the output value (driving duty ratio) to the motor  4  is gradually increased to the low speed target value (low speed target Duty) (S 530  to S 550 ). 
     Therefore, as shown in  FIG. 7 , after the motor  4  is started driven (time t 1 ), the rotation speed of the motor  4  is gradually increased to and held at the set rotational speed N 1 . 
     Therefore, when the screw is tightened using the driver  1  of the present embodiment, it is possible to inhibit increase in rotation speed of the motor  4  immediately after the tightening is started, and inhibit the driver bit from being disengaged from the screw, which may result in damaging the screw head and the workpiece. 
     Further, when a load begins to be applied to the motor  4  by the screw tightening (time t 2 ), the rotation speed of the motor  4  is reduced, and then hitting by the hitting mechanism  6  is started. 
     If the operation mode is the low speed mode, the output value (driving duty ratio) to the motor  4  is held in the low speed target Duty, and the hitting by the hitting mechanism  6  is continued. 
     On the other hand, if the operation mode is the high speed mode, when the number of hits by the hitting mechanism  6  reaches the specified number of times, and then, when the specified time elapses (time t 3 ), the control target is switched from the low speed target value (low speed target Duty) to the high speed target value (high speed target Duty). 
     When the control target is switched, the addition value B is added to the output value (driving duty ratio) whenever the stage time counted by the stage time counter reaches the set time, and the output value (driving duty ratio) increases to the high speed target value (high speed target Duty) in stages (S 560  to S 610 ). 
     That is, when the operation mode is the high speed mode, the driving power of the motor  4  is switched from low power to high power during a initial driving period from when the motor  4  is started driven until the number of hits by the hitting mechanism  6  reaches the specified number of time and the specified time elapses, and after the lapse of the initial driving period. 
     Therefore, in the high speed mode, the hitting force by the hitting mechanism  6  can be reduced more than normal during the initial driving period of the motor  4 . It is possible to inhibit the driver bit from being disengaged from the screw when hitting by the hitting mechanism  6  is started while the screw is not sufficiently inserted to the workpiece. 
     Further, when the initial driving period elapses in high speed mode, the control target of the motor  4  is switched to the high speed target value that is a final control amount (high speed target Duty) and the hitting force by the hitting mechanism  6  is increased. Thus, it is possible to shorten the time required to tighten the screw to the workpiece. 
     Further, upon switching the driving duty ratio that is the output value from the low speed target Duty to the high speed target Duty in the high speed mode, the driving duty ratio is increased in stages. Thus, it is also possible to inhibit the rotation speed of the motor  4  from being rapidly increased by the switching of the output value. 
     In particular, in this embodiment, the addition value B used to increase the driving duty ratio of the low speed target Duty to the high speed target Duty is less than the addition value A used to increases the driving duty ratio from the initial value “0” at the time when driving is stopped to the low speed target Duty. 
     Therefore, according to this embodiment, it is possible to inhibit increase in rotation speed of the motor  4  as compared to the increase at the time when the motor  4  is started driven, by reducing a rate of increase of the target Duty upon increasing the driving duty ratio from the low speed target Duty to the high speed target Duty as compared to a rate of increase immediately after the motor  4  is started driven (that is, initial rate of change). 
     Therefore, according to this embodiment, even with this configuration, it is possible to inhibit the driver bit from being disengaged from the screw after the initiation of hitting by the hitting mechanism  6 . 
     In the present embodiment, the speed selector switch  26  corresponds to an example of an operation unit of the present disclosure. The control circuit  46  corresponds to an example of a hitting detector and a control unit of the present disclosure. In particular, the number of hits process executed by the control circuit  46  functions as an example of the hitting detector of the present disclosure, and the motor control process executed by the control circuit  46  functions as an example of the control unit of the present disclosure. 
     One embodiment of the present disclosure has been described in the above. The present disclosure is not to be limited to the above embodiment and can take various forms within the scope not departing from the gist of the present disclosure. 
     (First Variation) 
     For example, in the above embodiment, it is described that, upon switching the output value that is the control amount of the motor  4  in the high speed mode, the driving duty ratio of the motor  4  is switched from the low speed target Duty to the high speed target Duty. 
     On the other hand, it is also possible to achieve the same effect as in the above embodiment, even if a conduction angle at the time of driving the motor  4  is changed as the control amount of the motor  4 , instead of the driving duty ratio of the PWM signal. For this purpose, the motor driving process may be executed according to the procedure shown in  FIG. 8 . 
     Note that, in the motor driving process, the target Duty to be set in accordance with the operation amount of the trigger switch  21  is used as the driving duty ratio of the motor  4 , so that the target Duty becomes 100% when the operation amount of the trigger switch  21  is the maximum operation amount. 
     Therefore, in the motor driving process of  FIG. 8 , a description is given in an assumption that an advance angle value that is a timing to start conduction to each phase winding when the motor  4  is driven and the conduction angle which is the conduction period are set as the output value. 
     As shown in  FIG. 8 , in the motor control process, it is determined first in S 710  whether the high speed switching flag is set. If the high speed switching flag is not set, the stage time counter and a conduction angle switching flag are cleared in S 720 . 
     In subsequent S 730 , the advance angle value to be used for the drive control of the motor  4  is updated by adding a predetermined addition value X to the current advance angle value. The process proceeds to S 740 . 
     In S 740 , it is determined whether the advance angle value has exceeded the low speed target value (e.g., 22.5°). If the advance angle value has exceeded the low speed target value, the advance angle value is set to be the low speed target value in S 750 . The motor driving process is then terminated. If the advance angle value has not exceeded the low speed target value, the motor driving process is terminated. 
     Note that an initial value of the advance angle value is less than the low speed target value (e.g., zero). Therefore, by the process of S 730 , the advance angle value is gradually increased when the motor  4  is driven, and the conduction start timing of the motor  4  moves toward the advance angle side, thereby to increase the driving power of the motor  4  (and thus driving torque). 
     Next, in S 710 , when it is determined that the high speed switching flag is set, the process proceeds to S 760 . It is determined whether the advance angle value is equal to the high speed target value (e.g., 37.5°). Unless the advance angle value is equal to the high speed target value, the process proceeds to S 770 . If the advance angle value is equal to the high speed target value, the process moves to S 820 . 
     In S 770 , the stage time counter is incremented (+1). The process moves to S 780 . In S 780 , it is determined whether the stage time counted by the stage time counter is equal to a predetermined set time. If the stage time is not equal to the set time, the motor driving process is terminated. If the stage time is equal to the set time, the process moves to S 790 . 
     In S 790 , the stage time counter is cleared. The advance angle value is updated by adding a predetermined addition value Y to the current advance angle value. The process proceeds to S 800 . 
     In S 800 , it is determined whether the advance angle value has exceeded the high speed target value. If the advance angle value has exceeded the high speed target value, the advance angle value is set to be the high speed target value in S 810 . The motor driving process is terminated. If the advance angle value has not exceeded the high speed target value, the motor driving process is terminated. 
     Next, in S 820 , it is determined whether the conduction angle switching flag is set. If the conduction angle switching flag is set, the motor driving process is terminated. Unless the conduction angle switching flag is set (that is, if it is cleared), the process proceeds to S 830 . 
     In S 830 , the stage time counter is incremented (+1). In subsequent S 840 , it is determined whether the stage time counted by the stage time counter is equal to the predetermined set time. Note that the set time may be the same as the set time for use in the determination of S 780 , or may be different. 
     In S 840 , if it is determined that the stage time is not equal to the set time, the motor driving process is terminated. If it is determined that the stage time is equal to the set time, the process proceeds to S 850 . The conduction angle at the time of driving the motor  4  is increased. 
     That is, in S 850 , a conduction end timing to the phase windings is delayed by a predetermined angle (e.g., 15°) so that the conduction angle at the time of driving the motor becomes equal to the high speed target value (e.g., 150°) that is greater than the low speed target value (e.g., 120°). 
     In subsequent S 860 , the stage time counter is cleared, and the conduction angle switching flag is set. The motor driving process is terminated. 
     Thus, in the motor driving process shown in  FIG. 8 , the advance angle value at the time of driving the motor  4  is gradually increased up to the low speed target value (low speed target advance angle value) after the motor  4  is started driven (time t 1 ) as shown in  FIG. 9 . Thereby, the rotation speed of the motor  4  is increased to the set rotation speed N 1  (S 720  to S 750 ). 
     When a load is started to be applied to the motor  4  by screw tightening (time t 2 ), the rotation speed of the motor  4  is reduced. Then, although the hitting by the hitting mechanism  6  is started, the advance angle value is held to the low speed target advance angle value if the operation mode is the low speed mode, and the conduction angle to the motor  4  is also fixed to a predetermined low speed target value. 
     On the other hand, if the operation mode is the high speed mode, and when the number of hits by hitting mechanism  6  reaches the specified number of times and when the specified time elapses (at time t 3 ), the control target of the advance angle value is switched from the low speed target advance angle value to a high speed target advance angle value. 
     When the control target of the advance angle value is switched, the addition value Y is added to the advance angle value each time the stage time counted by the stage time counter reaches the set time, and the advance angle value increases up to the high speed target advance angle value in stages (S 770  to S 810 ). 
     That is, in case that the operation mode is the high speed mode, when the number of hits by the hitting mechanism  6  reaches the specified number of times and the specified time elapses from when the motor  4  is started driven, the advance angle value at the time of driving the motor  4  is corrected toward the advance angle side than the advance angle value in the low speed mode, and the conduction angle to the motor  4  is gradually increased. 
     Therefore, in the high speed mode, the hitting force by the hitting mechanism  6  is reduced to the same extent as in the low speed mode during the initial driving period of the motor  4 . Even in the first variation, the same effect as in the above embodiment can be achieved. 
     In the high speed mode, as shown in  FIG. 9 , when the advance angle value reaches the high speed target advance angle value (at time t 4 ), the conduction end timing to each of the phase windings is delayed by a predetermined angle after a predetermined set time elapses, so that the conduction angle becomes equal to the high speed target value (e.g. 150°) (S 830  to S 860 ). 
     As a result, according to the first variation, the rotation speed of the motor  4  is further increased by switching the conduction angle in the high speed mode. It is possible to shorten the time required to tighten the screw to the workpiece. 
     In the first variation, if the operation amount of the trigger switch  21  is the maximum operation amount, the driving duty ratio of the motor  4  can be 100%. Thus, as compared with a case in which the driving duty ratio of the motor  4  is controlled, it is possible to reduce heat generation of the switching element inside the drive circuit  42 . 
     (Second Variation) 
     In the above embodiment and variation, when switching the control amount (driving duty ratio or advance angle value) to the final control amount (high speed target Duty or high speed target advance angle value) in the high speed mode, the control amount is gradually changed at a constant rate of change to be determined by the addition value B or Y. 
     However, upon switching the control amount to the final control amount as such, the rate of change (rate of increase) of the control amount may be changed so as to gradually increase in accordance with the elapsed time, as illustrated in  FIG. 10 . 
     That is, in  FIG. 10 , in the driver  1  of the embodiment, when the operation mode is the high speed mode, the initial driving period elapses, and the control target of the motor  4  is switched to the high speed target Duty, the driving duty ratio is changed (increased) in an arched shape. 
     Specifically, the rate of increase of the driving duty ratio after the lapse of the initial driving period (time t 3  or later) is less than the rate of increase immediately after the motor  4  is started driven immediately after the lapse of the initial driving period, and then gradually increases to eventually become more than the rate of increase immediately after the motor  4  is started driven. 
     Thus, if the driving duty ratio after the lapse of the initial driving period is increased in an arched shape up to the high speed target Duty, it is possible to inhibit the increase in rotation speed of the motor  4  immediately after the lapse of the initial driving period. 
     In particular, in the time chart shown in  FIG. 10 , the rate of increase of the driving duty ratio immediately after the lapse of the initial driving period is less than the rate of increase immediately after the motor  4  is started driven. It is possible to satisfactorily inhibit rapid increase in the rotation speed of the motor  4 . 
     Therefore, according to the second variation, it is possible to more reliably inhibit the driver bit from being disengaged from the screw after the hitting is started. 
     Furthermore, since the rotation speed of the motor is increased in an arched shape with the time elapsed from the initial driving period, it is possible to inhibit the time required for the tightening of the screw from being prolonged. 
     In order to change the driving duty ratio in an arched shape after the lapse of the initial driving period as such, the motor driving process shown in  FIG. 6  may be changed as shown in  FIG. 11 . This change will be explained below. 
     In the motor driving process shown in  FIG. 11 , the processes of S 582  and S 584  are executed from when it is determined in S 510  that the high speed switch flag is set until when it is determined in S 560  that the output value is equal to the high speed target value or it is determined in S 580  that the stage time is equal to the set time. 
     In S 582 , by updating (+1) a switching time counter which is cleared with the stage time counter in S 520 , time elapsed from when the high speed switching flag is set until when the output value reaches the high speed target value is counted. 
     In subsequent S 584 , on the basis of the count value of the switching time counter (that is, elapsed time after the end of the initial driving period), the addition value B is set. The process of S 584  is a process for changing the output value in an arched shape by increasing the addition value B in accordance with the elapsed time after the end of the initial driving period. To set the addition value B, an arithmetic expression having the count value of the switching time counter as a parameter, or the map is used. 
     In S 584 , when the addition value B is set, the process proceeds to S 590 . By adding the set addition value B to the output value, the output value is updated. The process of S 600  and the subsequent steps are executed. 
     Thus, after the lapse of the initial driving period, the driving duty ratio of the motor  4  that is the output value is increased in an arched shape as shown in  FIG. 10 . The above effect can be exhibited. 
     (Third Variation) 
     In the above embodiment and variations, the initial driving period after the motor  4  is started driven until the control target of the driving duty ratio is switched from the low speed target Duty to the high speed target Duty is a period until when the number of hits by the hitting mechanism  6  reaches the specified number of times and then the predetermined set time elapses. 
     However, the initial driving period may be defined simply by the number of times of hits by the hitting mechanism  6 , or may be defined only by the elapsed time after the motor  4  is started driven. 
     (Fourth Variation) 
     In the above embodiment and variations, it is described that the control amount (driving duty ratio or advance angle value) is not switched in the low speed mode. 
     This is because, in the low speed mode, the driving duty ratio or the advance angle value to be the final target is low, and it is considered that, even if the motor  4  is driven and controlled with the control amount, the driver bit will never be disengaged from the screw when hitting occurs. 
     However, even in the low speed mode, sometimes the driver bit is disengaged from the screw when hitting occurs. In such cases, the driving duty ratio may be switched, similar to the case in the high speed mode. 
     In the above embodiment and the variations, the rechargeable impact driver  1  is described in which the operation mode can be switched to two stages of low speed and high speed via the speed selector switch  26 . However, the present disclosure can be further applied, for example, to a rechargeable impact driver in which the operation mode can be switched to three stages of low speed, medium speed and high speed, or even more stages, similar to the case in the above embodiment. 
     Upon enabling the operation mode to be switched to three stages, the driving duty ratio may be switched even in the low speed mode and the medium speed mode, similar to the case in the high speed mode, as illustrated in  FIG. 12A or 13 . 
     Note that, upon switching the driving duty ratio to such a plurality of operation modes, a motor control process for each operation mode (see  FIG. 5 ) may be performed per operation mode, and the control amount (driving duty ratio or conduction angle) between the initial driving period and a final driving period after the lapse of the initial driving period may be switched. 
     Also in this case, as is apparent from the enlarged view shown in  FIG. 12A , upon changing the control amount up to the final control amount after the lapse of the initial driving period, the rate of change of the control amount for each operation mode may be set, for example, by setting the addition value B or Y different in each operation mode. 
     Note that in the enlarged view of  FIG. 12A , the rate of change (slope to a direction of increase) of the driving duty ratio upon changing the driving duty ratio up to the final target Duty is greater in the low speed mode than in the high speed mode. However, as shown in the enlarged view of  FIG. 12B , the rate of change in high speed mode may be greater than the rate of change in the low speed mode. In other words, a difference in the rate of change may be set as appropriate depending on the characteristics of the motor to be controlled (i.e., tool). 
     As described in the second variation,  FIG. 13  shows that the control amount is varied in an arched shape by increasing the rate of change of the control amount over time. In this case, a map used to set the addition values B and Y in accordance with the elapsed time may be prepared for each operation mode. 
     (Fifth Variation) 
     In the above embodiment and variations, it is described that, if the operation mode is the high speed mode, the control amount (driving duty ratio or conduction angle) of the motor  4  is always switched. 
     However, this switching is not necessary to be performed at all times. For example, the user may be able to select whether to perform similar switching control to that of the above embodiment, or to perform control with the final control amount in the high speed mode without performing the switching control. 
     In this way, it is possible to further improve usability of the rechargeable impact driver  1 . 
     (Sixth Variation) 
     In the above embodiment and the variations, it is described that the control circuit  46  sets the control amount (driving duty ratio or conduction angle) of the motor  4  based on the speed command value set in accordance with the operation mode (high speed or low speed). 
     However, the control circuit  46  may be configured such that the target rotation speed of the motor  4  is set in accordance with the operation mode (high speed or low speed), and, at the time of driving of the motor  4 , the motor  4  is driven and controlled so that the rotation speed of the motor  4  becomes equal to the target rotation speed. 
     For example, as shown in  FIG. 14 , the target rotation speeds NT 1 , NT 2  are set in accordance with the operation mode (low speed mode, high speed mode), and feedback control is applied to the motor  4  so that the rotation speed of the motor  4  becomes equal to the set target rotation speed NT 1 , NT 2 . 
     In this sixth variation as well, similar to the embodiment described above, the motor  4  may be driven and controlled, in the high speed mode, at the target rotation speed NT 1  in the low speed mode during the initial driving period of time t 1  to t 3 , and the target rotation speed may be switched to the target rotation speed NT 2  after the lapse of the initial driving period. 
     Note that  FIG. 14  shows that, after the target rotation speed is switched with the lapse of the initial driving period, then the driving duty ratio that is the control amount is gradually (specifically, in an arched shape, as in the second variation) changed until the rotation speed of the motor  4  reaches the target rotation speed NT 2 . 
     In the case of controlling the rotation speed of the motor  4  as such, the target rotation speed NT 1  may be set as a target rotation state of the motor  4  in S 340  of the motor control process shown in  FIG. 5 , and the target rotation speed NT 2  may be set as the target rotation state of the motor  4  in S 380 . 
     In this case, the motor driving process of S 350  may be implemented by the procedure shown in  FIG. 15 . Hereinafter, the motor driving the process of  FIG. 15  will be explained. 
     In the motor driving process of  FIG. 15 , similar to the motor driving process shown in  FIG. 11 , when it is determined in S 510  that the high speed switching flag is not set, the stage time counter and the switching time counter are cleared in S 520 . 
     In subsequent S 522 , it is determined whether a rotation speed command to the motor  4  is less than the target rotation speed NT 1 . If the rotation speed command is less than the target rotation speed NT 1 , the process proceeds to S 532 . The rotation speed command is updated by adding the addition value A to the rotation speed command. 
     If the rotation speed command is updated in S 532 , or it is determined that the rotation speed command has reached the target rotation speed NT 1  in S 522 , the process proceeds to S 620 . 
     On the other hand, if the high speed switching flag is set, it is determined in S 562  whether the rotation speed command is less than the target rotation speed NT 2 . 
     If the rotation speed command is less than the target rotation speed NT 2 , the stage time counter is incremented (+1) in S 570 . It is determined in S 580  whether the stage time counted by the stage time counter is equal to the set time. 
     If it is determined in S 562  that the rotation speed command has reached the target rotation speed NT 2 , or if it is determined in S 580  that the stage time is not equal to the set time, the process proceeds to S 620 . 
     If it is determined in S 580  that the stage time is equal to the set time, the switching time counter is updated (+1) in S 582 , and the addition value B is set in S 584  based on the count value of the switching time counter, similar to the case of the motor driving process shown in  FIG. 11 . 
     In S 592 , the stage time counter is cleared. In subsequent S 594 , the rotation speed command is updated by adding the addition value B to the rotation speed command. The process proceeds to S 620 . 
     In S 620 , on the basis of the speed difference between the rotation speed of the motor  4  and the rotation speed command, the output value (driving duty ratio) is updated (increased or reduced). The motor driving process is terminated. 
     As a result, the rotation speed of the motor  4  is controlled to the target rotation speed NT 1  or NT 2 . 
     The embodiment and variations of the present disclosure have been described in the above. However, the present disclosure is not to be limited to a rechargeable impact driver, and can be applied to all kinds of electric power tools provided with a hitting mechanism driven by a motor, such as, for example, an impact wrench. 
     In the above embodiment and variations, the motor  4  is described as a three-phase brushless motor, but may be any motor that can rotationally drive the hitting mechanism  6 . That is, for example, the present disclosure is not limited to battery powered tools, and may be applied to electric power tools that receive power supply via a code, or may be electric power tools configured to rotationally drive a tool element by an alternate current (AC) motor. 
     In the above embodiment and variations, the control circuit  46  is a microcomputer, but may be a combination of various individual electronic parts, may be an ASIC (Application Specified Integrated Circuit), may be a programmable logic device such as, for example, a FPGA (Field Programmable Gate Array), or may be a combination thereof.