Patent Publication Number: US-10780563-B2

Title: Electric power tool and method of controlling rotational speed of motor in electric power tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Patent Application No. 2016-199173, filed on Oct. 7, 2016; the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an electric power tool. 
     The hammer drill disclosed in Japanese Unexamined Patent Application Publication No. 2008-178935 is configured to perform so-called soft no load control. Under soft no load control, when a tip tool, such as a hammer bit, is out of contact with a work piece and a load is not imposed on the tip tool (i.e., no load is imposed on the tip tool), a motor is rotated at a low rotational speed independently of a command from a user. 
     SUMMARY 
     To perform such soft no load control, whether a tip tool is under load conditions should be detected. In addition, to detect a load imposed on the tip tool, as disclosed in the above mentioned Publication, current flowing through the motor is usually used. 
     To be specific, when the value of current flowing through the motor reaches a predetermined value, the hammer drill determines that the tip tool is under load conditions, and increases the rotational speed of the motor from a low rotational speed that is given when the tip tool is under no-load conditions. 
     However, when the drive mode of the hammer drill is set to the hammer mode in which the tip tool is not rotated but caused to only perform a hammering operation and the tip tool is under load conditions, current does not dramatically increase. 
     For this reason, in the hammer mode, a load on the tip tool due to hammering cannot be detected and the motor cannot be rotated at high speed for hammering in some cases. 
     It is preferable in one aspect of the present disclosure to detect a load imposed from a work piece to a tool bit and increase a rotational speed of a motor when an electric power tool is operated in a hammer mode in which the electric power tool performs only a hammering operation. 
     An electric power tool of one aspect of the present disclosure includes a main body, a motor, a tool holder, a hammer, a motion converter, a rotation transmitter, a first load detector, a second load detector, and a motor controller. The motor is provided to the main body. The tool holder is provided to the main body and holds a tool bit such that the tool bit is reciprocatable in an axial direction of the tool bit. The hammer is provided to the main body and reciprocates the tool bit held by the tool holder in the axial direction to hammer a work piece. 
     The motion converter is provided to the main body and converts rotation of the motor to linear motion and transmits the linear motion to the hammer. The rotation transmitter is provided to the main body and transmits the rotation of the motor to the tool holder and rotatively drives the tool bit in a circumferential direction of the tool bit. 
     The first load detector detects, based on information indicating a drive state of the motor, a load imposed from the work piece to the tool bit. The second load detector detects, based on information indicating the behavior of the main body, a load imposed from the work piece to the tool bit. 
     The motor controller controls drive of the motor based on a command rotational speed commanded from an outside of the electric power tool. The motor controller sets an upper limit of rotational speed of the motor to a predetermined no-load rotational speed in response to no-load on the tool bit being detected by both the first load detector and the second load detector. 
     In the electric power tool with this configuration, even when the electric power tool is operated in a drive mode in which the electric power tool performs only a hammering operation, a load imposed from the work piece to the tool bit is detected and the motor can be driven at the command rotational speed. 
     The electric power tool may include a mode switcher that is configured to selectively set a drive mode of the tool bit to any one of a hammer mode, a hammer drill mode, and a drill mode. The hammer mode is a mode in which the tool bit reciprocates in the axial direction, the hammer drill mode is a mode in which the tool bit reciprocates in the axial direction and rotates in the circumferential direction, and the drill mode is a mode in which the tool bit rotates in the circumferential direction. 
     During the drive mode is set to the hammer drill mode or the drill mode and the tool bit is in rotation, current flowing through the motor increases upon the tool bit being in contact with the work piece and imposed with a load. Consequently, the first load detector can detect a load on the tool bit based on the drive state of the motor. 
     In the hammer mode, the tool bit only reciprocates in the axial direction. Hence, even if the tool bit hammers the work piece and a load is imposed from the work piece to the tool bit, the drive state of the motor does not dramatically change. Accordingly, the first load detector cannot detect a load on the tool bit in some cases. 
     When the drive mode is set to the hammer mode or the hammer drill mode, hammering the work piece with the tool bit applies a reaction force to the main body due to hammering, so that the main body vibrates. 
     For this reason, the electric power tool includes the second load detector in addition to the first load detector. 
     In the electric power tool, in any drive mode selected from the group including the hammer mode, the hammer drill mode, and the drill mode, a load on the tool bit can be rapidly detected and a motor can be driven at a command rotational speed. 
     The mode switcher may be configured in any manner to selectively set the drive mode and may be, for example, configured to selectively transmit the rotation of the motor to the motion converter and/or the rotation transmitter to selectively set the drive mode. 
     The first load detector may include a current detector configured to detect a value of current flowing through the motor. In this case, the first load detector may detect a load on the tool bit in response to the value of the current detected by the current detector exceeding a predetermined first threshold. 
     The second load detector may include an acceleration sensor that is configured to detect at least acceleration of the main body in the axial direction of the tool bit. In this case, the second load detector may detect a load on the tool bit in response to acceleration detected by the acceleration sensor exceeding a predetermined second threshold. 
     The acceleration sensor may output a detection signal indicating the detected acceleration. In this case, the second load detector may detect a load on the tool bit based on an acceleration that is calculated based on the detection signal with an unwanted low-frequency signal component removed by a high-pass filter. 
     The high-pass filter may include an analog filter or digital filter. 
     If the high-pass filter includes the digital filter, a higher accuracy of detecting the acceleration can be obtained than in the case where the analog filter removes the unwanted signal component from the detection signal. 
     To be more specific, if the high-pass filter includes the analog filter, it may take time to stabilize the detection signal outputted from a circuit including the high-pass filter, since a reference voltage of the circuit may rapidly increase from 0 V to a specified voltage immediately after the electric power tool is supplied with electric power. 
     If the detection signal is subjected to the filtering process by the digital filter, a signal level of the detection signal immediately after the supply of electric power can be set to an initial value, so that fluctuations in the detection signal (data) can be reduced. 
     Consequently, the acceleration can be accurately detected from immediately after the supply of electric power to the electric power tool. Thus, the risk that a load on the tool bit cannot be detected due to a detection error of the acceleration can be reduced. 
     The first load detector may measure a first time and a second time, detect a load on the tool bit in response to the first time reaching a first threshold time, and detect no-load on the tool bit in response to the second time reaching a second threshold time. The first time is a time period during which the value of the current exceeds the first threshold, the second time is a time period during which the value of the current is at or below the first threshold, and the first threshold time and the second threshold time are different from each other. 
     The second load detector may measure a third time and a fourth time, detect a load on the tool bit in response to the third time reaching a third threshold time, and detect no-load on the tool bit in response to the fourth time reaching a fourth threshold time. The third time is a time period during which the acceleration exceeds the second threshold, the fourth time is a time period during which the acceleration is at or below the second threshold, and the third threshold time and the fourth threshold time are different from each other. 
     Setting a time required for determining a load or no-load on the tool bit as described above can reduce failures in determination of a load or no-load on the tool bit caused by noise and the like. 
     The first threshold time may be shorter than the second threshold time. The third threshold time may be shorter than the fourth threshold time. In this case, a load on the tool bit can be detected earlier than no-load on the tool bit. Thus, the delay time upon switching of the rotational speed of the motor from the no-load rotational speed to the command rotational speed can be shortened. 
     Accordingly, the rotational speed of the motor rapidly rises when a load is imposed on the tool bit, allowing chipping or drilling of the work piece to be satisfactorily performed. In addition, for example, switching of the rotational speed of the motor to low speed due to the detection of no-load on the tool bit can be restrained in the middle of chipping operation. In other words, in the electric power tool, a reduction in work efficiency can be restrained. 
     The second load detector may be separated from the motor controller. For example, the second load detector may be installed in a portion of the main body where large vibration occurs, while the motor controller may be installed in a portion of the main body where large vibration is less likely to occur. In this case, the second load detector can readily detect vibrations of the main body, while the motor controller can restrain degradations due to vibrations of the main body. 
     The motor controller may rotate the motor at a constant speed corresponding to the command rotational speed or the no-load rotational speed. 
     The electric power tool may include an upper-limit speed setter that is configured to be operated by an operator of the electric power tool and set an upper limit of the command rotational speed, and a speed change commander that is configured to be operated by the operator and change the command rotational speed in accordance with an amount of operation. 
     In this case, the motor controller may set the command rotational speed according to the amount of operation of the speed change commander, using the upper limit, which is set by the upper-limit speed setter, as a maximum rotational speed. 
     With such a configuration, the operator can set the maximum rotational speed of the motor through the upper-limit speed setter, and command to set a given rotational speed at or below the set maximum rotational speed as the command rotational speed, thereby increasing usability of the electric power tool. 
     The no-load rotational speed may be a constant rotational speed. In this case, the upper-limit speed setter may be able to set upper limit of the command rotational speed to a rotational speed in a range of a rotational speed higher than the no-load rotational speed to a rotational speed lower than the no-load rotational speed. 
     With such a configuration, the operator can set the command rotational speed to a rotational speed lower than the no-load rotational speed, thereby setting the operating characteristics of the electric power tool variously. 
     The motor controller may gradually change the rotational speed of the motor upon switching from no-load conditions to load conditions and/or from the load conditions to the no-load conditions. The no-load conditions are conditions in which no-load on the tool bit is detected, and the load conditions are conditions in which a load on the tool bit is detected. 
     With such a configuration, when the tool bit is made in contact with the work piece or separated from the work piece, a rapid change in the rotational speed of the motor and thus a strangeness that the operator feels can be restricted. 
     To gradually change the rotational speed of the motor, for example, either a rate (i.e., gradient) of change of the command rotational speed or a rate (i.e., gradient) of change of a duty ratio of a Pulse Width Modulation (PWM) signal used to drive the motor may be limited. Also, a rate (i.e., gradient) of change of current flowing through the motor may be limited. 
     The main body may be able to be attached with an external unit. Examples of the external unit may include a dust collector device, a water injection device, a lighting device, and a blower. Attaching the external unit to the main body hinders vibrations of the main body in some cases. In other words, attaching the external unit to the main body decreases a load detection sensitivity of the second load detector. 
     Accordingly, the motor controller may change, in response to the external unit being attached to the main body, conditions under which the upper limit of the rotational speed of the motor is set to the no-load rotational speed. 
     The motor controller may change the conditions such as the threshold and the like for load determination such that the upper limit of the rotational speed of the motor is barely set to the no-load rotational speed. 
     The motor controller may control, in response to the external unit being attached to the main body, drive of the motor in accordance with the command rotational speed independently of detection results from the first load detector and the second load detector. 
     In this case, since the external unit is attached to the main body, when the second load detector cannot detect a load on the tool bit based on the behavior of the main body, the risk that the motor cannot be driven at the command rotational speed can be reduced. 
     Another aspect of the present disclosure is a method of controlling a rotational speed of a motor of an electric power tool. The method includes detecting, based on first information indicating a drive state of the motor, a load imposed from a work piece to the tool bit, the motor being provided to a main body of the electric power tool, the tool bit being provided to the main body so as to reciprocate in an axial direction of the tool bit and so as to rotate in a circumferential direction of the tool bit; detecting, based on second information indicating a behavior of the main body, a load imposed from the work piece to the tool bit; and setting an upper limit of rotational speed of the motor to a predetermined no-load rotational speed in response to no-load imposed from the work piece to the tool bit being detected based on both the first information and the second information. 
     Such a method can provide effects similar to those provided by the above-described electric power tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example embodiment of the present disclosure will be described hereinafter with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a structure of a hammer drill of one embodiment; 
         FIG. 2  is a perspective view of the external view of the hammer drill; 
         FIG. 3  is a side view of the hammer drill with a dust collector device attached thereto; 
         FIG. 4  is a block diagram showing an electrical configuration of a drive system of the hammer drill; 
         FIG. 5  is a flow chart of control process executed in a control circuit in a motor controller; 
         FIG. 6  is a flow chart showing details of an input process shown in  FIG. 5 ; 
         FIG. 7  is a flow chart showing details of a motor control process shown in  FIG. 5 ; 
         FIG. 8  is a flow chart showing details of a soft no load process shown in  FIG. 7 ; 
         FIG. 9  is a flow chart of a current load detection process executed in an A/D conversion process shown in  FIG. 5 ; 
         FIG. 10  is a flow chart showing details of an output process shown in  FIG. 5 ; 
         FIG. 11  is a flow chart showing details of a motor output process shown in  FIG. 10 ; 
         FIG. 12  is a flow chart of an acceleration load detecting process executed in an acceleration detecting circuit in a twisted-motion detector; 
         FIG. 13A  is a flow chart of a part of a twisted-motion detecting process executed in the acceleration detecting circuit in the twisted-motion detector; 
         FIG. 13B  is a flow chart showing the rest of the twisted-motion detecting process; 
         FIG. 14  is a diagram for explaining an operation of a high-pass filter in detection process shown in  FIGS. 12, 13A, and 13B  by a comparison with that of an analog filter; and 
         FIG. 15  is a time chart showing determination of a load and no-load and an operation of rotational speed control. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A hammer drill  2  of this embodiment is configured to perform chipping or drilling on a work piece (e.g., concrete) by a hammering by a tool bit  4 , such as a hammer bit, along the longer axis of the tool bit  4  or rotating it about the longer axis. 
     As shown in  FIG. 1 , the hammer drill  2  includes a main body housing  10  defining the contour of the hammer drill  2 . The tool bit  4  is detachably attached to the tip of the main body housing  10  through a tool holder  6 . The tool holder  6  has a cylindrical shape. 
     The tool bit  4  is inserted in a bit insertion hole  6   a  in the tool holder  6  and held by the tool holder  6 . The tool bit  4  can reciprocate along the longer axis of the tool bit  4  against the tool holder  6  but its rotational motion about the longer axis of the tool bit  4  against the tool holder  6  is restricted. 
     The main body housing  10  includes a motor housing  12  and a gear housing  14 . The motor housing  12  houses a motor  8 . The gear housing  14  houses a motion converting mechanism  20 , a hammering element  30 , a rotation transmitting mechanism  40 , and a mode switching mechanism  50 . 
     The main body housing  10  is connected to a hand grip  16  on the opposite side to the tool holder  6 . The hand grip  16  includes a hold part  16 A which is held by an operator. This hold part  16 A extends in a direction orthogonal to the longer axis of the tool bit  4  (i.e., the center shaft of the tool holder  6 ) (the vertical direction in  FIG. 1 ), and a part of the hold part  16 A is on the extension (i.e., the longer axis) of the tool bit  4 . 
     A first end of the hold part  16 A (i.e., the end adjacent to the longer axis of the tool bit  4 ) is connected to the gear housing  14 , and a second end of the hold part  16 A (i.e., the end remote from the longer axis of the tool bit  4 ) is connected to the motor housing  12 . 
     The hand grip  16  is fixed to the motor housing  12  such that it can swing about a support shaft  13 . The hand grip  16  and the gear housing  14  are connected to each other through a vibration-insulating spring  15 . 
     The spring  15  restrains vibrations that occur in the gear housing  14  (i.e., the main body housing  10 ) due to a hammering operation of the tool bit  4 , so that vibrations from the main body housing  10  to the hand grip  16  are restrained. 
     In the description below, for convenience of description, the side on which the tool bit  4  is disposed along the longer axis direction parallel with the longer axis of the tool bit  4  is defined as the front side. The side on which the hand grip  16  is disposed along the longer axis direction is defined as the back side. The side on which a joint between the hand grip  16  and the gear housing  14  is disposed along a direction which is orthogonal to the longer axis direction and in which the hold part  16 A extends (i.e., the vertical direction of  FIG. 1 ) is defined as the upper side. The side on which a joint between the hand grip  16  and the motor housing  12  is disposed along the vertical direction of  FIG. 1  is defined as the lower side. 
     Further, in the description below, the Z axis is defined as an axis that extends along the longer axis of the tool bit  4  (i.e., the center shaft of the tool holder  6 ), the Y axis is defined as an axis that is orthogonal to the Z axis and extends in the vertical direction, and the X axis is defined as an axis that is orthogonal to the Z axis and the Y axis and extends in the horizontal direction (i.e., the width direction of the main body housing  10 ) (see  FIG. 2 ). 
     In the main body housing  10 , the gear housing  14  is disposed on the front side and the motor housing  12  is disposed on the lower side of the gear housing  14 . In addition, the hand grip  16  is joined to the back side of the gear housing  14 . 
     In this embodiment, the motor  8  housed in the motor housing  12  is a brushless motor but not limited to a brushless motor in the present disclosure. The motor  8  is disposed such that the output shaft  8 A (rotation shaft) of the motor  8  intersects the longer axis of the tool bit  4  (i.e., the Z axis). In other words, the output shaft  8 A extends in the vertical direction of the hammer drill  2 . 
     As shown in  FIG. 2 , in the gear housing  14 , a holder grip  38  is attached to the outer area of the tip region from which the tool bit  4  protrudes, through an annular fixer member  36 . Like the hand grip  16 , the holder grip  38  is configured to be gripped by the user. To be specific, the user grips the hand grip  16  with one hand and the holder grip  38  with the other hand, thereby securely holding the hammer drill  2 . 
     As shown in  FIG. 3 , a dust collector device  66  is mounted to the front side of the motor housing  12 . To mount the dust collector device  66 , as shown in  FIGS. 1 and 2 , a depressed portion is provided on the lower and front portion of the motor housing  12  (i.e., the lower and front portion of the motor  8 ) for fixation of the dust collector device  66 . A connector  64  for electrical connection to the dust collector device  66  is provided in the depressed portion. 
     Further, a twisted-motion detector  90  is accommodated in a lower portion of the motor housing  12  (i.e., in a lower portion of the motor  8 ). When the tool bit  4  is rotated for a drilling operation and the tool bit  4  fits in the work piece, the twisted-motion detector  90  detects twisting of the main body housing  10 . 
     Battery packs  62 A and  62 B serving as the power source of the hammer drill  2  are provided on the back side of the container region of the twisted-motion detector  90 . The battery packs  62 A and  62 B are detachably attached to a battery port  60  provided on the lower side of the motor housing  12 . 
     The battery port  60  is higher than the lower end surface of the container region of the twisted-motion detector  90  (i.e., the bottom surface of the motor housing  12 ). The lower end surfaces of the battery packs  62 A and  62 B attached to the battery port  60  flush with the lower end surface of the container region of the twisted-motion detector  90 . 
     A motor controller  70  is provided on the upper side of the battery port  60  in the motor housing  12 . The motor controller  70  controls drive of the motor  8 , receiving electric power from the battery packs  62 A and  62 B. 
     The rotation of the output shaft  8 A of the motor  8  is converted to a linear motion by the motion converting mechanism  20  and then transmitted to the hammering element  30 . The hammering element  30  generates impact force in the direction along the longer axis of the tool bit  4 . The rotation of the output shaft  8 A of the motor  8  is decelerated by the rotation transmitting mechanism  40  and transmitted also to the tool bit  4 . In other words, the motor  8  rotatively drives the tool bit  4  about the longer axis. The motor  8  is driven in accordance with the pulling operation on a trigger  18  disposed on the hand grip  16 . 
     As shown in  FIG. 1 , the motion converting mechanism  20  is disposed on the upper side of the output shaft  8 A of the motor  8 . 
     The motion converting mechanism  20  includes a countershaft  21 , a rotating object  23 , a swing member  25 , a piston  27 , and a cylinder  29 . The countershaft  21  is disposed to intersect the output shaft  8 A and is rotatively driven by the output shaft  8 A. The rotating object  23  is attached to the countershaft  21 . The swing member  25  is swung in the back and forth direction of the hammer drill  2  with the rotation of the countershaft  21  (the rotating object  23 ). The piston  27  is a bottomed cylindrical member slidably housing a striker  32  which will be described later. The piston  27  reciprocates in the back and forth direction of the hammer drill  2  with the swing of the swing member  25 . 
     The cylinder  29  is integrated with the tool holder  6 . The cylinder  29  houses the piston  27  and defines a back region of the tool holder  6 . 
     As shown in  FIG. 1 , the hammering element  30  is disposed on the front side of the motion converting mechanism  20  and on the back side of the tool holder  6 . The hammering element  30  includes the above-described striker  32  and an impact bolt  34 . The striker  32  serves as a hammer and strikes the impact bolt  34  disposed on the front side of the striker  32 . 
     The space in the piston  27  on the back side of the striker  32  defines an air chamber  27   a , and the air chamber  27   a  serves as an air spring. Accordingly, the swing of the swing member  25  in the back and forth direction of the hammer drill  2  causes the piston  27  to reciprocate in the back and forth direction, thereby driving the striker  32 . 
     In other words, the forward motion of the piston  27  causes the striker  32  to move forward by the act of the air spring and strike the impact bolt  34 . Accordingly, the impact bolt  34  is moved forward and strikes the tool bit  4 . Consequently, the tool bit  4  hammers the work piece. 
     In addition, the backward motion of the piston  27  moves the striker  32  backward and thereby makes the pressure of the air in the air chamber  27   a  positive with respect to atmospheric pressure. Further, reaction force generated when the tool bit  4  hammers the work piece also moves the striker  32  and the impact bolt  34  backward. 
     This causes the striker  32  and the impact bolt  34  to reciprocate in the back and forth direction of the hammer drill  2 . The striker  32  and the impact bolt  34 , which are driven by the act of the air spring of the air chamber  27   a , move in the back and forth direction, following the motion of the piston  27  in the back and forth direction. 
     As shown in  FIG. 1 , the rotation transmitting mechanism  40  is disposed on the front side of the motion converting mechanism  20  and on the lower side of the hammering element  30 . The rotation transmitting mechanism  40  includes a gear deceleration mechanism. The gear deceleration mechanism includes a plurality of gears including a first gear  42  rotating with the countershaft  21  and a second gear  44  to be engaged with the first gear  42 . 
     The second gear  44  is integrated with the tool holder  6  (specifically, the cylinder  29 ) and transmits the rotation of the first gear  42  to the tool holder  6 . Thus, the tool bit  4  held by the tool holder  6  is rotated. The rotation of the output shaft  8 A of the motor  8  is decelerated by, in addition to the rotation transmitting mechanism  40 , a first bevel gear that is provided at the front tip of the output shaft  8 A and a second bevel gear that is provided at the back tip of the countershaft  21  and engages with the first bevel gear. 
     The hammer drill  2  of this embodiment has three drive modes including a hammer mode, a hammer drill mode, and a drill mode. 
     In the hammer mode, the tool bit  4  performs a hammering operation along the longer axis direction, thereby hammering the work piece. In the hammer drill mode, the tool bit  4  performs a rotation operation about the longer axis in addition to a hammering operation, so that the work piece is drilled while being hammered by the tool bit  4 . In the drill mode, the tool bit  4  does not perform a hammering operation and only performs a rotation operation, so that the work piece is drilled. 
     The drive mode is switched by the mode switching mechanism  50 . The mode switching mechanism  50  includes rotation transmitting members  52  and  54  shown in  FIG. 1  and a switching dial  58  shown in  FIG. 3 . 
     The rotation transmitting members  52  and  54  are generally cylindrical members and movable along the countershaft  21 . The rotation transmitting members  52  and  54  are spline-engaged with the countershaft  21  and rotate in cooperation with the countershaft  21 . 
     The rotation transmitting member  52  moving toward the back side of the countershaft  21  is engaged with an engagement groove on the front of the rotating object  23  and transmits the rotation of the motor  8  to the rotating object  23 . Consequently, the drive mode of the hammer drill  2  is set to the hammer mode or the hammer drill mode. 
     The rotation transmitting member  54  moving toward the front side of the countershaft  21  is engaged with the first gear  42  and transmits the rotation of the motor  8  to the first gear  42 . Consequently, the drive mode of the hammer drill  2  is set to the hammer drill mode or the drill mode. 
     The switching dial  58  turned by the user displaces the rotation transmitting members  52  and  54  on the countershaft  21 . The switching dial  58  is turned and set to any of the three positions shown in  FIG. 3 , thereby setting the drive mode of the hammer drill  2  to any of the modes: the hammer mode, the hammer drill mode, and the drill mode. 
     The structures of the motor controller  70  and the twisted-motion detector  90  will now be described with reference to  FIG. 4 . 
     The twisted-motion detector  90  includes an acceleration sensor  92  and an acceleration detecting circuit  94 . The acceleration sensor  92  and the acceleration detecting circuit  94  are mounted on a common circuit board and contained in a common case. 
     The acceleration sensor  92  detects accelerations (more specifically, values of accelerations) in the directions along three axes (i.e., the X axis, the Y axis, and the Z axis). 
     The acceleration detecting circuit  94  subjects detection signals from the acceleration sensor  92  to process to detect twisting of the main body housing  10 . 
     To be specific, the acceleration detecting circuit  94  includes a micro controller unit (MCU) including a CPU, a ROM, and a RAM. The acceleration detecting circuit  94  executes a twisted-motion detecting process, which will be described later, to detect the rotation of the main body housing  10  about the Z axis (i.e., the longer axis of the tool bit  4 ) over a predetermined angle, in accordance with detection signals (specifically, an output based on acceleration in the direction of the X axis) from the acceleration sensor  92 . 
     The acceleration detecting circuit  94  further executes an acceleration load detecting process to detect, using the acceleration sensor  92 , vibrations (more specifically, magnitude of vibrations) that occur in the main body housing  10  in the directions of the three axes due to a hammering operation of the tool bit  4 . In this acceleration load detecting process, the acceleration detecting circuit  94  detects imposition of a load on the tool bit  4  if a vibration in the main body housing  10  (i.e., acceleration) exceeds a threshold. 
     The motor controller  70  includes a drive circuit  72  and a control circuit  80 . The drive circuit  72  and the control circuit  80  are mounted on another common circuit board together with various detection circuits, which will be described later, and contained in another common case. 
     The drive circuit  72  includes switching devices Q 1  to Q 6  and is configured to receive electric power from a battery pack  62  (specifically, series-connected battery packs  62 A and  62 B) and feed current to a plurality of phase windings in the motor  8  (which is, specifically, a three-phase brushless motor). The switching devices Q 1  to Q 6  in this embodiment are FETs but not limited to FETs in the present disclosure. The switching devices Q 1  to Q 6  in another embodiment may be switching devices other than FETs. 
     The switching devices Q 1  to Q 3  are each provided as a so-called high side switch between a power source line and one corresponding terminal selected from the terminals U, V, and W of the motor  8 . The power source line is coupled to the positive terminal of the battery pack  62 . 
     The switching devices Q 4  to Q 6  are each provided as a so-called low side switch between a ground line and one corresponding terminal selected from the terminals U, V, and W of the motor  8 . The ground line is coupled to the negative terminal of the battery pack  62 . 
     A capacitor C 1  for restraining fluctuations in battery voltage is provided in a power supply path from the battery pack  62  to the drive circuit  72 . 
     Like the acceleration detecting circuit  94 , the control circuit  80  includes an MCU including a CPU, a ROM, and a RAM. The control circuit  80  feeds current to a plurality of phase windings in the motor  8  by turning on and off the switching devices Q 1  to Q 6  in the drive circuit  72 , and rotates the motor  8 . 
     To be specific, the control circuit  80  sets the command rotational speed and rotation direction of the motor  8  in accordance with commands from a trigger switch  18   a , a speed change commander  18   b , an upper-limit speed setter  96 , and a rotation direction setter  19 , and controls drive of the motor  8 . 
     The trigger switch  18   a  is turned on by pulling the trigger  18  and is configured to input a drive command for the motor  8  to the control circuit  80 . The speed change commander  18   b  is configured to generate a signal depending on the amount of pulling operation of the trigger  18  (i.e., the operation rate) and vary the command rotational speed depending on this amount of operation. 
     The upper-limit speed setter  96  includes a not-shown dial. The operational position of the dial is switched by the user of the hammer drill  2  stage by stage. The upper-limit speed setter  96  is configured to set the upper limit of rotational speed of the motor  8  depending on the operational position of the dial. 
     To be specific, the upper-limit speed setter  96  is configured to be able to set the upper limit of the rotational speed of the motor  8  between a rotational speed higher than a no-load rotational speed under soft no load control, which will be described later, and a rotational speed lower than the no-load rotational speed. 
     The rotation direction setter  19  is configured to set the rotation direction of the motor  8  to a normal or opposite direction through the operation by the user, and is provided, in this embodiment, on the upper side of the trigger  18  as shown in  FIGS. 2 and 3 . Rotating the motor  8  in a normal direction enables drilling of the work piece. 
     The control circuit  80  sets the command rotational speed of the motor  8  in accordance with a signal from the speed change commander  18   b  and an upper limit rotational speed set through the upper-limit speed setter  96 . In particular, the control circuit  80  sets a command rotational speed dependent on the amount of the operation (the operation rate) of the trigger  18  such that the rotational speed of the motor  8  reaches the upper limit rotational speed set by the upper-limit speed setter  96 , when the trigger  18  is pulled to a maximum extent. 
     The control circuit  80  sets a drive duty ratio among the switching devices Q 1  to Q 6  rotatively drives the motor  8  by transmitting a control signal based on the drive duty ratio to the drive circuit  72 , in accordance with the set command rotational speed and rotation direction. 
     An LED  84  serving as a lighting (hereinafter referred to as “lighting LED  84 ”) is provided in the front side of the motor housing  12 . When the trigger switch  18   a  is turned on, the control circuit  80  turns on the lighting LED  84  to illuminate a portion of the work piece to be processed with the tool bit  4 . 
     Rotational position sensors  81  are provided to the motor  8 . The rotational position sensors  81  detect the rotational speed and rotational position of the motor  8  (to be specific, the rotational position of the rotor of the motor  8 ), and transmit detection signals to the motor controller  70 . The motor controller  70  includes a rotational position detection circuit  82 . The rotational position detection circuit  82  detects the rotational position needed for setting the timing of energization of each phase winding in the motor  8 , in accordance with detection signals from the rotational position sensors  81 . 
     The motor controller  70  further includes a voltage detection circuit  78 , a current detection circuit  74 , and a temperature detection circuit  76 . 
     The voltage detection circuit  78  detects the value of a battery voltage supplied from the battery pack  62 . The current detection circuit  74  detects the value of a current flowing through the motor  8  via a resistor R 1  provided in a current path to the motor  8 . The current detection circuit  74  corresponds to one example of the current detector in the present disclosure. 
     The temperature detection circuit  76  detects the temperature of the motor controller  70 . 
     The control circuit  80  receives detection signals from the voltage detection circuit  78 , the current detection circuit  74 , the temperature detection circuit  76 , and the rotational position detection circuit  82 , and detection signals from the twisted-motion detector  90 . 
     The control circuit  80  restricts the rotational speed of the motor  8  that is being driven or stops the drive of the motor  8 , in accordance with detection signals from the voltage detection circuit  78 , the current detection circuit  74 , the temperature detection circuit  76 , and the rotational position detection circuit  82 . 
     The motor controller  70  includes a not-shown regulator for receiving power from the battery pack  62  and generating a constant power source voltage Vcc. 
     The power source voltage Vcc generated by the regulator is supplied to the MCU of the control circuit  80  and the acceleration detecting circuit  94  of the twisted-motion detector  90 . In addition, upon detection of twisting of the main body housing  10  from the acceleration in the direction of the X axis, the acceleration detecting circuit  94  transmits an error signal to the control circuit  80 . 
     This error signal is transmitted for stopping drive of the motor  8 . When the main body housing  10  is not twisted, the acceleration detecting circuit  94  transmits a no-error signal to the control circuit  80 . 
     Upon detection of imposition of a load to the tool bit  4  from vibration (i.e., acceleration) of the main body housing  10 , the acceleration detecting circuit  94  transmits a load signal to the control circuit  80 . The load signal indicates the fact that the tool bit  4  is in a load state. When the acceleration detecting circuit  94  does not detect imposition of a load to the tool bit  4 , the acceleration detecting circuit  94  transmits a no-load signal to the control circuit  80 . The no-load signal indicates the fact that the tool bit  4  is in a no-load state. 
     The dust collector device  66  mounted on the front side of the motor housing  12  collects, by suction, dust particles that occur from the work piece upon chipping and drilling. 
     As shown in  FIG. 4 , the dust collector device  66  includes a dust collector motor  67  and a circuit board  69 . The dust collector motor  67  is driven by the circuit board  69 . The dust collector device  66  includes a lighting LED  68  that has a function of illuminating a portion of the work piece to be processed, instead of the lighting LED  84  provided to the motor housing  12 . This is because the lighting LED  84  is covered when the dust collector device  66  is mounted to the motor housing  12 . 
     When the dust collector device  66  is mounted to the motor housing  12 , drive current is fed from the battery pack  62  to the dust collector motor  67  through the current path on the circuit board  69 . 
     When the dust collector device  66  is mounted to the motor housing  12 , the circuit board  69  is coupled to the control circuit  80  through the connector  64 . The circuit board  69  includes the switching device Q 7  and turns on and off the switching device Q 7  to open and close the current path to the dust collector motor  67 . The lighting LED  68  can be turned on by a drive signal from the control circuit  80 . 
     Control process performed in the control circuit  80  will now be explained with the flow charts of  FIGS. 5 to 11 . It should be noted that this control process is implemented when the CPU in the control circuit  80  executes a program stored in the ROM which is a nonvolatile memory. 
     As shown in  FIG. 5 , in this control process, whether a given time base has elapsed is first determined in S 110  (S represents Step) and a waiting time lasts until the elapse of the time base from the execution of the previous process from S 120 . This time base corresponds to the cycle for controlling drive of the motor. 
     If it is determined that the time base has elapsed in S 110 , input process in S 120 , A/D conversion process in S 130 , motor control process in S 140 , and output process in S 150  are sequentially executed and the process goes to S 110  again. In other words, in this control process, the CPU in the control circuit  80  executes a series of process in S 120  to S 150  each elapse of the time base, that is, in a cyclical fashion. 
     Here, in input process in S 120 , as shown in  FIG. 6 , trigger switch (trigger SW) input process is first executed in S 210  for retrieving the operation state of the trigger  18  from the trigger switch  18   a . In the following S 220 , rotation direction input process is executed for retrieving the direction of the rotation of the motor  8  from the rotation direction setter  19 . 
     In the following S 230 , a twisted-motion detection input process is executed for retrieving the results of detection (an error signal or no-error signal) of a twisted-motion from the twisted-motion detector  90 . In the following S 240 , acceleration load detection input process is executed for retrieving the results of detection of an acceleration load from the twisted-motion detector  90  (a load signal or no-load signal). 
     Finally, in S 250 , dust collector device input process is executed for detecting the value of the battery voltage through the connector  64  of the dust collector device  66 , and the input process in S 120  is terminated. It should be noted that the dust collector device input process in S 250  detects the value of the battery voltage in order to determine whether the dust collector device  66  is mounted to the motor housing  12 . 
     In the following A/D conversion process in S 130 , detection signals (voltage signals) related to the amount of pulling operation of the trigger  18  and upper-limit speed, or a voltage value, a current value, a temperature, and the like are retrieved, through A/D conversion, from the speed change commander  18   b , the upper-limit speed setter  96 , the voltage detection circuit  78 , the current detection circuit  74 , the temperature detection circuit  76  and the like. 
     As shown in  FIG. 7 , in motor control process in S 140 , whether the motor  8  should be driven based on motor drive conditions is first determined in S 310 . 
     In this embodiment, the motor drive conditions are satisfied when the trigger switch  18   a  is in the on state, the voltage value, the current value, and the temperature retrieved in S 130  are normal, and no twisted-motion of the main body housing  10  is detected by the twisted-motion detector  90  (no-error signal input). 
     When the motor drive conditions are satisfied and if it is determined that the motor  8  should be driven in S 310 , the process proceeds to S 320  and command rotational speed setting process is executed. In this command rotational speed setting process, the command rotational speed is set in accordance with a signal from the speed change commander  18   b  and an upper limit rotational speed set through the upper-limit speed setter  96 . 
     In the following S 330 , soft no load process is executed. In soft no load process, when the tool bit  4  is in the no load state, the command rotational speed of the motor  8  is limited below a predetermined no-load rotational speed Nth. 
     In the following S 340 , control amount setting process is executed. In this control amount setting process, the drive duty ratio for the motor  8  is set according to the command rotational speed set in S 320  or limited below the predetermined no-load rotational speed Nth in S 330 . Upon completion of this control amount setting process, the motor control process is terminated. 
     It should be noted that in S 340 , the drive duty ratio is set such that the drive duty ratio does not rapidly change in accordance with a change of the command rotational speed from the rotational speed set by a trigger operation or the like to the no-load rotational speed or toward the side opposite to this. 
     In other words, in S 340 , the rate of change in the drive duty ratio (i.e., the gradient of change) is limited so that the rotational speed of the motor  8  can gradually change. This is for restraining a rapid change in the rotational speed of the motor  8  when the tool bit  4  is made in contact with the work piece or separated from the work piece. 
     When the motor drive conditions are not satisfied and if it is determined that the motor  8  should not be driven in S 310 , the process proceeds to S 350  and a motor stop setting process for setting a stop of drive of the motor  8  is executed and the motor control process is terminated. 
     As shown in  FIG. 8 , in soft no load process in the following S 330 , whether soft no load control execution conditions (soft no load conditions) are satisfied is first determined in S 332 . Under soft no load control, the command rotational speed of the motor  8  is limited at or below the no-load rotational speed Nth. 
     In this embodiment, soft no load conditions are satisfied in a current load detection process shown in  FIG. 9  and in the acceleration detecting circuit  94  in the twisted-motion detector  90 , when the tool bit  4  is determined to be in the no-load state and the dust collector device  66  is not mounted to the hammer drill  2 . 
     If it is determined that the soft no load conditions are satisfied in S 332 , the process proceeds to S 334  and whether the command rotational speed exceeds the no-load rotational speed Nth (e.g., 11000 rpm) is determined. This no-load rotational speed Nth corresponds to the upper limit rotational speed of soft no load control. 
     If the command rotational speed is determined to exceed the no-load rotational speed Nth in S 334 , the process proceeds to S 336  in which the no-load rotational speed Nth is applied to the command rotational speed, and the soft no load process is terminated. 
     If it is determined that the soft no load conditions are not satisfied in S 332  or that the command rotational speed does not exceed the no-load rotational speed Nth in S 334 , the soft no load process is immediately terminated. 
     To summarize, in the soft no load process, the command rotational speed is limited at or below the no-load rotational speed Nth if the tool bit  4  is determined to be in the no-load state in both the current load detection process in  FIG. 9  and the acceleration detecting circuit  94 , and when the dust collector device  66  is not mounted to the hammer drill  2 . 
     In the A/D conversion process in S 130 , the current load detection process in  FIG. 9  is executed for determining whether the tool bit  4  is in the no-load state in accordance with the current value retrieved from the current detection circuit  74 . 
     In this current load detection process, first, in S 410 , whether the value retrieved through A/D conversion (detect current value) exceeds a current threshold Ith is determined. This current threshold Ith is a value predetermined to determine whether the tool bit  4  is in the load state. 
     If the detected current value exceeds the current threshold Ith, a load counter for load determination is incremented (+1) in S 420 , a no-load counter for no-load determination is decremented (−1) in S 430 , and the process proceeds to S 440 . 
     In S 440 , whether the value of the load counter exceeds a load determination value T 1  is determined. The load determination value T 1  is a value predetermined to determine whether the tool bit  4  is in the load state. If the value of the load counter exceeds the load determination value T 1 , the process proceeds to S 450  and a current load detecting flag is set, and the current load detection process is then terminated. 
     If the value of the load counter does not exceed the load determination value T 1 , the current load detection process is immediately terminated. The current load detecting flag indicates that the tool bit  4  is in the load state, and is used to detect the fact (a current load) that the load state of the tool bit  4  is detected from a current value in S 332  of the soft no load process. 
     If the detected current value is determined to be at or below the current threshold Ith in S 410 , the process proceeds to S 460  in which the no-load counter is incremented (+1), and to the following S 470  in which the load counter is decremented (−1). 
     In the following S 480 , whether the value of the no-load counter exceeds a no-load determination value T 2  is determined. The no-load determination value T 2  is a value predetermined to determine whether the tool bit  4  is in the no-load state. If the value of the no-load counter exceeds the no-load determination value T 2 , the process proceeds to S 490  and the tool bit  4  is determined to be in the no-load state, so that the current load detecting flag is cleared and the current load detection process is terminated. 
     If the value of the no-load counter does not exceed the no-load determination value T 2 , the current load detection process is immediately terminated. 
     The load counter measures the time during which the detected current value exceeds the current threshold Ith. In the current load detection process, whether the time measured by the load counter has reached a predetermined time is determined by using the load determination value T 1 . The no-load counter measures the time during which the detected current value does not exceed the current threshold Ith. In the current load detection process, whether the time measured by the no-load counter has reached a predetermined time is determined by using the no-load determination value T 2 . The load determination value T 1  corresponds to one example of the first threshold time in the present disclosure. The no-load determination value T 2  corresponds to one example of the second threshold time in the present disclosure. 
     In this embodiment, the load determination value T 1  is smaller than the no-load determination value T 2  (i.e., the time measured by the load counter is shorter than the time measured by the no-load counter). This is for detecting the load state of the tool bit  4  more rapidly so that the rotational speed of the motor  8  can be set to a command rotational speed dependent on the amount of the operation of the trigger. The load determination value T 1  is set to a value corresponding to, for example, 100 ms, and the no-load determination value T 2  is set to a value corresponding to, for example, 500 ms. 
     As shown in  FIG. 10 , in output process in S 150 , motor output process is first executed in S 510 . In the motor output process, a control signal for driving the motor  8  at the command rotational speed, and a rotation direction signal for designating the rotation direction are transmitted to the drive circuit  72 . 
     In the following S 520 , a dust collection output process is executed for transmitting a drive signal for the dust collector motor  67  to the dust collector device  66  mounted to the hammer drill  2 . Subsequently, a lighting output process is executed for transmitting a drive signal to the lighting LED  84  to turn on the lighting LED  84  in S 530 , and the output process is terminated. 
     In S 530 , if the dust collector device  66  is mounted to the hammer drill  2 , a drive signal is transmitted to the lighting LED  68 , which is provided to the dust collector device  66 , to turn on the lighting LED  68 . 
     As shown in  FIG. 11 , in motor output process in S 510 , whether the motor  8  should be driven is first determined in S 511 . Process in S 511  is executed in a manner similar to that for S 310  in the motor control process. 
     In other words, in S 511 , whether the motor drive conditions are satisfied is determined. These motor drive conditions are satisfied when the trigger switch  18   a  is in the on state, the voltage value, the current value, and the temperature retrieved in S 130  are normal, and no twisted-motion of the main body housing  10  is detected by the twisted-motion detector  90  (no-error signal input). 
     When the motor drive conditions are satisfied and if it is determined that the motor  8  should be driven in S 511 , the process proceeds to S 512  and transmission of a control signal to the drive circuit  72  is started. 
     In the following S 513 , whether the direction of the rotation of the motor  8  is the normal direction (forward direction) is determined. If the direction of the rotation of the motor  8  is the normal direction (forward direction), the process proceeds to S 514  in which a rotation direction signal that designates the “forward direction” as the direction of the rotation of the motor  8  is transmitted to the drive circuit  72 , and the motor output process is terminated. 
     If it is determined that the direction of the rotation of the motor  8  is not the normal direction in S 513 , the process proceeds to S 515  in which a rotation direction signal that designates the “reverse direction” as the direction of the rotation of the motor  8  is transmitted to the drive circuit  72 , and the motor output process is terminated. 
     When the motor drive conditions are not satisfied and if it is determined that the motor  8  should not be driven in S 511 , the process proceeds to S 516  and transmission of a control signal to the drive circuit  72  is stopped. 
     Next, an acceleration load detecting process and twisted-motion detecting process executed in the acceleration detecting circuit  94  of the twisted-motion detector  90  will be explained with reference to the flow charts of  FIGS. 12, 13A, and 13B . 
     As shown in  FIG. 12 , for the acceleration load detecting process, in S 610 , whether a sampling time predetermined to judge load on the tool bit  4  has elapsed is determined. In other words, a waiting time lasts until the elapse of the given sampling time since the previous process executed S 620 . 
     If it is determined that the sampling time has elapsed in S 610 , the process proceeds to S 620  in which whether the trigger switch  18   a  is in the on state (i.e., whether there is an input of a drive command of the motor  8  from the user) is determined. 
     If it is determined that the trigger switch  18   a  is in the on state in S 620 , the process proceeds to S 630 . Accelerations in the directions of the three axes (X, Y, and Z) is retrieved from the acceleration sensor  92  through A/D conversion in S 630 , and the retrieved acceleration data is subjected to a filtering process for removing gravity acceleration components from acceleration data related to the directions of the three axes in the following S 640 . 
     The filtering process in S 640  functions as a high-pass filter (HPF) with a cut-off frequency of about 1 to 10 Hz for removing low-frequency components corresponding to gravity acceleration. 
     After the accelerations in the directions of the three axes is subjected to the filtering process in S 640 , the process proceeds to S 650  in which the accelerations in the directions of the three axes after the filtering process is D/A converted and, for example, acceleration signals in the directions of the three axes after D/A conversion are subjected to full-wave rectification to obtain the absolute values of the respective accelerations [G] in the directions of the three axes. 
     The absolute values obtained in S 650  are smoothed using a low-pass filter (LPF) to obtain the respective smoothed accelerations in the following S 660 , and the process proceeds to S 670 . 
     In S 670 , the respective smoothed accelerations are compared with a threshold predetermined to determine whether the tool bit  4  is in the load state, and whether the state where any of the smoothed accelerations exceeds the threshold has continued for over a given time is determined. 
     If it is determined that the state where any of the smoothed accelerations exceeds the threshold has continued for over the given time in S 670 , the tool bit  4  is determined to be in the load state and the process proceeds to S 680 . Subsequently, a load signal is transmitted to the control circuit  80  in S 680 , and the process proceeds to S 610 . 
     If it is determined that the state where any of the smoothed accelerations exceeds the threshold has not continued for over the given time in S 670  or if it is determined that the trigger switch  18   a  is in the off state in S 620 , the process proceeds to S 690 . 
     In S 690 , a no-load signal is transmitted to the control circuit  80  to notify the control circuit  80  that the tool bit  4  is in the no-load state. The process then proceeds to S 610 . 
     Consequently, the control circuit  80  retrieves a load signal or no-load signal from the acceleration detecting circuit  94  and can therefore determine whether the load state (acceleration load) of the tool bit  4  is detected or whether the soft no load conditions are satisfied. 
     As shown in  FIGS. 13A and 13B , in the twisted-motion detecting process, whether a sampling time predetermined to detect a twisted-motion has elapsed is determined in S 710 . In other words, a waiting time lasts until the elapse of the given sampling time since the previous process executed S 720 . 
     Subsequently, if it is determined that the sampling time has elapsed in S 710 , the process proceeds to S 720  in which whether the trigger switch  18   a  is in the on state is determined. If the trigger switch  18   a  is in the on state, the process proceeds to S 730 . 
     In S 730 , twisting of the hammer drill  2  is detected in the twisted-motion detecting process and whether the error state is currently occurring is determined. If the error state is occurring, the process proceeds to S 710 . If the error state is not occurring, the process proceeds to S 740 . 
     In S 740 , the acceleration in the direction of the X axis is retrieved from the acceleration sensor  92  through A/D conversion. In the following S 750 , as in the above-described S 640 , gravity acceleration components are removed from the retrieved data of the acceleration in the direction of the X axis in a filtering process functioning as an HPF. 
     Subsequently, in S 760 , the angular acceleration [rad/s 2 ] about the Z axis is calculated from the acceleration [G] in the direction of the X axis after the filtering process by using the following expression. The process then proceeds to S 770 .
 
Expression: angular acceleration=acceleration  G× 9.8/distance  L  
 
     In this expression, distance L is the distance between the acceleration sensor  92  and the Z axis. 
     In S 770 , the angular acceleration obtained in S 760  is integrated for a sampling time. In the following S 780 , the initial integral of the angular acceleration is updated. This initial integral is the integral of the angular acceleration for a given past time. Since the angular acceleration has been additionally calculated in S 760 , the integral of the angular acceleration that has been sampled for a sampling time more than a given time ago is removed from the initial integral in S 780 . 
     In the following S 790 , the angular velocity [rad/s] about the Z axis is calculated by addition of the initial integral of the angular acceleration updated in S 780  and the latest integral of the angular acceleration calculated in S 770 . 
     In S 800 , the angular velocity calculated in S 790  is integrated for a sampling time. In the following S 810 , the initial integral of the angular velocity is updated. This initial integral is the integral of the angular velocity for a past given time. Since the angular velocity has been additionally calculated in S 790 , the integral of the angular velocity that has been obtained for a sampling time more than a given time ago is removed from the initial integral in S 810 . 
     In the following S 820 , the first rotation angle [rad] about the Z axis related to the hammer drill  2  is calculated by addition of the initial integral of the angular velocity updated in S 810  and the latest integral of the angular velocity calculated in S 800 . 
     In S 830 , the second rotation angle of the hammer drill  2  required for actually stopping the motor  8  after twisting of the hammer drill  2  about the Z axis is detected is calculated based on the current angular velocity determined in S 790 . The process then proceeds to S 840 . This rotation angle is calculated by multiplying the angular velocity by a predetermined estimated time (rotation angle=angular velocity×estimated time). 
     In S 840 , an estimated angle is calculated by adding the second rotation angle calculated in S 830  to the first rotation angle about the Z axis calculated in S 820 . This estimated angle corresponds to the rotation angle about the Z axis including the rotation angle after a stop of drive of the motor  8  (i.e., the second rotation angle). 
     In S 850 , whether the state where the estimated angle calculated in S 840  exceeds a threshold angle predetermined to detect a twisted-motion has continued for more than a given time is determined. 
     If yes in S 850 , the process proceeds to S 860  to transmit an error signal to the control circuit  80 . In other words, the fact that the tool bit  4  fits the work piece during drilling of the work piece and a twisted-motion of the hammer drill  2  has started is notified to the control circuit  80 . 
     Consequently, the control circuit  80  determines that the motor drive conditions are not satisfied and stops drive of the motor  8 , thereby restraining a large amount of twisting of the hammer drill  2 . After execution of the process in S 860 , this process proceeds to S 710  again. 
     On the contrary, if no in S 850 , the process proceeds to S 870  to transmit a no-error signal to the control circuit  80 . In other words, the fact that the hammer drill  2  is not twisted is notified to the control circuit  80 . After execution of the process in S 870 , this process proceeds to S 710  again. 
     In S 720 , if it is determined that the trigger switch  18   a  is not in the on state, the operation of the hammer drill  2  stops; thus, the process proceeds to S 880  to reset the integrals and the initial integrals of angular acceleration and angular velocity. The process then proceeds to S 870 . 
     As described above, in the hammer drill  2  in this embodiment, the control circuit  80  in the motor controller  70  executes the current load detection process shown in  FIG. 9  to determine whether the tool bit  4  is in the no-load state or the load state, in accordance with the current flowing through the motor  8  (load imposition or no-load imposition is detected in accordance with a current). 
     Since the acceleration detecting circuit  94  of the twisted-motion detector  90  executes the acceleration load detecting process shown in  FIG. 12 , whether the tool bit  4  is in the no-load imposed state or the load imposed state is determined in accordance with accelerations detected in the directions of the X axis, the Y axis, and the Z axis by the acceleration sensor  92  (load imposition or no-load imposition is detected in accordance with accelerations). 
     When load imposition is not detected in accordance with a current or accelerations and the dust collector device  66  is not mounted to the hammer drill  2 , the control circuit  80  limits the rotational speed of the motor  8  at or below the no-load rotational speed Nth in the soft no load process shown in  FIG. 8 . 
     Accordingly, in the hammer drill  2  of this embodiment, if the drive mode is in the hammer mode, load imposition on the tool bit  4  can be detected in the acceleration load detecting process. If the drive mode is in the drill mode, load imposition on the tool bit  4  can be detected in the current load detection process. If the drive mode is in the hammer drill mode, load imposition on the tool bit  4  can be detected in both the acceleration load detecting process and the current load detection process. 
     Hence, in the hammer drill  2  of this embodiment, in any drive mode selected from the group including the hammer mode, the hammer drill mode, and the drill mode, load imposition from the work piece to the tool bit  4  can be rapidly detected and the motor  8  can be driven at a command rotational speed. 
     Further, in the hammer drill  2  of this embodiment, when the dust collector device  66  is mounted to the hammer drill  2 , the motor  8  is driven at a command rotational speed even if at least one of the detection of a load based on current flowing through the motor  8  and the detection of a load based on acceleration detected by the acceleration sensor  92  is not performed. In this hammer drill  2 , mounting the dust collector device  66  to the hammer drill  2  restrains vibrations of the main body housing  10 , so that soft no load control is not executed even if the detection of the load conditions of the tool bit  4  is difficult in the acceleration load detecting process. Accordingly, the user can perform chipping or drilling of the work piece as usual. 
     In this embodiment, the current load detection process executed in the control circuit  80  functions as one example of a first load detector of the present disclosure, and the acceleration load detecting process executed by the acceleration detecting circuit  94  functions as one example of a second load detector of the present disclosure. 
     In the acceleration load detecting process, accelerations in the directions of the three axes (X, Y, and Z) sent from the acceleration sensor  92  is subjected to A/D conversion, and the obtained acceleration data is subjected to a filtering process. Through this filtering process, a gravity acceleration component is removed from acceleration data related to each axis direction. 
     This filtering process yields high accuracy of acceleration detection, compared with removing a gravity acceleration component through transmission of a detection signal from the acceleration sensor  92  to an analog filter (a high-pass filter). 
     To be specific, upon generation of acceleration due to the vibration of the main body housing  10 , a detection signal from the acceleration sensor  92  fluctuates according to the acceleration, whereas when no electric power is supplied to the hammer drill  2 , the fluctuation of the detection signal is centered around the ground potential. 
     As shown in the upper diagram in  FIG. 14 , when the hammer drill  2  is supplied with electric power, the fluctuation of the detection signal is centered around a raised voltage determined by adding a gravity acceleration component (Vg) to the reference voltage of the input circuit (typically the middle voltage of the power source voltage Vcc: Vcc/2). 
     Since the motor  8  is stopped immediately after the hammer drill  2  is supplied with electric power, no acceleration is assumed to occur in the main body housing  10 . Accordingly, an input signal (a detection signal) from the acceleration sensor  92  rises to a constant voltage of “(Vcc/2)+Vg”. 
     In this case, a detection signal is input to an analog filter (high-pass filter: HPF) to remove gravity acceleration components (Vg); thus, as shown in the middle drawing of  FIG. 14 , the output of the analog filter rapidly rises immediately after supply of electric power and exceeds the reference voltage (Vcc/2). Afterwards, the output of the analog filter eventually decreases to the reference voltage (Vcc/2) and goes into the stable state but after a certain period of time. 
     On the contrary, if a detection signal is subjected to a filtering process using a digital filter as in this embodiment, as shown in the lower drawing of  FIG. 14 , the signal level of the detection signal can be set to the initial value immediately after supply of electric power, thereby restraining or preventing the fluctuation of the detection signal (data). 
     Accordingly, in this embodiment, accelerations can be accurately detected from immediately after supply of electric power to the hammer drill  2 , thereby the risk that the load imposition on the tool bit, which are caused by acceleration detection errors, cannot be detected is restrained. 
     Further, the twisted-motion detector  90  is separate from the motor controller  70 , which leads to a smaller size than that given by integration of these components. Accordingly, the twisted-motion detector  90  can be disposed in a position where it can easily detect the behavior (acceleration) of the main body housing  10 , using a space in the main body housing  10 . 
     In addition, in the current load detection process, the time during which the detected current value exceeds the current threshold Ith and the time during which the detected current value does not exceed the current threshold Ith are measured using the load counter and the no-load counter. A load or no-load on the tool bit  4  is confirmed based on the detected current when these times reach given set times depending on the determination values T 1  and T 2 . For this reason, in this embodiment, failures in determination of a current load caused by noise and the like can be restrained. 
     In particular, in this embodiment, as shown in  FIG. 15 , the load determination value T 1  used to determine the load conditions after the detected current exceeds the current threshold Ith is set lower (shorter) than the no-load determination value T 2  used to determine the no-load conditions after the detected current becomes at or below the current threshold Ith. 
     For this reason, in this embodiment, the load conditions of the tool bit  4  can be detected earlier than the no-load conditions, thereby shortening the delay time upon switching of the rotational speed of the motor  8  from the no-load rotational speed Nth to a command rotational speed. 
     Accordingly, in this embodiment, the rotational speed of the motor  8  rapidly rises when a load is imposed on the tool bit  4 , so that the user can satisfactorily perform chipping or drilling of the work piece. In addition, in this embodiment, switching of the motor&#39;s rotational speed to low speed due to the detection of the no-load conditions can be restrained in the middle of chipping operation. 
     In the acceleration load detecting process, as in the current load detection process, the time during which average acceleration exceeds a threshold and the time during which the average acceleration is at or below the threshold may be measured, and when they reach given set times, a load or no-load on the tool bit  4  may be confirmed. 
     Further, in this embodiment, the user can set the upper limit rotational speed through a dial operation on the upper-limit speed setter  96 . In addition, the user can set the command rotational speed of the motor  8 , which is determined according to a signal from the speed change commander  18   b , lower than the no-load rotational speed Nth through a trigger operation. 
     When the command rotational speed is set lower than the no-load rotational speed Nth, the rotational speed of the motor  8  becomes the command rotational speed as indicated by the dotted line in  FIG. 15 . Hence, the user can operate the motor  8  at a speed lower than the no-load rotational speed Nth, thereby widening the range of use of the hammer drill  2  and improving usability. 
     Although the embodiment for implementing the present disclosure has been described so far, the present disclosure is not limited to the above-described embodiment and various modifications can be made for implementation. 
     For example, in the above-described embodiment, a load on the tool bit  4  is detected using current flowing through the motor  8 , which is information about the drive state of the motor, and acceleration imposed on the main body housing  10 . 
     However, the present disclosure is not limited to this and the load conditions of the tool bit may be determined using motor&#39;s rotational speed (specifically variations in speed), motor&#39;s drive voltage (specifically variations in voltage), or the like instead of current flowing through the motor  8 . In addition, an angular velocity sensor may be used as a sensor for detecting the behavior of the hammer drill main body, instead of the acceleration sensor  92 . An angle sensor may detect vibrations of the main body housing  10  to detect load imposition on the tool bit. 
     In addition, in the above-described embodiment, the acceleration load detecting process uses all the accelerations in the directions of the three axes (X, Y, and Z) detected by the acceleration sensor  92 . However, load imposition on the tool bit  4  due to a hammering operation can be detected using at least the acceleration of the direction of the Z axis. 
     Further, in the above-described embodiment, the hammer drill  2  that can be attached with the dust collector device  66  which is an external unit is described. However, the hammer drill may be attached with a water injection device for injecting water to a portion of a work piece being processed, a lighting device for illuminating a work piece, a blower for giving air to blow dust particles, or the like that is an external unit. When attached with any of these external units, such a hammer drill may be configured to drive the motor  8  at a command rotational speed even if neither the current-based detection of a load nor the acceleration-based detection of a load has been performed. 
     When any of these external units is attached to the hammer drill, the main body housing  10  barely vibrates, hindering the detection of load imposition on the tool bit. For this reason, when any external unit is attached, soft no load control may not be executed and the motor  8  may be driven in accordance with a command from the user. Consequently, when any external unit is attached, the risk that the motor  8  cannot be driven at a command rotational speed set by the user due to the execution of soft no load control can be reduced. 
     Multiple functions of one component in the above-described embodiment may be implemented by multiple components, or one function of one component may be implemented by multiple components. In addition, multiple functions of multiple components may be implemented by one component, or one function implemented by multiple components may be implemented by one component. Further, part of the structure of the above-described embodiment can be omitted. Moreover, at least part of the above-described embodiment can be added to or replaced by another structure of the above-described embodiment. It should be noted that any mode included in technical ideas specified by the words in the claims is the embodiment of the present disclosure.