Patent Publication Number: US-2015084554-A1

Title: Electric power tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2013-198995, filed on Sep. 25, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to an electric power tool that is actuated by a rechargeable battery. 
     BACKGROUND 
     In an electric power tool in which a bit is actuated by driving power of a motor, a large inrush current may be generated when the motor is started. To reduce the large inrush current, a soft start control, which gradually increases the voltage applied to the motor, has been proposed. A smaller load increases the inrush current. Japanese Laid-Open Patent Publication No. 2011-240441 describes an electric power tool including a load detection unit that changes the increase rate of the voltage applied to a motor based on the detected load amount. For example, when a small load is detected, the rotation produced by the motor is accelerated from null to a high speed within a short period. 
     SUMMARY 
     In a rechargeable battery, overdischarging occurs when the output voltage of a rechargeable battery excessively falls. This is a factor that shortens the life of the rechargeable battery. The inventors of the present invention have recognized that the soft start control described in Japanese Laid-Open Patent Publication No. 2011-240441 does not take into consideration the overdischarging of the rechargeable battery. Thus, it is the goal of the inventors to prevent or inhibit the occurrence of overdischarging in a rechargeable battery when a motor is started. 
     One aspect of the present invention is an electric power tool including a rechargeable battery, a motor, a detector configured to detect a parameter that affects a voltage drop of the rechargeable battery and output a detection signal corresponding to the detected parameter, a controller configured to receive the detection signal and generate a control signal in accordance with the detection signal, and a power switching unit operated in accordance with the control signal from the controller to switch between a situation in which the rechargeable battery supplies electric power to the motor and a situation in which the rechargeable battery stops supplying electric power to the motor. The controller is configured to control the power switching unit using a pulse width modulation (PWM) control signal when the electric power tool is activated. The controller is configured to change a duty ratio of the PWM control signal in accordance with the detection signal of the detector. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a block diagram of an electric power tool of a first embodiment; 
         FIG. 2  is a timing chart illustrating activation of the electric power tool of the first embodiment; 
         FIG. 3  is a timing chart illustrating activation of an electric power tool of a second embodiment; and 
         FIG. 4  is a timing chart illustrating activation of an electric power tool of a third embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An electric circuit of an electric power tool  1  will now be described with reference to  FIG. 1 . 
     The electric power tool  1  is, for example, a drill driver. The electric power tool  1  includes an electric power tool main body  10  and a battery pack  20 . Preferably, the electric power tool main body  10  and the battery pack  20  are structured to be attachable to and detachable from each other. The electric power tool  1  transmits torque through a bit attached to the electric power tool main body  10  (not shown in the drawings) to a task subject component. The task subject component is, for example, a screw or a bolt. 
     The electric power tool main body  10  includes a motor  11 , a motor driver  30 , an activation switch  12 , a controller  13 , a power circuit  14 , a voltage detector  15 , a temperature detector  16 , and a current detector  17 . The electric power tool main body  10  includes a positive terminal  41 , a negative terminal  42 , and a signal terminal  43 . The motor driver  30  includes a power switching unit  31 . 
     The activation switch  12  includes two terminals. The activation switch  12  is switched on and off. When the activation switch  12  is switched on, the two terminals of the activation switch  12  are closed. When the activation switch  12  is switched off, the two terminals of the activation switch  12  open. 
     The battery pack  20  may include a rechargeable battery  21 , a battery voltage detector  22 , a battery temperature detector  23 , and a battery current detector  24 . For example, the rechargeable battery  21  may include a plurality of cells connected in series. The rechargeable battery  21  may be a lithium-ion battery. The battery pack  20  includes a positive terminal  51 , a negative terminal  52 , and a signal terminal  53 . When the battery pack  20  is attached to the electric power tool main body  10 , the terminals  41 ,  42 , and  43  of the electric power tool main body  10  are electrically connected to the terminals  51 ,  52 , and  53  of the battery pack  20 , respectively. 
     A positive terminal of the motor  11  is connected to the positive terminal  41  of the electric power tool main body  10  through the motor driver  30 , the activation switch  12 , and the current detector  17 . A negative terminal of the motor  11  is connected to the negative terminal  42  of the electric power tool main body  10 . A positive terminal of the controller  13  is connected to the positive terminal  41  of the electric power tool main body  10  through the power circuit  14 , the activation switch  12 , and the current detector  17 . A negative terminal of the controller  13  is connected to the negative terminal  42  of the electric power tool main body  10 . The controller  13  provides the motor driver  30  with a control signal Cs. 
     The current detector  17  detects an internal current of the electric power tool main body  10  and provides the controller  13  with a detection signal  110  based on the amount of the detected current. The internal current may be, for example, the current flowing from the positive terminal  41  of the electric power tool main body  10  to the negative terminal  42  of the electric power tool main body  10  through the activation switch  12 . For example, the current detector  17  may be located between the positive terminal  41  of the electric power tool main body  10  and the motor driver  30 . 
     The voltage detector  15  is connected between the positive terminal  41  and the negative terminal  42  of the electric power tool main body  10 . The voltage detector  15  detects the voltage between the positive terminal  41  and the negative terminal  42  of the electric power tool main body  10 , that is, the voltage provided by the rechargeable battery  21 . Then, the voltage detector  15  provides the controller  13  with a detection signal V 10  based on the detected voltage. 
     The temperature detector  16  detects the temperature of the electric power tool  1  and provides the controller  13  with a detection signal T 10  based on the detected temperature. For example, the temperature detector  16  may be located in the proximity of the motor  11  to detect the temperature at the proximity of the motor  11 . 
     A positive electrode of the rechargeable battery  21  is connected to the positive terminal  51  of the battery pack  20  through the battery current detector  24 . A negative electrode of the rechargeable battery  21  is connected to the negative terminal  52  of the battery pack  20 . 
     The battery current detector  24  detects the current output from the rechargeable battery  21  (also referred to as the battery current) and provides the controller  13  with a detection signal I 20  based on the amount of the detected current through, for example, the signal terminals  53  and  43 . In the illustrated example, the battery current detector  24  detects the current between the positive terminal  51  of the battery pack  20  and the rechargeable battery  21 . 
     The battery voltage detector  22  is connected between the positive electrode of the rechargeable battery  21  and the negative electrode of the rechargeable battery  21 . The battery voltage detector  22  is driven by the voltage generated with the rechargeable battery  21 . The battery voltage detector  22  detects the voltage of each cell forming the rechargeable battery  21  and the voltage of the rechargeable battery  21 , which is the total voltage of the cells, and provides the controller  13  with a detection signal V 20  based on the detected voltages through the signal terminals  53  and  43 . 
     The battery temperature detector  23  detects the internal temperature of the battery pack  20  and provides the controller  13  with a detection signal T 20  through, for example, the signal terminals  53  and  43 . 
     The operation of the electric power tool  1  shown in FIG.  1  will now be described. 
     The battery pack  20  provides the electric power tool main body  10  with electric power from the rechargeable battery  21  through the terminals  41  and  51  and the terminals  42  and  52 . The electric power tool  1  is activated when a user switches on the activation switch  12 . 
     When the activation switch  12  is switched on, the controller  13  is provided with the electric power from the power circuit  14  and starts to operate. The electric power or signal that is supplied to the controller  13  from the power circuit  14  when the activation switch  12  is switched on may be referred to as a motor start-up request. 
     The motor driver  30  provides the motor  11  with electric power in accordance with a control signal Cs from the controller  13 . The motor  11  starts to generate rotation when supplied with electric power from the motor driver  30 . 
     An inrush current flows to the motor  11  when the motor  11  is started and the rotation speed increases from null. The rechargeable battery  21  includes internal resistance. The voltage of the rechargeable battery  21  may drop depending on the amount of the inrush current generated during the activation of the motor  11 . 
     The power switching unit  31  may include, for example, a field effect transistor (FET). The FET may be in a first state (for example, on state) or a second state (for example, off state). The FET switches between the first state and the second state in accordance with a control signal Cs from the controller  13 . When the FET is in the first state, the power switching unit  31  supplies the motor  11  with electric power from the rechargeable battery  21 . When the FET is in the second state, the power switching unit  31  stops supplying the motor  11  with electric power. 
     The controller  13  stores an overdischarge reference voltage to prevent or inhibit overdischarging of the rechargeable battery  21 . Based on the detection signal V 20  received from the battery voltage detector  22  and the overdischarge reference voltage, the controller  13  sets the power switching unit  31  to the second state when the voltage of the rechargeable battery  21  is less than the overdischarge reference voltage. This stops the supply of electric power to the motor  11 , which in turn, stops generating rotation. Accordingly, further discharging of the rechargeable battery  21  is stopped. 
     When a user wishes to continuously uses the electric power tool  1  though the motor  11  has stopped generating rotation, the user first switches off the activation switch  12  and then switches on the activation switch  12  again. By performing such operations, the electric power tool  1  repeats the activation operation described above. 
     In the electric power tool  1 , voltage drop caused by the internal resistance of the rechargeable battery  21  may be decreased by lowering the inrush current flowing to the motor  11  when the motor  11  is started. This prevents or inhibits the voltage of the rechargeable battery  21  from decreasing below the overdischarge reference voltage. 
     The control signal Cs of the controller  13  includes a pulse width modulation (PWM) control signal, which switches the FET of the power switching unit  31  between the first state and the second state. The PWM control signal includes a duty ratio, which is the ratio of the period when the FET is in the first state relative to a single cycle of a pulse signal between a high level and a low level. The amount of the electric power supplied to the motor  11  from the rechargeable battery  21  changes based on the duty ratio of the PWM control signal. 
     Based on a parameter that affects the voltage drop of the rechargeable battery  21 , the controller  13  changes the duty ratio of the PWM control signal to be provided to the power switching unit  31  when the electric power tool  1  is activated to change the electric power supplied to the motor  11 . The controller  13  employs battery temperature, which is indicated by the detection signal T 20  of the battery temperature detector  23 , as a parameter affecting the voltage drop of the rechargeable battery  21 . 
     The internal resistance of the rechargeable battery  21  has a temperature dependency. For example, the internal resistance of the rechargeable battery  21  increases as the temperature of the rechargeable battery  21  increases. The detection signal T 20  of the battery temperature detector  23 , which corresponds to the temperature in the battery pack  20 , may reflect the amount of the internal resistance of the rechargeable battery  21 . 
     The controller  13  changes the duty ratio of the PWM control signal corresponding to the temperature in the battery pack  20  of which range is a normal temperature, a high temperature, or a low temperature. The range of the normal temperature in the battery pack  20  is, for example, between 5 and 35° C. The range of the high temperature is, for example, greater than 35° C. The range of the low temperature is, for example, less than 5° C. 
     The activation of the electric power tool  1  will now be described with reference to  FIG. 2 . In  FIG. 2 , the waveforms in solid lines indicate the operation when the temperature of the battery pack  20  is normal, the waveforms in double-dashed lines indicate the operation when the temperature is high, and the waveforms in single-dashed lines indicate the operation when the temperature is low. 
     In accordance with the temperature of the battery pack  20  when the electric power tool  1  is activated at time to, the controller  13  sets the duty ratio of the PWM control signal for a start-up period from time t0 to time t10. For example, the controller  13  sets the duty ratio of the PWM control signal to 50% when the temperature in the battery pack  20  is normal. The controller  13  sets the duty ratio of the PWM control signal to a value greater than 50% when the temperature in the battery pack  20  is low. The controller  13  sets the duty ratio of the PWM control signal to a value less than 50% when the temperature in the battery pack  20  is high. That is, the controller  13  sets the duty ratio of the PWM control signal in such a manner that inhibits the fluctuation of the voltage drop of the rechargeable battery  21  depending on the temperature. Time t0 may be when a user switches on the activation switch  12 . Time t0 may be referred to as the motor start-up request time. The start-up period (t0-t10) may be referred to as the motor start-up period until which the duty ratio of the PWM control signal reaches 100%. In the motor start-up period (t0-t10), the electric power that is supplied to the motor  11  from the motor driver  30  may be referred to as the motor start-up power. 
     The start-up period ends at time t10. At time t10, the controller  13  sets the duty ratio of the PWM control signal to 100% and the FET of the power switching unit  31  to the first state. 
     During the start-up period, in which the rotation speed of the motor  11  gradually increases, the inrush current flows in correspondence with the rotation speed. As shown in  FIG. 2 , a motor current Im flowing to the motor  11  increases from the starting time t0 of the start-up period along a rising curve. The motor  11  generates electromotive force in correspondence with the rotation speed. The electromotive force of the motor  11  acts to decrease the inrush current. Thus, the motor current Im increases along the rising curve from the time t0 and then decreases. The motor current Im decreases as the rotation speed increases. 
     By setting the duty ratio of the PWM control signal for the start-up period, as shown in  FIG. 2 , the motor current Im is relatively large when the internal resistance of the rechargeable battery  21  is small and the temperature is low, and relatively small when the internal resistance of the rechargeable battery  21  is large and the temperature is high. 
     By setting the duty ratio of the PWM control signal to 100%, the motor current Im increases, and the rotation speed in the motor  11  increases. When the rotation speed in the motor  11  increases, an inrush current flows again. As shown in  FIG. 2 , the rotation speed of the motor  11  increases immediately after time t10. However, the rate of the increase is relatively small. Therefore, the inrush current flowing to the motor  11  immediately after the time t10 is smaller than the inrush current flowing to the motor  11  immediately after time t0. 
     The control of the duty ratio of the PWM control signal during the start-up period reduces the temperature dependency of the voltage drop of the rechargeable battery  21 . As a result, the voltage of the rechargeable battery  21  during the start-up period remains the same regardless of the temperature. That is, even when the internal resistance changes as the temperature of the battery pack  20  changes, the amount of the voltage drop due to the internal resistance remains the same. 
     During the start-up period, the temperature dependency of the voltage drop is reduced and the inrush current is decreased. This inhibits excessive voltage drop of the rechargeable battery  21 . 
     The electric power tool  1  has the advantages described below. 
     (1) The electric power tool  1  includes the controller  13  and the power switching unit  31 . During the start-up period, the controller  13  performs PWM control on the power switching unit  31 . The controller  13  changes the duty ratio of PWM control signal based on the detection signal T 20  received from the battery temperature detector  23 . The amount of the electric power supplied to the motor  11  changes in correspondence with the amount of the internal resistance of the rechargeable battery  21 . This prevents or inhibits excessive voltage drop of the rechargeable battery  21  during the start-up period of the electric power tool  1 . For example, even when the temperature in the battery pack  20  changes, overdischarging of the rechargeable battery  21  may be prevented or inhibited when the power tool  1  is activated. 
     (2) The controller  13  changes the duty ratio of the PWM control signal based on the detection signal T 20  received from the battery temperature detector  23 . This prevents or inhibits the voltage of the rechargeable battery  21  from decreasing below the overdischarge reference voltage. For example, forced suspension of the rotation in the motor  11  becomes limited immediately after start-up of the electric power tool  1 . This limits interruptions resulting from forced operation suspension of the motor  11  and improves the working efficiency. 
     (3) The controller  13  sets the duty ratio of the PWM control signal for the start-up period to a large value when the temperature in the battery pack  20  is lower than the normal temperature. Thus, the rotation speed in the motor  11  increases when the battery pack  20  is in the low temperature. This improves the working efficiency when the temperature in the battery pack  20  is low. 
     An electric power tool  1  of a second embodiment will now be described. The controller  13  of the second embodiment changes the duty ratio of the PWM control signal during the start-up period based on the detection signal V 10  of the voltage detector  15  instead of the detection signal T 20  of the battery temperature detector  23 . 
     The current including the motor current Im that flows to the electric power tool main body  10  changes in correspondence with the voltage applied between the terminal  41  and the terminal  42  of the electric power tool main body  10 . An increase in the voltage increases the current flowing to the electric power tool main body  10 . 
     The controller  13  employs voltage, which is indicated by the detection signal V 10  of the voltage detector  15 , as a parameter affecting the voltage drop of the rechargeable battery  21 . 
     The amount of the current supplied by the rechargeable battery  21  is in correspondence with the voltage between the terminal  41  and the terminal  42 , which is detected by the voltage detector  15 . Thus, the detection signal V 10  of the voltage detector  15  may reflect the internal resistance of the rechargeable battery  21  and/or the amount of the voltage drop. 
     The controller  13  changes the duty ratio of the PWM control signal in correspondence with whether the voltage of the rechargeable battery  21  is a standard voltage range, a high voltage range, or a low voltage range. For example, the standard voltage range is a range of ±10% of the standard voltage. The high voltage range is a range greater than the standard voltage range. The low voltage range is a range less than the standard voltage range. 
     In  FIG. 3 , the waveforms in solid lines indicate the operation when the voltage of the rechargeable battery  21  is standard, the waveforms in double-dashed lines indicate the operation when the voltage of the rechargeable battery  21  is high, and the waveforms in single-dashed lines indicate the operation when the voltage of the rechargeable battery  21  is low. 
     When the electric power tool  1  is activated at time to, the controller  13  determines the voltage of the rechargeable battery  21  based on the detection signal V 10  received from the voltage detector  15 . Then, the controller  13  sets the duty ratio of the PWM control signal for the start-up period based on the detection signal V 10  received from the voltage detector  15 . For example, the controller  13  sets the duty ratio of the PWM control signal to 50% when the rechargeable battery  21  has the standard voltage. The controller  13  sets the duty ratio of the PWM control signal to a value less than 50% when the voltage of the rechargeable battery  21  is high. The controller  13  sets the duty ratio of the PWM control signal to a value greater than 50% when the voltage of the rechargeable battery  21  is low. That is, the controller  13  sets the duty ratio of the PWM control signal to limit changes in the voltage of the rechargeable battery  21  during the start-up period resulting from the voltage prior to time t0. 
     By setting the duty ratio of the PWM control signal for the start-up period as shown in  FIG. 3 , the motor current Im is large when the voltage of the rechargeable battery  21  is relatively low, and small when the voltage of the rechargeable battery  21  is relatively high. 
     For example, even when the voltage of the rechargeable battery  21  differs before the activation switch  12  is switched on at time t0, the voltage of the rechargeable battery  21  gradually converges to the same voltage by controlling the duty ratio of the PWM control signal based on the voltage of the rechargeable battery  21 . That is, even when the voltage of the rechargeable battery  21  differs prior to time t0, the voltage of the rechargeable battery  21  during the start-up period remains the same. 
     Here, the duty ratio is controlled based on the voltage of the rechargeable battery  21  prior to the activation, and a re-inrush current is small. This limits excessive voltage drops in the rechargeable battery  21 . 
     The electric power tool  1  has advantages (1) to (3) of the first embodiment. 
     An electric power tool  1  of a third embodiment will now be described. The controller  13  of the third embodiment changes the duty ratio of the PWM control signal during the start-up period based on the detection signal I 20  of the battery current detector  24  instead of the detection signal T 20  of the battery temperature detector  23 . 
     During the start-up period, the motor current Im changes as the property of the motor  11  or the like vary. The change in the motor current Im changes the amount of the voltage drop of the rechargeable battery  21 . 
     The controller  13  employs battery current, which is indicated by the detection signal I 20  of the battery current detector  24 , as a parameter affecting the voltage drop of the rechargeable battery  21 . 
     The rechargeable battery  21  supplies the electric power tool main body  10  with current including the motor current Im. Therefore, the detection signal I 20  of the battery current detector  24 , which corresponds to the amount of the current flowing to the electric power tool main body  10 , may reflect the internal resistance of the rechargeable battery  21  and/or the amount of the voltage drop. 
     The controller  13  changes the duty ratio of the PWM control signal in accordance with whether the current flowing to the electric power tool main body  10  is in a standard, large, or small range. For example, a standard current range is ±10% of the standard current of the electric power tool main body  10 . The large current range is greater than the standard current range. The small current range is smaller than the standard current range. 
     When the electric power tool  1  is activated, the controller  13  determines the increase rate of the current flowing to the electric power tool main body  10  based on the detection signal I 20  received from the battery current detector  24 . Then, the controller  13  determines the amount of the current flowing to the electric power tool main body  10  based on the current increase rate. During the start-up period, the controller  13  changes the duty ratio of the PWM control signal based on the amount of the current flowing to the electric power tool main body  10 , which is determined using the detection signal I 20  of the battery current detector  24 . 
     For example, the controller  13  sets the duty ratio of the PWM control signal to 50% when the amount of the current flowing to the electric power tool main body  10  is standard. The controller  13  sets the duty ratio of the PWM control signal to a value less than 50% when the amount of the current flowing to the electric power tool main body  10  is large. The controller  13  sets the duty ratio of the PWM control signal to a value greater than 50% when the amount of the current flowing to the electric power tool main body  10  is small. That is, the controller  13  sets the duty ratio of the PWM control signal to limit changes in the voltage drop of the rechargeable battery  21  that occur in accordance with the amount of the current flowing to the electric power tool main body  10 . 
     During the start-up period, the duty ratio of the PWM control signal is controlled based on the amount of the current flowing to the electric power tool main body  10 . As a result, the voltage of the rechargeable battery  21  remains the same regardless of the increase rate of the current flowing to the electric power tool main body  10  during the start-up period. 
     Here, the duty ratio is controlled based on the current flowing to the electric power tool main body  10  during the start-up period, and a re-inrush current is reduced. This limits excessive voltage drops in the rechargeable battery  21 . 
     The electric power tool  1  has advantages (1) to (3) of the first embodiment. 
     An electric power tool  1  of a fourth embodiment differs from the electric power tool  1  of the first embodiment in the following aspect. The controller  13  of the first embodiment sets the duty ratio of the PWM control signal for the start-up period based on the detection signal T 20  received from the battery temperature detector  23 . In contrast, the controller  13  of the fourth embodiment continuously, or linearly, increases the duty ratio of the PWM control signal during the start-up period and changes the increase rate of the duty ratio based on the detection signal T 20  of the battery temperature detector  23 . 
       FIG. 4  shows an example of the operation that continuously increases the duty ratio of the PWM control signal during the start-up period when the temperature in the battery pack  20  varies. In  FIG. 4 , the waveforms in solid lines indicate the operation performed when the temperature of the battery pack  20  is normal, the waveforms in double-dashed lines indicate the operation when the temperature of the battery pack  20  is high, and the waveforms in single-dashed lines indicate the operation when the temperature of the battery pack  20  is low. 
     The controller  13  continuously increases the duty ratio of the PWM control signal in the range from 0% to 100% in accordance with the time elapsed from time t0, which is when the electric power tool  1  is activated. In the electric power tool  1 , a drastic increase in the rotation speed of the motor  11  is limited by continuously increasing the duty ratio of the PWM control signal during the start-up period. This limits the flow of a large inrush current to the motor  11 . 
     The controller  13  determines the temperature of the battery pack  20  at time t0, which is when the electric power tool  1  is activated, and sets the increase rate of the duty ratio of the PWM control signal in accordance with the temperature of the battery pack  20  detected at time t0. For example, the controller  13  sets the increase rate for the duty ratio of the PWM control signal to a larger value under a low temperature than under a normal temperature. For example, the controller  13  sets a smaller increase rate for the duty ratio of the PWM control signal at a high temperature than at a normal temperature. That is, the controller  13  sets the increase rate of the duty ratio of the PWM control signal to limit changes in the voltage drop of the rechargeable battery  21  caused by the temperature. 
     The length of the start-up period varies depending on the increase rate of the duty ratio of the PWM control signal. In the illustrated example, the start-up period ends when the duty ratio of the PWM control signal reaches 100%. When the temperature in the battery pack  20  is low, the duty ratio of the PWM control signal reaches 100% at time t01. Accordingly, the controller  13  shortens the start-up period (t0-t01). When the temperature in the battery pack  20  is high, the duty ratio of the PWM control signal reaches 100% at time t11. Accordingly, the controller  13  prolongs the start-up period (t0-t11). 
     During the start-up period, by setting the increase rate of the duty ratio of the PWM control signal based on the internal resistance of the rechargeable battery  21 , the maximum motor current Im remains the same even when the temperature of the battery pack  20  changes. 
     The temperature dependency of the voltage drop of the rechargeable battery  21  is reduced during the start-up period. This stabilizes the amount of voltage drop even when the temperature of the battery pack  20  changes. 
     Here, the temperature dependency of the voltage drop during the start-up period and an inrush current are reduced. This limits excessive voltage drops in the rechargeable battery  21 . 
     The electric power tool  1  of the fourth embodiment has the following advantage in addition to advantages (1) to (3) of the first embodiment. 
     (4) During the start-up period, the controller  13  continuously increases the duty ratio of the PWM control signal and determines the increase rate based on the detection signal indicating the operation parameter that affects the voltage drop of the rechargeable battery  21 . Thus, in the electric power tool  1 , during the start-up period, the inrush current of the motor  11  may be reduced in comparison with when the duty ratio of the PWM control signal is not continuously increased. This further effectively limits excessive voltage drops in the rechargeable battery  21  during the start-up period, and limits the occurrence of overdischarging of the rechargeable battery  21  during the start-up period. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
     In a modified example, the controller  13  may change the duty ratio of the PWM control signal for starting the motor  11  based on the detection signal T 10  received from the temperature detector  16 . 
     In a modified example, the controller  13  may change the duty ratio of the PWM control signal for starting the motor  11  based on the detection signal V 20  received from the battery voltage detector  22 . 
     In a modified example, the controller  13  may change the duty ratio of the PWM control signal for starting the motor  11  based on the detection signal I 10  received from the current detector  17 . 
     In a modified example, the controller  13  may change the duty ratio of the PWM control signal for starting the motor  11  based on the combination of the detection signals provided by at least two of the battery temperature detector  23 , the voltage detector  15 , the battery current detector  24 , the temperature detector  16 , the battery voltage detector  22 , and the current detector  17 . 
     In a modified example, the controller  13  may calculate the internal resistance of the rechargeable battery  21  based on the temperature in the battery pack  20  that is detected by the battery temperature detector  23 , and set the duty ratio of the PWM control signal based on the calculated internal resistance. 
     In a modified example, the controller  13  may set the duty ratio of the PWM control signal based on the voltage of the rechargeable battery  21  that is detected by the voltage detector  15 . 
     In a modified example, the controller  13  may set the duty ratio of the PWM control signal based on the current that is provided by the rechargeable battery  21  and detected by the battery current detector  24 . 
     In a modified example, the controller  13  may set the duty ratio of the PWM control signal based on the temperature in the vicinity of the motor that is detected by the temperature detector  16 . 
     In a modified example, the controller  13  may set the duty ratio of the PWM control signal based on the amount of the voltage of the rechargeable battery  21  that is detected by the battery voltage detector  22 . 
     In a modified example, the controller  13  may set the duty ratio of the PWM control signal based on the amount of the current that flows to the electric power tool main body  10  and is detected by the current detector  17 . 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. Also, in the above description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the claims are hereby incorporated into the description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.