Abstract:
An electric working machine comprises a motor, a switch, a drive device, a brake device, and a failure determiner. The switch is configured for operation by a user, has an on-state and an off-state. The drive device is configured to drive the motor in response to the switch being placed in the on-state. The brake device is configured to control deceleration of the motor to a stopped state in response to the switch being placed in the off-state. The failure determiner is configured to monitor deceleration of the motor during controlled deceleration and to determine whether the brake device has failed based on the monitored deceleration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Japanese Patent Application No. 2016-084683 filed on Apr. 20, 2016 with the Japan Patent Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present invention is related to an electric working machine having a brake function to generate braking force when the driving of a motor stops. 
         [0003]    For example, an electric working machine, such as an electric power tool, is known, which comprises a brake function to supply braking current to a motor to immediately stop the motor when the power source driving the motor stops (Japanese Patent Publication No. 5777924, and the corresponding U.S. Pat. No. 8,766,569). 
         [0004]    With this type of electric working machine, safety can be improved wherein a tool bit, such as a grinding stone or rotational blades, is inhibited from being continuously rotated by inertia when a user switches off an operation switch to stop the driving of the motor. 
       SUMMARY 
       [0005]    As disclosed in, for example, JP 5777924, the above-described brake function can be achieved by a brake device including an electric circuit that supplies braking current to a motor to generate braking force and a brake that applies braking force directly to the rotational shaft of the motor. However, if the brake device fails, the desired braking force cannot be generated. 
         [0006]    Moreover, for example, if the breaking force, generated when the driving of the motor is stopped, is large, a sudden brake is applied to the tool bit, which gives the user discomfort and may cause loosening of the tool bit. On the other hand, if the breaking force, generated when the driving of the motor is stopped, is small, the rotation of the tool bit cannot be quickly stopped, which prevents the brake function from being fully implemented. 
         [0007]    According to one aspect of the present disclosure, an electric working machine is desirably configured to detect failure of a brake device that generates braking force when the driving of a motor is stopped and further to notify a user of the failure. 
         [0008]    An electric working machine according to one aspect of the present disclosure comprises a motor, a switch, a drive device, a brake device, and a failure determiner. 
         [0009]    The switch is configured for operation by a user, has an on-state and an off-state. The drive device is configured to drive the motor in response to the switch being placed in the on-state. The brake device is configured to control deceleration of the motor to a stopped state in response to the switch being placed in the off-state. 
         [0010]    The failure determiner is configured to monitor deceleration of the motor during controlled deceleration and to determine whether the brake device has failed based on the monitored deceleration. 
         [0011]    Accordingly, the electric working machine according to the present disclosure can detect failure of the brake device by the failure determiner and notify the user of the failure. 
         [0012]    The failure determiner may be further configured to determine whether the brake device has failed based on whether the monitored deceleration falls outside a predetermined range of values. 
         [0013]    The failure determiner may be further configured to notify the user whether the brake device has failed. 
         [0014]    The failure determiner may be further configured to notify the user in the event the failure determiner determines that the brake device has failed. 
         [0015]    Another aspect of the present disclosure provides an electric working machine comprising a motor, a drive device configured to drive the motor, a brake device configured to generate braking force to stop rotation of the motor when driving of the motor by the drive device is finished, and a failure determiner. 
         [0016]    The failure determiner is configured to detect deceleration of the motor when the driving of the motor by the drive device is stopped and to determine whether the brake device has failed based on the deceleration. 
         [0017]    Accordingly, the electric working machine according to the present disclosure can detect failure of the brake device by the failure determiner and notify the user of the failure. 
         [0018]    “Driving of the motor by the drive device is finished” and “driving of the motor by the drive device is stopped” described herein may be to stop supplying driving current from the drive device to the motor. 
         [0019]    The failure determiner may be configured to determine that the brake device has failed when the deceleration of the motor exceeds a specified maximum value and the motor is suddenly decelerated. 
         [0020]    Moreover, the failure determiner may be configured to determine that the brake device has failed when the deceleration of the motor falls below a specified minimum value and the motor cannot be sufficiently braked. 
         [0021]    Furthermore, the failure determiner may be configured to notify a user of failure of the brake device when the brake device is determined to have failed, or to disable the drive device so as not to drive the motor. 
         [0022]    When the failure determiner detects failure of the brake device, if the drive device is immediately disabled so as not to drive the motor, the electric working machine cannot be operated for any work. Thus, the failure determiner may be configured to allow the drive device to drive the motor for a specific period of time when the brake device is determined to have failed, and then disable the drive device so as not to drive the motor when the specific period of time passes. 
         [0023]    Moreover, the failure determiner may be configured to sample the number of revolutions of the motor at specific intervals and to detect the deceleration of the motor based on a difference between a most recent sampling result and a sampling result obtained a specified period of time before. 
         [0024]    Accordingly, by suitably setting the interval (in other words, time difference) to obtain two sampling results used for detecting the deceleration of the motor, the deceleration corresponding to the braking force generated by the brake device can be detected without being influenced by the rotational variation. Therefore, in this case, failure of the brake device can be more accurately determined in accordance with the detected deceleration. 
         [0025]    Further aspect of the present disclosure provides a method for operating an electric working machine having a motor and a switch configured for operation by a user, the switch having an on-state and an off-state. The method comprises driving the motor in response to the switch being placed in the on-state, controlling deceleration of the motor to a stopped state in response to the switch being placed in the off-state, monitoring deceleration of the motor during controlled deceleration, and determining whether the brake device has failed based on the monitored deceleration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The present invention will now be described by way of example with reference to the accompanying drawings, in which: 
           [0027]      FIG. 1  is an external view showing the structure of an alternating-current driven type grinder according to an embodiment; 
           [0028]      FIG. 2  is a block diagram showing the overall configuration of a drive system of the grinder shown in  FIG. 1 ; 
           [0029]      FIG. 3  is a flowchart illustrating a failure determination process executed by a MCU shown in  FIG. 2 ; 
           [0030]      FIG. 4  is a time chart illustrating deceleration and failure determination operations used for failure determination; and 
           [0031]      FIG. 5  is a block diagram showing the configuration of a direct-current driven type drive system of an electric working machine. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    The following will describe an embodiment of the present disclosure with reference to the drawings. 
         [0033]    As shown in  FIG. 1 , a grinder  2  of the present embodiment mainly comprises a motor housing  4 , a gear housing  6 , and rear cover  8 . 
         [0034]    The motor housing  4  is approximately cylindrical in shape, and accommodates a motor  20 . The motor  20  is housed in the motor housing  4  such that the rotational shaft of the motor  20  is disposed in parallel to the central axis of the motor housing  4 . One end of the rotational shaft protrudes toward the gear housing  6 . 
         [0035]    The rotational shaft of the motor  20  is coupled with spindle  12 , externally protruding from the gear housing  6 , via a gear mechanism disposed in the gear housing  6 . 
         [0036]    The spindle  12  is rotatably disposed in the gear housing  6  such that the central shaft of the spindle  12  perpendicularly crosses the rotational shaft of the motor  20 . The gear mechanism in the gear housing  6  is configured, for example, with a bevel gear so as to convert the rotation of the motor  20  into the rotation of the spindle  12 . Since the gear mechanism is similarly configured to common grinders, the detailed description the gear mechanism is omitted here. 
         [0037]    The spindle  12 , protruding from the gear housing  6 , is provided with an inner flange  14  for positioning and securing a circular plate-shaped tool bit  16 . Toward the distal end of the spindle  12  further than the inner flange  14 , a locknut  18  is threadedly engaged to hold the tool bit  16  between the locknut  18  and the inner flange  14 . 
         [0038]    Accordingly, disposing the tool bit  16  between the inner flange  14  and the locknut  18  and fastening the locknut  18  toward the inner flange  14  enables the tool bit  16  to be tightly secured. For the grinder  2  in the present embodiment, the tool bit  16  may be, for example, a grinding stone, a cutting-off wheel, or a wire brush. 
         [0039]    Moreover, around the protruding portion of the spindle  12  in the gear housing  6 , a wheel cover  19  is secured to protect a user from bits of a workpiece or the tool bit  16  scattering when, for example, grinding, polishing, or cutting is performed. 
         [0040]    On the lateral wall of the gear housing  6 , a hole  7  is formed for externally attaching a grip that is held in a user&#39;s hand. A rear cover  8  is disposed in the opposite side of the gear housing  6  of the motor housing  4 . From the rear end, which is in the opposite side of the gear housing  6 , a power cord  9  is drawn out to receive power supply from a commercial power source, which is an alternating current (AC) power source  10  (see  FIG. 2 ). 
         [0041]    The power cord  9  comprises, at the distal end thereof, a power plug that can be connected to an electric outlet of the AC power source  10  and is configured to supply AC power from the AC power source  10  to the grinder  2  when the power plug is inserted into the electric outlet. 
         [0042]    Inside the rear cover  8 , a controller  40  (see  FIG. 2 ) is housed to perform drive control of the motor  20  with the AC power supplied from the AC power source  10 . On the lateral wall of the motor housing  4 , an operation switch  30  is disposed to complete or interrupt the electric conduction path for supplying electric power from the AC power source  10  to the motor  20 . 
         [0043]    The motor  20  is a commutator motor (so-called brushed motor) having a mechanical commutator and a brush to switch electric current flowing through an armature in accordance with the rotational phase and to maintain the rotational moment in a specific direction. In the present embodiment, a single-phase series-wound commutator motor (so-called universal motor) is used. 
         [0044]    The controller  40  comprises a pair of drive switches  32 ,  34 , each of the switches  32 ,  34  being disposed on the opposite sides of the connecting point of a pair of field windings (so-called field coils)  24 ,  26  (that is, at both ends of the pair of field windings  24 ,  26 ) of the motor  20 . 
         [0045]    The pair of the drive switches  32 ,  34  is operated in association with the operation of the operation switch  30 . When the operation switch  30  is in the off-state, the drive switches  32 ,  34  are each switched to a contact point “a”, and when the operation switch  30  is in the on-state, the drive switches  32 ,  34  are each switched to a contact point “b”. 
         [0046]    When the operation switch  30  is in the on-state, in which the drive switches  32 ,  34  are switched to the contact points “b”, the field winding  24  is connected to one end of the AC power source  10  via the drive switch  32 , and the field winding  26  is connected to the armature  22  via the drive switch  34 . 
         [0047]    The armature  22  includes a commutator in the present embodiment. One end of the armature  22 , on the opposite side of the field windings  26 , is connected to the other end of AC power source  10  via a bidirectional thyristor  36 . 
         [0048]    Accordingly, when the drive switches  32 ,  34  are switched to the contact points “b” (drive position), a drive current path of the motor  20  is formed that includes the AC power source  10 , the bidirectional thyristor  36 , the armature  22 , and the field windings  26 ,  24 . When the bidirectional thyristor  36  is in the on-state, drive current flows through the armature  22  and the field windings  26 ,  24 , and the motor  20  is rotated in one direction. 
         [0049]    The bidirectional thyristor  36  is a known thyristor configured with an electric current drive type semiconductor device, and used, in the present embodiment, to control conduction current (driving current) to the motor  20  when the operation switch  30  is in the on-state. 
         [0050]    On the other hand, when the operation switch  30  is in the off-state and the drive switches  32 ,  34  are switched to the contact points “a” (brake position), the field windings  24 ,  26  are connected to the armature  22  in a direction opposite to the direction when the motor  20  is driven. 
         [0051]    In other words, to one end of the armature  22 , connected to the AC power source  10 , the field winding  26  is connected via the drive switch  34 . To the other end of the armature  22 , where the field winding  26  is connected when the motor  20  is driven, the field winding  24  is connected via the drive switch  32 . 
         [0052]    On the path connecting one end of the armature  22  to the contact point “a” of the drive switch  34 , a diode D 1 , an FET  38  (field effect transistor), a resistor R 1 , and a fuse  39  are disposed in this order from the side of the armature  22 . 
         [0053]    The diode D 1  is provided to run brake current from the armature  22  toward the drive switch  34 . The FET  38  is provided to complete and interrupt the current path. Moreover, the resistor R 1  for current detection is provided to detect the brake current flowing through the current path. The fuse  39  is provided to fuse and disconnect the current path when electric current equal to or larger than a specified value flows in the current path. 
         [0054]    Moreover, the current path between the diode D 1  and the FET  38  is connected to the other end of the armature  22  via a capacitor C 1  for charge accumulation. The other end of the armature  22  is also connected to the current path between the FET  38  and the resistor R 1  for current detection via a diode D 2 . 
         [0055]    The anode of the diode D 2  is directly connected to the other end of the armature  22  (in other words, the contact point “a” of the drive switch  32 ), while the cathode is directly connected to the current path between the FET  38  and the resistor R 1  for electric current detection. 
         [0056]    Between the current path extending between the diode D 1  and the FET  38  and the current path extending between the diode D 2  and from the resistor R 1  to the FET  38 , a brake control circuit  54  is connected to drive and control the FET  38  upon receiving power supply from each of the above-described current paths. 
         [0057]    To the brake control circuit  54 , a current detection circuit  56  is connected to detect electric current from the voltage at both ends of the resistor R 1  and to output a detection signal to the brake control circuit  54 . 
         [0058]    The brake control circuit  54  detects brake current flowing in the field windings  24 ,  26  via the current detection circuit  56  when the drive switches  32 ,  34  are in the brake position, that is, to the contact point “a”, and switches the on-off-state of the FET  38  such that the value of the electric current becomes a desired value. 
         [0059]    Consequently, when the operation switch  30  is switched from the on-state to the off-state, a brake current flows in the motor  20  and the value of the current is controlled via the brake control circuit  54 . The control operation of the brake control circuit  54  is described in detail in U.S. Pat. No. 8,766,569, the disclosure of which is incorporated herein by reference. 
         [0060]    The controller  40  comprises a drive circuit  42 , configured to drive the bidirectional thyristor  36 , a revolution detection circuit  44 , configured to detect the number of revolutions of the motor  20  (specifically, rotational speed), a switching detection circuit  46 , and a zero-cross detection circuit  48 . 
         [0061]    The switching detection circuit  46  is provided to detect the on-state of the operation switch  30  and configured to detect that the operation switch  30  is switched to the on-state from a voltage change at a connecting portion between the drive switch  32  and the field winding  24 . 
         [0062]    The zero-cross detection circuit  48  is provided to detect zero-cross points of the AC voltage supplied from the AC power source  10  and is configured to be connected to the electric conduction path between the contact point “b” of the drive switch  32  and the AC power source  10  to detect the zero-cross points from a voltage change in the path. 
         [0063]    The drive circuit  42 , the revolution detection circuit  44 , the switching detection circuit  46 , and the zero-cross detection circuit  48  are connected to a MCU (that is, Micro Control Unit)  50 . The MCU  50  is a known control unit comprising a CPU, a ROM, and a RAM and is operated upon receiving drive voltage (direct current (DC) constant voltage) from a power source circuit  52 . 
         [0064]    The power source circuit  52  generates drive voltage (DC constant voltage) from AC power supplied from the AC power source  10  and supplies the drive voltage to the internal circuits of the controller  40  including the MCU  50 . Thus, the controller  40  is operated upon receiving power supply from the AC power source  10  even when the operation switch  30  is in the off-state. To the MCU  50 , an indicator (such as LEDs)  58  is also connected to indicate the state of the grinder  2 . 
         [0065]    When the operation switch  30  is in the on-state, the MCU  50  controls motor current by adjusting the time from when a zero-cross point is detected by the zero-cross detection circuit  48  until when the bidirectional thyristor  36  is switched on in accordance with a specified driving speed. 
         [0066]    Moreover, when the operation switch  30  is switched from the on-state to the off-state, the MCU  50  operates the brake control circuit  54  to generate braking force for the motor  20  until the rotation of the motor  20  stops. 
         [0067]    While the braking force is generated for the motor  20 , the MCU  50  monitors the deceleration state (deceleration) of the motor  20  based on the output from the revolution detection circuit  44  and determines a malfunction of the brake system if the deceleration of the motor  20  falls out of a tolerance range. 
         [0068]    The following describes the control process executed in the MCU  50  in accordance with the flowchart shown in  FIG. 3 . 
         [0069]    As shown in  FIG. 3 , in S 110 , the MCU  50  determines whether the operation switch  30  is in the on-state and waits for the operation switch  30  to be switched into the on-state. When the operation switch  30  is determined to be in the on-state in S 110 , the process proceeds to S 120 . In S 120 , the drive circuit  42  is controlled to drive the bidirectional thyristor  36  to start driving the motor  20  and detecting the number of revolutions by the revolution detection circuit  44 . 
         [0070]    The number of revolutions is detected, as shown in  FIG. 4 , by periodically sampling the number of revolutions of the motor  20  (specifically, rotational speed) N detected by the revolution detection circuit  44  at every specific sampling time is and by storing the sampling results for the past specific period of time in the RAM. 
         [0071]    Once the driving of the motor  20  and the detecting of the rotation of the motor  20  are initiated in S 120 , the process proceeds to S 130  wherein the MCU  50  determines whether the operation switch  30  is switched into the off-state and waits until the operation switch  30  is switched into the off-state. 
         [0072]    When it is determined in S 130  that the operation switch  30  is in the off-state, the process proceeds to S 140  wherein the drive circuit  42  is controlled to stop (finish) the driving of the motor  20 , that is, to stop supplying driving current from the drive circuit  42  to the motor  20 , and the brake control circuit  54  is controlled to initiate brake control. 
         [0073]    In S 150 , from the revolution detection data of the motor  20  obtained from the sampling initiated in S 120 , the most recent number of revolutions N 2  and the number of revolutions N 1  sampled a specified period of time before are read, and the difference (N 1 -N 2 ) is calculated as the deceleration of the motor  20 . 
         [0074]    In S 160 , it is determined whether the deceleration (N 1 -N 2 ) calculated in S 150  is within the tolerance range defined by the specified maximum and minimum values. As shown in the upper portion of  FIG. 4 , when the deceleration (N 1 -N 2 ) is determined to be within the tolerance range, it is determined that the brake control is performed in a normal state. Then, the process proceeds to S 170 . 
         [0075]    In S 170 , it is determined, from the most recent number of revolutions N 2  of the motor  20 , whether the rotation of the motor  20  has stopped. If it is determined that the rotation of the motor  20  has stopped, it is determined that the motor  20  has been normally driven and stopped, and the control process is terminated in a normal state. When the process is terminated, the process goes back to S 110 , and the MCU  50  waits for the operation switch  30  to be operated again. Although the upper portion of  FIG. 4  may seem to show that the time interval between time t 1  for sampling the number of revolutions N 1  and time t 2  for sampling the number of revolutions N 2  is six cycles of the sampling cycles ts, the interval between t 1  and t 2  may be set for a suitable length. For example, t 2  may be defined to be at one sampling cycle after t 1 . 
         [0076]    In S 170 , it is determined that the rotation of the motor  20  has not stopped (in other words, the motor  20  is decelerating), the process goes back to S 150  wherein the deceleration of the motor  20  is calculated. Then, in S 160 , it is determined whether the deceleration is within the tolerance range. 
         [0077]    In S 160 , if it is determined that the deceleration of the motor  20  is not within the tolerance range, the process proceeds to S 180  wherein, as shown in the middle portion in  FIG. 4 , it is determined whether the deceleration (N 1 -N 2 ) exceeds the maximum value. 
         [0078]    If it is determined that the deceleration of the motor  20  exceeds the maximum value, the process proceeds to S 190 . In S 190 , it is determined that sudden brake has been applied to the motor  20  due to brake failure of the motor  20  and the determination is stored. 
         [0079]    The brake failure may be stored in S 190  and S 220 , which will be described later, only temporarily in the RAM of the MCU  50 , or may be stored in a nonvolatile memory of the ROM in the MCU  50  so that the failure history can be checked later. 
         [0080]    As described above, when a brake failure (sudden brake) is detected in S 190 , the process proceeds to S 200  wherein the brake failure is notified by indicating the brake failure on the indicator  58 . In S 210 , further driving of the motor  20  is disabled and then the control process is terminated in an abnormal state. 
         [0081]    After the process is terminated in an abnormal state, since the driving of the motor  20  is disabled, power supply at least to the MCU  50  is stopped until the MCU  50  is restarted. Thus, the control process is not performed even if a user operates the operation switch  30 , and the stopped state of the motor  20  is maintained. 
         [0082]    In S 180 , if it is determined that the deceleration of the motor  20  does not exceed the maximum value, in other words, the deceleration (N 1 -N 2 ) falls below the minimum value as shown in the lower portion in  FIG. 4 , the process proceeds to S 220 . 
         [0083]    In S 220 , it is determined that, due to the brake failure of the motor  20 , the motor  20  is uncontrollable, and such determination is stored. When brake failure (uncontrollable braking) is detected in S 220  as described above, the process proceeds to S 230  wherein the brake failure is shown on the indicator  58  and the user is notified of the brake failure. In S 240 , it is determined whether the driving of the motor  20  is disabled. 
         [0084]    In S 240 , if it is determined that the driving the motor  20  is not disabled, the control process is terminated. On the other hand, if it is determined in S 240  that the driving of the motor  20  is disabled, a timer is set to disable the driving of the motor  20  when a specific period of time passes after the brake failure is currently detected, and the control process is terminated. 
         [0085]    As described above, in the present embodiment, when brake failure (uncontrollable braking) is detected in S 220 , the control process is terminated either after the brake failure (uncontrollable braking) is notified, or after the timer is set to the timing to disable the driving of the motor  20 . 
         [0086]    This is because, even if brake failure (uncontrollable braking) takes place, the motor  20  can still be driven via the bidirectional thyristor  36  and the drive circuit  42 , serving as the drive device, and the rotation of the motor  20  can be stopped by stopping the driving of the motor  20 . 
         [0087]    In other words, if brake failure (uncontrollable braking) takes place, the user is notified of the failure in the present embodiment. The user can be aware that braking force cannot be generated when the driving of the motor  20  is stopped and it would take time until the motor  20  stops. Since the user can rotate the motor  20  while being aware of such state, the user can continue to work with the grinder  2  in consideration of safety. 
         [0088]    Moreover, in the present embodiment, by establishing the setting to disable the driving the motor  20  after a specific period of time, the driving of the motor  20  after the work with the grinder  2  finishes can be disabled. Consequently, a user who is unaware of brake failure can be inhibited from using the grinder  2 . 
         [0089]    On the other hand, if brake failure takes place and sudden brake is applied to the motor  20 , the driving of the motor  20  is immediately disabled in addition to the user being notified of the brake failure. This is because, if the motor  20  suddenly is decelerated, the spindle  12  is also suddenly decelerated, which may cause loosening of the tool bit  16  fastened by the locknut  18 . 
         [0090]    In other words, in the present embodiment, when sudden brake is applied to the motor  20  due to brake failure, the driving of the motor  20  after the sudden brake is disabled so that the motor  20  is inhibited from being driven while the fastening of the tool bit  16  by the locknut  18  is loosened. 
         [0091]    As described above, according to the grinder  2  of the present embodiment, when an abnormality takes place in the FET  38 , the brake control circuit  54 , or the circuits around these devices, which serve as the brake device, brake failure can be detected, the user informed, and, if necessary, the driving of the motor  20  can be disabled. Therefore, according to the present embodiment, the grinder  2  can be inhibited from being unavailable due to brake failure and safety in using the grinder  2  can be improved. 
         [0092]    In the present embodiment, the processes in S 150  to S 250  in the control process executed by the MCU  50  are one example of the failure determiner of the present disclosure. 
         [0093]    Although the above has described one embodiment of the present disclosure, the present disclosure is not limited to the above-described embodiment, but may be carried out in various ways. 
         [0094]    For example, in the above-described embodiment, the motor  20  is configured with a single-phase series-wound commutator motor, and the controller  40  is configured to control driving of the motor  20  upon receiving power supply from the AC power source  10 . 
         [0095]    However, the technique of the present disclosure can be applied in a similar manner as in the above-described embodiment to, for example an electric working machine, as shown in  FIG. 5 , that comprises a brushed DC motor (to be referred to as a motor  60 ) and is configured to drive the motor  60  at DC power voltage Vdc supplied from a DC power source, such as a battery. 
         [0096]    That is, as shown in  FIG. 5 , a switching element  62 , serving as the drive device, is serially coupled to the motor  60 . When a switching detection circuit  72  detects that an operation switch (not shown) is switched on, a MCU  70  switches on the switching element  62  to run drive current through the motor  60  and to rotate the motor  60  in one direction. 
         [0097]    Moreover, a series circuit, having a switching element  64  and a fuse  66  serving as the brake device, and a diode (so-called flywheel diode)  68  for reflux are coupled to the motor  60  in parallel. When a switching detection circuit  72  detects that the operation switch is switched from the on-state to the off-state, a MCU  70  switches off the switching element  62  and switches on the switching element  64 . As a result, brake current runs into the motor  60  and braking force is generated. 
         [0098]    The electric working machine configured as above can also achieve the same effect as in the above-described embodiment if the configuration is such that a detection signal, outputted from the revolution detection circuit  74  that detects the number of revolutions of the motor  60 , is input into the MCU  70  and the control process shown in  FIG. 3  is executed by the MCU  70 . 
         [0099]    On the other hand, in the above-described embodiment, the number of revolutions of the motor  20  is sampled and the deceleration of the motor  20  is obtained from the difference between the most recent sampling value (the number of revolutions N 2 ) and the sampling value (the number of revolutions N 1 ) obtained a specified period of time before. 
         [0100]    However, the deceleration of the motor  20  may be, for example, calculated from the number of revolutions of the spindle  12 , or the deceleration may be detected with, for example, an acceleration speed sensor. In other words, the deceleration of the motor  20  may be obtained with various conventionally-known techniques. 
         [0101]    Furthermore, in the above-described embodiment, brake failure is determined when deceleration of the motor  20  falls outside the tolerance range defined by the maximum and the minimum values. Brake failure may be determined only when the deceleration exceeds the maximum value. Alternatively, brake failure may be determined only when the deceleration falls below the minimum value. When brake failure is determined, the brake failure may be only brought to the user&#39;s attention, or the driving of the motor  20  may be simply stopped. 
         [0102]    Moreover, in the above-described embodiment, brake function of the motor  20  is achieved by running brake current to the motor windings and generating braking force in the motor  20 . The configuration may be such that the brake function is achieved by providing a motor with windings for braking. Alternatively, the configuration may be such that brake function is achieved by a brake device that applies braking force directly to the rotational shaft of a motor. 
         [0103]    Still furthermore, in the above-described embodiment, the grinder  2  is provided as one example of the electric working machine. The electric working machine according to the present disclosure may be any type of electric working machine that has brake function to generate braking force when the motor, serving as a power source, is stopped. An example of the electric working machine includes electric power tools for stone work, electric power tools for metal work, electric power tools for woodwork, and electric power tools for gardening. 
         [0104]    More specifically, the present disclosure may be applied to various electric working machines including electric hammers, electric hammer drills, electric drills, electric drivers, electric wrenches, electric circular saws, electric reciprocating saws, electric jigsaws, electric hammers, electric cutters, electric chain saws, electric planes, electric nailguns (including reveting machines), electric hedge trimmers, electric lawn mowers, electric lawn clippers, electric trimmers, electric cleaners, and electric blowers. 
         [0105]    A plurality of functions possessed by one component in the above-described embodiment may be achieved by a plurality of components, or one function possessed by one component may be achieved by a plurality of components. Furthermore, a plurality of functions possessed by a plurality of components may be achieved by one component, or one function achieved by a plurality of components may be achieved by one component. Moreover, the configuration of the above-described embodiment may be partially omitted. At least a part of the configuration of the above-described embodiment may be added to or altered with the configurations of other embodiments. Various aspects included in the technical ideas specified by the expressions used in the claims correspond to the embodiments of the present disclosure. 
         [0106]    Still furthermore, the technique according to the present disclosure can be achieved by, in addition to the electric working machine, various ways including a system comprising the electric working machine as a component, a program for a computer to function as the electric working machine, a recording medium, such as a semiconductor memory, having this program recorded therein, or a method for controlling an electric working machine.