Abstract:
A dust collection device has a tank, a suction unit, a filtering unit, a dust removal unit, a first detection unit, and a control unit. The tank has an air inlet for accommodating dust. The suction unit aspirates the dust through the air inlet. The filtering unit captures the dust through the air inlet. The dust removal unit removes the dust from the filtering unit. The first detection unit detects an operational state of the suction unit. The control unit controls the suction unit and the dust removal unit. The control unit activates the dust removal unit, when the first detection unit detects that the suction unit operates for a first predetermined time period and then stop operating.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a dust collection device having a dust removal unit for cleaning a filter unit of the dust collection device. 
       BACKGROUND 
       [0002]    Generally, dust collectors are designed to aspirate external air into a tank having an air inlet port by means of a suction device, filter off powdery dust from the aspirated air including the dust, collect the filtered powdery dust and discharge the air cleaned by the filtering. 
         [0003]    The filter fitted to a dust collector collects the powdery dust produced by an electric tool. When the entire surface of the filter is clogged with the powdery dust, the filter can quickly become fully loaded with the dust to reduce the air suction effect thereof. Then, the filter needs to be cleaned. The filter is generally required to be removed from the dust collector before the filter is cleaned. 
         [0004]    With conventional dust collectors, each time the operator visually finds that the filter is clogged, the dust clogging the filter needs to be manually removed, which is a cumbersome operation. Additionally, since a unit is not provided for the operator to directly recognize a clogged condition of the filter, the operator is required to judge the clogged condition of the filter on the basis of his or her experience and/or the phenomenon that the suction force of the filter has fallen and the filter can no longer aspirate the powdery dust effectively. 
       SUMMARY 
       [0005]    An object of the present invention is to provide a dust collection device from which dust can be easily removed in accordance with the operating condition of the dust collection device. 
         [0006]    The present invention provides a dust collection device having a tank, a suction unit, a filtering unit, a dust removal unit, a first detection unit, and a control unit. The tank has an air inlet for accommodating dust. The suction unit aspirates the dust through the air inlet. The filtering unit captures the dust through the air inlet. The dust removal unit removes the dust from the filtering unit. The first detection unit detects an operational state of the suction unit. The control unit controls the suction unit and the dust removal unit. The control unit activates the dust removal unit, when the first detection unit detects that the suction unit operates for a first predetermined time period and then stop operating. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a vertical sectional view showing a dust collection device according to the present invention; 
           [0009]      FIG. 2  is a partial side view showing an upper portion of the dust collection device of  FIG. 1 ; 
           [0010]      FIG. 3  is an overall view showing the dust collection device of  FIG. 1 ; 
           [0011]      FIG. 4  is an overall view showing the dust collection device connected with an electric tool; 
           [0012]      FIG. 5  is a circuit diagram showing a control circuit of the dust collection device; 
           [0013]      FIG. 6  is a flowchart illustrating a former portion of an operation by the dust collection device; and 
           [0014]      FIG. 7  is a flowchart illustrating a latter portion of the operation by the dust collection device shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    A dust collection device according to an embodiment of the present invention will be described by referring to the accompanying drawings. 
         [0016]    Referring to  FIG. 1 , a dust collector  1  has a housing  100  including a cylindrical tank  2  having a air inlet port  3  for containing dust, a main motor base  4  attached with a main motor cover  5  and a head cover  6 . The main motor base  4  and the head cover  6  are clamped by a clamp member (not shown) at an upside aperture of the tank  2 . The dust collector  1  includes an air suction device  7  arranged between the main motor base  4  and the motor cover  5  for aspirating powdery dust from the air inlet port  3 , a filter unit  10  for capturing the aspirated powdery dust, a dust removal device  14  for removing the powdery dust deposited to the filter  13  and a control circuit  20  for controlling the air suction device  7  and the dust removal device  14 , all of which are contained in the housing  100 . 
         [0017]    The air suction device  7  has a main motor  9  and a suction fan  8  driven by the main motor  9 . The air suction device  7  takes in external air from the air inlet port  3  through an air intake port  4   a  formed in the main motor base  4 . Additionally, the air suction device  7  guides the external air introduced in the device  7  through the discharge route defined by the main motor base  4 , the main motor cover  5  and the head cover  6 , and discharges the external air through an exhaust port  4   b  to the outside of the housing  100 . 
         [0018]    A filter housing  11  is fixed and attached between the upside aperture of the tank  2  and the main motor base  4 . A filter device  10  is formed by the filter housing  11 , the filter  13  for capturing dust and a filter cover  12 , the filter  13  and the filter cover  12  being fitted to the filter housing  11 . The filter cover  12  is provided to prevent the filter  13  from being crushed due to the negative pressure produced in the inside of the filter  13  as a result of the operation of the air suction device  7 . 
         [0019]    The dust removal device  14  is arranged in the space surrounded by the filter housing  11  and the filter  13 . The dust removal device  14  is fitted to a dust removal device base  15  and has a DC power supply  37  (see  FIG. 5 ), a second motor  16  driven by the DC power supply  37 , an anti-dust cover  19  for protecting the second motor  16  against powdery dust, a reduction gear unit  17  coupled to the output shaft of the second motor  16  and a dust removal member  18  arranged at the output shaft of the reduction gear unit  17  and driven by the second motor  16 . The dust removal member  18  removes the powdery dust deposited to the filter  13  by vibrating the filter  13 , and clean the filter  13 . 
         [0020]    The air suction device  7 , the filter  13 , the filter cover  12  and the dust removal device  11  are aligned on a vertical line M 1  extending in the vertical direction, passing through each own center. The vertical line M 1  is displaced from the center line M 2  of the tank  2  to the opposite side of the air inlet port  3 . 
         [0021]    An operation panel  21  and a control circuit  20  are provided to the main motor cover  5  at respective positions located above the air inlet port  3 . As shown in  FIG. 2 , the operation panel  21  has a plug socket  24  for supplying electric power to an external device such as an electric tool  43  (see  FIG. 4 ), a main switch  26  for causing the dust collector  1  to start and stop operating, an interlocking mode/single mode selection switch  27  and a dust removal device switch  28 . Note that the main switch  26  may have a function for selecting the dust collecting power. 
         [0022]    As shown in  FIG. 3 , a hose  38 , a hose connection handle  39 , an extension tube  40  and a floor air inlet member  41  can be connected to the air inlet port  3 . In this case, dust including fragments produced by cutting wood plates, stones, concrete or plaster boards can be aspirated with air. 
         [0023]    As shown in  FIG. 4 , the plug of an electric tool  43  can be inserted to the plug socket  24  of the operation panel  21  and the dust discharge port  42  of an electric tool  43  is connected to the air inlet port  3  by way of a hose  38 . With this arrangement, the operation of the dust collector  1  can be interlocked with the operation of the electric tool  43  to collect the powdery dust discharged from the electric tool  43  into the tank  2 . 
         [0024]    Now, the control circuit  20  will be described below by referring to  FIG. 5 . The power source plug  22  includes a pair of terminal pieces  22   a ,  22   b  and AC 100V is supplied from a commercial power source. 
         [0025]    The main switch  26  includes two sub-switches  26   a  and  26   b  for switching simultaneously. The plug  22  is connected to the sub-switch  26   a , the main motor  9  and the main motor drive circuit  34  for controlling the rotary motion of the main motor  9 . 
         [0026]    The plug  22  is connected to the input side of a diode bridge  23  for full wave rectification. The sub-switch  26   b  and the DC power supply  37  are connected in series to the output side of the diode bridge  23 . A relay  29  is connected in parallel with the sub-switch  26   b . The output side of the DC power supply  37  is connected to the second motor  16 . 
         [0027]    The plug  22  is connected to the plug socket  24  that is connectable to the electric tool  43 . The current detector  25  detects the electric current flowing through the plug socket  24 . 
         [0028]    The control circuit  20  includes a microcomputer  36 . The output port P 02  of the microprocessor  36  is connected to the relay  29  through a relay drive circuit  30 . The opening and closing of the relay  29  is controlled according to the output signal from the output port P 02 . When the plug  22  is connected to a commercial power source and the main switch  26  is turned on, the main motor  9  starts rotating and electric power is supplied to the DC power supply  37  through the diode bridge  23  and the sub-switch  26   b . When the main switch detection circuit  31  detects the on-status of the main switch  26 , the microcomputer  36  transmits an output signal to the relay drive circuit  30  to turn on the relay  29 . Then, electric power is supplied to the DC power supply  37  from two routes. 
         [0029]    When the main switch  26  is turned off, the supply of electric power to the main motor  9  is stopped. On the other hand, the supply of electric power to the DC power supply  37  is continued as long as the relay  29  is on. Therefore, while electric power is supplied to the DC power supply  37 , the second motor  16  for driving the dust removal device  14  can be operable. 
         [0030]    The microcomputer  36  confirms the operation of the electric tool  43  connected to the plug socket  24  as follows: the microcomputer  36  first receives the input signal indicating the detection of the electric current by the current detector  25  at port Ain 1  from the current detection circuit  33  and then determines based on the input signal how the electric tool  43  is operating. The electric tool  43  may be provided with a DC power source mounted therein. Therefore, the microcomputer  36  may have a function of recognizing the electric current of the DC power source mounted in the electric tool  43  and the electric current of the series commutator motor that is a drive source of the electric tool  43 . Alternatively, the microcomputer  36  can determine the operating condition of the electric tool  43  by detecting the power consumption of the electric tool  43 . 
         [0031]    The microcomputer  36  determines the operating condition of the air suction device  7  on the basis of the input signal indicating the voltage of the main switch  26  when the microcomputer  36  receives the input signal at ports P 13 /INT through the main switch detection circuit  31 . Alternatively, the microcomputer  36  can determine the operating condition of the air suction device  7  by detecting the electric current flowing through the main motor  9 , the number of revolutions per unit time of the main motor  9 , the internal pressure of the tank  2 , the air flow rate in the hose or the wind velocity in the hose. 
         [0032]    The suction device  7  continues operating due to the inertia of the main motor  9  after the supply of electric power to the main motor  9  is stopped. Therefore, the microcomputer  36  determines that the air suction device  7  actually stops operating, when the preset time has elapsed since the supply of electric power to the main motor  9  is stopped. The microcomputer  36  determines the stop of operation of the air suction device  7  in a similar manner when the main motor  9  is stopped by the stop signal of the air suction device  7  that is output from the port P 01 . The time period during which the air suction device  7  continues to operate due to the inertia is about 10 seconds. 
         [0033]    The interlocking mode/single mode selection switch  27  is a switch for selecting either an interlocking mode or a single mode of the air suction device  7 . The single mode is a mode of operation where the air suction device  7  is operated by means of the main switch  26 . The interlocking mode is a mode of operation where the air suction device  7  is operated in response to the operation of the electric tool  43  connected to the plug socket  24 . The interlocking mode/single mode selection switch  27  has a common terminal  27   a  and a normally open terminal  27   b . The interlocking mode is selected when the common terminal  27   a  and the normally open terminal  27   b  are open. The single mode is selected when the common terminal  27   a  and the normally open terminal  27   b  are closed. In the interlocking mode, the air suction device  7  continues the operation thereof for a predetermined time, after the electric tool  43  stops operating and then the operation of the suction device  7  is stopped. This arrangement is aimed at preventing the hose connected between the electric tool  43  and the dust collector  1  from being clogged in an interlocking mode. 
         [0034]    The dust removal device switch  28  is a switch for operating the dust removal device  14 . When the dust removal device switch  28  is on, the dust removal device  14  is driven for a predetermined time to automatically remove the dust in the filter  13  after the supply of electric power to the air suction device  7  is stopped. In other words, the dust removal device switch  28  is effective only when the microcomputer  36  determines that the operation of the air suction device  7  has been stopped. On the other hand, when the microcomputer  36  determines that the operation of the air suction device  7  has not been stopped, the dust removal device  14  is not allowed to operate even if the dust removal device switch  28  is on. The dust deposited to the filter  13  is not removed due do the negative pressure until the air suction device  7  completely stops. Therefore, with this arrangement of the switch  28 , ineffective operation of the dust removal device  15  is prevented in order to prevent waste of electric power, and the filter  13  is protected against damages. 
         [0035]    The operation of the main motor  9  can be switched by the output signal sent from the ports P 00  and P 01  of the microcomputer  36 . When a triac is used as an electric power control device for the main motor drive circuit  34 , the suction power of the air suction device  7  can be changed by phase control of the main motor  9 . Alternatively, the number of revolutions per unit time of the main motor  9  can be changed by connecting a pair of field windings to the main motor  9  and selectively operating the field windings. 
         [0036]    The second motor  16  is operated in accordance with the output signal generated from the port P 03  of the microcomputer  36  through the second motor drive circuit  35 . The second motor  16  can be softly started by driving the power control device in the second motor drive circuit  35  in a chopping mode. With this arrangement, the voltage fall of the DC power supply  37  due to the starting current of the second motor can be avoided. 
         [0037]    The dust collector  1  operates either in the single mode or in the interlocking mode. 
         [0038]    In the single mode, the air suction device  7  is driven when the interlocking mode/single mode selection switch  27  is switched to the single mode and the main switch  26  is turned on. When the main switch  26  is turned on, the microcomputer  36  closes the contacts of the relay  29 . When the main switch  26  is turned off subsequently, the supply of electric power to the main motor  9  is stopped. At this time, the supply of electric power to the DC power supply  37  is continued since the relay  29  is held on. 
         [0039]    In the interlocking mode, the interlocked mode/single mode selection switch  27  is switched to the interlocking mode and the electric tool  43  is connected to the plug socket  24 . When the main switch  26  is turned on and the microcomputer  36  determines that the electric tool  43  starts operating, the air suction device  7  starts to operate. At this time, the microcomputer  36  generates a signal from the ports P 00  and P 01  to drive the main motor  9  through the main motor drive circuit  34 . Additionally, when the main switch  26  is turned on, the microcomputer  36  turns on the relay  29 . 
         [0040]    Thereafter, when the electric tool  43  is stopped and the electric current flowing through the plug socket  24  disappears, the microcomputer  36  determines that the electric tool  43  has stopped. Then, after the elapse of a predetermined time, the microcomputer  36  generates a signal from the ports P 00  and P 01  to the main motor drive circuit  34  to stop the operation of the main motor  9 . Since the main switch  26  remains on at this time, the supply of electric power to the DC power supply  37  is continued. 
         [0041]    The microcomputer  36  monitors the operation time of the air suction device  7 , and determines that a dust removal operation is necessary if the operation time period of the air suction device  7  exceeds the predetermined time. On the other hand, the microcomputer  36  determines that the dust removal operation is not necessary if the operation time period of the air suction device  7  does not exceed the predetermined time. When the air suction device  7  stops, the microcomputer  36  drives the second motor  16  for several seconds to remove the dust deposited to the filter  13  by using the dust removal member  18 , if dust removal for the filter  13  is necessary. The microcomputer  36  does not drive the second motor  16  when the dust removal is not necessary. 
         [0042]    The dust removal device  14  is operated when the air suction device  7  is at rest. Therefore, when the main switch  26  is turned off, the on-status of the relay  29  is maintained for a while so that electric power is kept supplying to the DC power supply  37  to keep the dust removal device  14  ready for operation. Ten and several minutes is selected as the time period in which the on-status of the relay  29  is maintained after the stop of the power supply to the air suction device  7 , considering the condition where the user manually operates the dust removal device  14 . This time period corresponds to the self-holding time period of the relay  29 . When the self-maintaining time period elapses, the relay  29  is turned off and the standby power of the dust collector  1  becomes practically disappeared. 
         [0043]    Now, the operation of the dust collector  1  will be described below by referring to  FIGS. 6 and 7 . 
         [0044]    When the power supply plug  22  of the dust collector  1  is connected to a commercial power source and the main switch  26  is turned on, electric power is supplied to the control circuit  20  and the microcomputer  36  turns off a dust removal evaluation timer and turns off a dust removal effective flag (S 100 ). The dust removal evaluation timer is a timer for measuring the operation time of the main motor  9 . The dust removal evaluation timer is used to determine whether the operation time of the main motor  9  exceed a predetermined time period such as 10 minutes. When a process for turning off the motor  9  has not performed within the predetermined time period, the dust removal effective flag is changed from off to on. Then, the microcomputer  36  turns on the relay  29  (S 101 ). The microcomputer  36  examines the status of the main switch  26  (S 102 ). If the main switch  26  is on, the microcomputer  36  turns off a 15 minute timer (S 103 ). The 15 minute timer is a timer for measuring the time after the power supply to the main motor  9  is stopped, in other words, the power supply suspension time. The dust removal effective flag indicates whether dust removal should be performed or not. Then, the microcomputer  36  determines whether the operation mode is a single mode or not (S 104 ). 
         [0045]    If the operation mode is the interlocking mode (S 104 : interlocking), the microcomputer  36  then determines whether the plug socket current flowing through the plug socket  24  is more than or equal to 1 A or not (S 105 ). If the plug socket current is less than 1 A (S 105 : NO), the microcomputer  36  examines the condition of the dust removal device switch  28  (S 106 ). If the dust removal device switch  28  is off (S 106 : NO), the microcomputer  36  returns to Step S 102 . The plug socket current of 1 A is the reference value for determining whether the electric tool  43  connected to the plug socket  24  is operating or not. Therefore, if the plug socket current is more than or equal to 1 A, the microcomputer  36  determines that the electric tool  43  is operating. On the other hand, if the plug socket current is lower than 1 A, the microcomputer  36  determines that the electric tool  43  is at rest. 
         [0046]    Then, if the main switch  26  is off in Step S 102  (S 102 : NO), the microcomputer  36  proceeds to the next step, or Step S 107 , where the microcomputer  36  determines whether the 15 minute timer is on or off. If the 15 minute timer is off (S 107 : NO), the microcomputer  36  turns off the 15 minute timer (S 108 ). In Step S 109 , the microcomputer  36  determines whether the power supply suspension time is more than or equal to 15 minutes or not. If the power supply suspension time is less than 15 minutes (S 109 : NO), the microcomputer  36  proceeds to Step S 106 . On the other hand, if the power supply suspension time is determined to be more than or equal to 15 minutes (S 109 : YES), the microcomputer  36  turns off the relay  29  and stops the operation of the dust collector  1  in Step S 110 . 
         [0047]    The microcomputer  36  determines in Step S 106  whether the dust removal device switch  28  is on or off. At this case, the supply of electric power to the main motor  9  is stopped. Accordingly, the dust removal device switch  28  is effective for operating the dust removal device. If the dust removal device switch  28  is turned on (S 106 : YES), the dust removal device can be started. Therefore, the microcomputer  36  starts the second motor  16  in Step S 111  and then starts a 3 second timer in Step S 112 . The 3 second time is a timer for measuring the operation time of the dust removal device  14 . 
         [0048]    Then, the microcomputer  36  sequentially determines whether the operation time of the dust removal device  14  exceeds 3 seconds or not (S 113 ), whether the main switch  26  is on or not (S 114 ), whether the current operation mode is an interlocking mode or a single mode (S 115 ), and whether the plug socket current is more than or equal to 1 A or not (S 116 ) when the current mode of operation is an interlocking mode. The above steps are a process for driving the dust removal device  14  for 3 seconds and then stopping the dust removal device  14 . 
         [0049]    While the steps S 113 -S 116  are processed, if it is determined that the main switch  26  is on (S 114 : YES) and that the current mode of operation is a single mode (S 115 : YES), the microcomputer  36  proceeds to Step S 117 , where the microcomputer  36  stops the second motor  16 . On the other hand, if the current mode of operation is the interlocking mode (S 115 : interlocking), and the plug socket current is more than or equal to 1 A (S 116 : YES), the microcomputer  36  proceeds to Step S 117 , where the microcomputer  36  stops the second motor  16 . With these steps, the operation of the dust removal device  14  can be suspended when the air suction device  7  starts operating while the second motor  16  is in operation. When the operation time of the dust removal device  14  exceeds 3 seconds (S 113 : YES), the microcomputer  36  stops the rotation of the second motor  16  (S 117 ). 
         [0050]    On the other hand, if the microcomputer  36  determines in Step S 102  that the main switch  26  is on, that the current mode is the interlocking mode in Step S 104  (S 104 : interlocking) through Step S 103  and that the plug socket current is more than or equal to 1 A in Step S 105  (S 105 : YES), the microcomputer  36  proceeds to Step S 118 . The subsequent process is a process for the interlocking mode. Then, the microcomputer  36  turns on the main motor  9  in Step S 118  and starts the dust removal evaluation timer in Step S 119 . The microcomputer  36  determines whether the main switch  26  is on or off (S 120 ) and whether the plug socket current is more than or equal to 1 A or not (S 121 ). 
         [0051]    If the microcomputer  36  determines in Step S 120  that the main switch  26  is off, the microcomputer  36  proceeds to Step S 132 . If the microcomputer  36  determines in Step S 121  that the plug socket current is less than 1 A, the microcomputer  36  proceeds to Step S 122 , where the microcomputer stops the dust removal evaluation timer and starts a five second timer. The five second timer is a timer for measuring the operation time during which the air suction device  7  is driven after stopping the electric tool  43 . The microcomputer  36  determines whether the reading of the five second timer exceeds five seconds or not in S 123 , whether the main switch  26  is on or not in S 124 , whether the current mode of operation is a single mode or an interlocking mode in S 125  and whether the plug socket current is not less than 1 A or not in S 126 . The steps from S 123  to S 126  are a process for waiting for the elapse of the preset time from the time when the air suction device  7  starts operating and to the time when the air suction device  7  stops operating in the interlocking mode. 
         [0052]    If the microcomputer  36  determines in Step S 104  that the current mode of operation is the single mode, the microcomputer  36  proceeds to Step S 127  ( FIG. 7 ). The subsequent process is a process for a single mode. The microcomputer  36  turns on the main motor  9  in S 127  and starts the dust removal evaluation timer in S 128 . Then, the microcomputer  36  determines whether the main switch  26  is on or off in Step S 129  and which the current mode of operation is in Step S 130 . The steps of S 129  and S 130  are a process for determining whether the main switch  26  for stopping the air suction device  7  is off or not and whether the operation mode is switched or not. When the microcomputer  36  determines in Step S 129  that the main switch  26  is turned off (S 129 : YES), the microcomputer  36  proceeds to Step S 132 . If the microcomputer  36  determines in Step S 130  that the current mode of operation has been switched to the interlocking mode (S 130 : interlocking), the microcomputer proceeds to Step S 131 , where the microcomputer determines whether the plug socket current is more than or equal to 1 A or not. When the plug socket current is not less than 1 A (S 131 : YES), the microcomputer  36  proceeds to Step S 118 . On the other hand, when the plug socket current is less than 1 A (S 131 : NO), the microcomputer proceeds to Step S 132 . 
         [0053]    The following steps from Step S 132  are a process for stopping the air suction device  7 . The microcomputer  36  turns off the main motor  9  in Step S 132 , turns off the dust removal timer, and starts the five second timer in Step S 133 . Then, in Step S 134 , the microcomputer  36  starts a 9 second timer. The 9 second timer is a timer for measuring the time elapsed from the time when the supply of electric power to the main motor  9  is stopped to the time when the internal pressure of the filter  13  becomes equal to the atmospheric pressure from the previous negative pressure. Then, the microcomputer  36  determines whether the reading of the 9 second timer exceeds 9 seconds or not in Step S 135  and whether the main switch  26  is on or off in Step S 136 . When the microcomputer determines that the main switch  26  is not on in Step S 136 , the microcomputer  36  returns to Step S 135 . 
         [0054]    On the other hand, when the microcomputer determines in Step S 136  that the main switch  26  is on, the microcomputer  36  determines in Step S 138  whether the current mode of operation is a single mode or not. If the microcomputer  36  determines in Step S 138  that the current mode of operation is not the single mode, the microcomputer proceeds to Step S 139 , where the microcomputer measures the plug socket current. When the plug socket current is less than 1 A (S 139 : NO), the microcomputer  36  returns to Step S 135 . The steps from S 135  to S 139  are a process for standing by the dust removal device  14  until the internal pressure of the filter restores the atmospheric pressure. 
         [0055]    If the microcomputer  36  determines in Step S 135  that the reading of the 9 second timer exceeds 9 seconds (S 135 : YES), the microcomputer proceeds to Step S 140 , where the microcomputer executes an automatic dust removal process. When the microcomputer determines in Step S 136  that the main switch  26  is on and in Step S 138  that the current mode of operation is a single mode (S 138 : YES), the microcomputer  36  proceeds to Step S 148 . However, when the microcomputer determines in Step S 138  that the current mode of operation is the interlocking mode (S 138 : interlocking) and in Step S 139  that the plug socket current is more than or equal to 1 A (S 139 : YES), the microcomputer  36  also proceeds to Step S 148 . 
         [0056]    The following process starting from Step S 140  is the automatic dust removal process for removing dust form the filter  13  to clean the filter  13 . The microcomputer  36  examines the dust removal effective flag in Step S 140 . If the dust removal effective flag is not ON, the microcomputer  36  returns to Step S 102  and does not execute the automatic dust removal operation. On the other hand, if the dust removal effective flag is ON, the microcomputer  36  starts rotating the second motor  16  in Step S 141  and also starts a 3 second timer in Step S 142 . The 3 second timer is a timer for measuring the operation time of the dust removal device  14 . In this embodiment, the operation time of the dust removal device  14  is set as 3 seconds. Then, the microcomputer  36  determines whether the reading of the 3 second timer exceeds 3 seconds or not in Step S 143 . If the microcomputer  36  determines in Step S 143  that the reading of the 3 second timer exceeds 3 seconds (S 143 : YES), the microcomputer proceeds to Step S 147 , where the microcomputer stops the rotation of the second motor  16  to finish the automatic dust removal process. 
         [0057]    On the other hand, if the microcomputer  36  determines in Step S 143  that the reading of the 3 second timer is less than 3 seconds (S 143 : NO), the microcomputer determines whether the main switch  26  is on or off in Step S 144 . when the main switch  26  is off (S 144 : NO), the microcomputer  36  returns to Step S 143 . On the other hand, when the main switch  26  is on (S 144 : YES), the microcomputer  36  examines the current mode of operation in Step S 145 . If the current mode of operation is the interlocking mode (S 145 : interlocking), the microcomputer  36  determines whether the plug socket current is more than or equal to 1 A or not in Step S 146 . If the main switch  26  is turned on, or the current mode of operation is switched to the interlocking mode and the operation of the electric tool  43  is confirmed while the dust removal device  14  is automatically operating, the microcomputer  36  proceeds to Step S 147 , where the microcomputer stops the second motor  16  to finish the automatic dust removal process. 
         [0058]    After S 147 , the microcomputer  36  turns off the dust removal effective flag in Step S 148  and returns to Step S 102 . 
         [0059]    A swinging vibrator may be used as the drive source of the dust removal member  18  in other embodiments. 
         [0060]    According to the present invention, the dust removal device is automatically activated when the air suction device operates for a predetermined time period and stops operating. Accordingly, dust is reliably removed from the filter device. A reduction in the suction force can be prevented effectively. 
         [0061]    The dust removal device automatically starts operating only when the operation time of the air suction device exceeds a predetermined time. In other words, the filter is cleaned by the dust removal process when the filter device is considered to be significantly clogged. Thus, the dust removal device can enjoy a prolonged service life because the dust removal device is operated only in limited occasions. 
         [0062]    The dust removal device starts operating when a predetermined time has elapsed since the supply of electric power to the air suction device is stopped. This structure ensures that dust can be removed from the filter after the internal pressure of the filter restores the atmospheric pressure. Therefore, dust can be removed efficiently from the filter. 
         [0063]    It is understood that the foregoing description and accompanying drawings set forth the embodiments of the invention at the present time. Various modifications, additions and alternative designs will, of course, become apparent to those skilled in the art in light of the foregoing teachings without departing from the spirit and scope of the disclosed invention. Thus, it should be appreciated that the invention is not limited to the disclosed embodiments but may be practiced within the full scope of the appended claims.