Patent Publication Number: US-9845560-B2

Title: Washing machine with buoyancy clutch and controlling method thereof

Description:
This application is a 35 USC §371 National Stage entry of International Application No. PCT/KR2011/010085 filed Dec. 26, 2011, and claims priority of Korean Application No. 10-2010-0135854 filed Dec. 27, 2010, which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a washing machine and, more particularly, to a washing machine having a buoyancy clutch that operates when wash water is filled in an inner tub and a method of controlling the washing machine. 
     BACKGROUND ART 
     A washing machine generally refers to various devices for processing the laundry by applying a physical and chemical action to the laundry, such as a laundry machine for detaching a contaminant from the clothes, bedclothes, and the like, (referred to as the ‘laundry’, hereinafter) by using a chemical decomposition operation between water and a detergent and a physical operation such as friction between water and the laundry. 
     The related art washing machine includes a pulsator that is rotatably provided in an inner tub in which laundry is loaded. The inner tub and/or pulsator rotate by a motor. Here, rotational force generated by the motor is selectively transferred to the inner tub and/or pulsator by a clutch. 
     The clutch moves up and down by buoyancy created by the wash water. When the clutch is in an upper position, a driving shaft of the motor is engaged with the pulsator to rotate only the pulsator. When the clutch is in a lower position, the driving shaft of the motor is engaged with both the pulsator and inner tub to rotate the pulsator and inner tub together. 
     However, the clutch that operates by the buoyancy has a problem in that, since it is always in the upper position when the wash water is filled in the inner tub, it cannot rotate the inner tub when the wash water is filled in the inner tub. When the wash water is filled in the inner tub, the prior art buoyancy clutch can rotate only the pulsator. That is, the clutch is engaged with not only the pulsator but also the inner tub only in a spin cycle that is performed after the wash water is drained out of the inner tub. 
     In addition, when the clutch is accurately shifted from the upper position to the lower position, the driving shaft of the motor cannot be accurately engaged with the inner tub. When the motor is driven with a high speed in this state, a shaft-engaging portion of the inner tub may be damaged. This results in the deterioration of the durability of the washing machine. Therefore, there is a need for a method for detecting if the clutch operates accurately and, when the clutch inaccurately operates, correcting the inaccurate operation of the clutch. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     Thus, an object of the present invention is to provide a washing machine that is designed to rotate an inner tub by operating a buoyancy clutch even when wash water is filled in the inner tub. 
     Another object of the present invention is to provide a washing machine that is designed to remove buoyancy acting on a buoyancy clutch even when wash water is filled in the inner tub. 
     Still another object of the present invention is to provide a method of controlling a washing machine, which can improve operational accuracy of a buoyancy clutch. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     Solution to Problem 
     According to an aspect of the present invention, there is provided a washing machine including: an outer tub holding wash water; an inner tub rotatably provided in the outer tub and receiving laundry; a pulsator rotatably provided in the inner tub; a power transmission shaft including a first driving shaft rotating the pulsator and a second driving shaft engaged with the inner tub; a buoyancy clutch that is disposed between the pulsator and the outer tub, integrally rotate with the first driving shaft, moves up and down in accordance with buoyancy created by the wash water, is separated from the second driving shaft in an upper position, and is engaged with the second driving shaft in a lower position to allow the pulsator to rotate together with the inner tub; a circulation passage allowing the wash water drained out of the outer tub to be returned into the outer tub; a pump provided on the circulation passage; a hub that is disposed between the buoyancy clutch and the outer tub, coupled to the second driving shaft to support the inner tub, disposed under the buoyancy clutch, and provided with a plurality of through holes; and a passage guide that is disposed between the hub and the outer tub to direct the wash water drained through the through holes formed on the hub to a drain hole of the outer tub, the drain hole being connected to the circulation passage. 
     According to another aspect of the present invention, there is provided a washing machine including: an outer tub holding wash water; an inner tub rotatably provided in the outer tub and receiving laundry; a pulsator rotatably provided in the inner tub; a power transmission shaft rotating the pulsator; a buoyancy clutch that that moves up and down by buoyancy created by the wash water and transfers, when the buoyancy clutch is in a lower position, rotational force from the power transmission shaft to the inner tub to allow the pulsator and the inner tub to rotate integrally with each other; a circulation passage that is provided on an outer portion of the outer tub to allow the wash water drained out of the outer tub to be returned into the outer tub; a pump provided on the circulation passage and a drain guide unit that directs the wash water under the buoyancy clutch to a drain hole of the outer tub so that the buoyancy clutch moves downward. 
     According to another aspect of the present invention, there is provided a method of controlling a washing machine including an inner tub, an outer tub, a motor, a first driving shaft rotating by the motor, a second driving shaft, and a buoyancy clutch that integrally rotates with the first driving shaft, moves up and down along the first driving shaft by buoyancy creased by the wash water, and is engaged with the second driving shaft in a lower position of the buoyancy clutch to rotate the inner tub, the method including: alternately rotating the motor in both direction in a state where the wash water is filled in the inner tub; stopping the motor and operating the pump provided on the circulation passage; continuously driving the motor in one direction during the operation of the pump; and alternately rotating the motor in the both direction on the basis of a detection value of a driving detecting unit for detecting a driving state of the motor. 
     Advantageous Effects of Invention 
     According to the aspects, even when the wash water is filled in the inner tub, the buoyancy clutch can operate to rotate the inner tub. 
     Since the buoyancy clutch is provided in the outer tub, the space between the cabinet and the outer tub can be increased as compared with the prior art washing machine in which a clutch unit is provided at an outer side of the outer tub, thereby increasing the volume of the outer tub. 
     Since both the driving for rotating only the pulsator and the driving for integrally rotating the pulsator and the inner tub can be performed in the rinse and wash cycles, the washing pattern can be variously realized. 
     In addition, the operating accuracy of the buoyancy clutch can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is a schematic view of a washing machine according to an exemplary embodiment of the present invention; 
         FIG. 2  is a schematic view of a circulation passage and a pump of the washing machine of  FIG. 1 ; 
         FIGS. 3 a  and 3 b    are enlarged views of a portion A of  FIG. 1 , in which  FIG. 3 a    illustrates a state where a buoyancy clutch is in an upper position and  FIG. 3 b    illustrates a state where the buoyancy clutch is in a lower position; 
         FIG. 4  is an exploded perspective view illustrating a buoyancy clutch, a hub, and a passage guide that are illustrated in  FIGS. 3 a    and  3   b;    
         FIG. 5 a    is a schematic view illustrating a centrifugal circulation water stream; 
         FIG. 5 b    is a schematic view illustrating a pressurized water stream; 
         FIG. 6  is a block diagram illustrating a control relationship between major parts of the washing machine according to an exemplary embodiment of the present invention; and 
         FIG. 7  is a flowchart of a washing machine control method according to an exemplary embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the shapes and dimensions may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like components. 
       FIG. 1  is a schematic view of a washing machine according to an exemplary embodiment of the present invention and  FIG. 2  is a schematic view of a circulation passage and a pump of the washing machine of  FIG. 1 . 
     Referring to  FIG. 1 , a washing machine W according to an exemplary embodiment of the present invention includes a cabinet  1 , a top cover  2  disposed on a top of the cabinet  1  and provided with a laundry loading hole through which the laundry is loaded and unloaded, a door  3  that is coupled to the top cover  2  to open and close the laundry loading hole, and a control panel  4  providing a user interface for allowing a user to input a variety of control commands and displaying operational information of the washing machine W. 
     An outer tub  12  is suspended in the cabinet  1  by a supporting member  8 . The supporting member  8  is connected to the outer tub  12  by a suspension  9  to attenuate vibration generated during washing. An inner tub  14  is rotatably disposed in the outer tub  12 . 
     The water is supplied from an external water source into the outer and inner tubs  12  and  14  through a water supply passage  5  via a washing aid box  7 . Various washing aid agents such as detergent, softener, whitener, and/or the like are stored in the washing aid box  7 . The washing aid agents are mixed with the water and supplied into the outer and inner tubs  12  and  14 . Therefore, hereinafter, “wash water” may be defined as any one of water and water mixed with the washing aid agents. 
     An outer tub cover  13  is provided on a top of the outer tub  12 . The outer tub cover  13  is formed in a ring-shape having a central opening through which the laundry is loaded and unloaded. The outer tub over  13  prevents the wash water from scattering into the outer tub  12  and guides the wash water, which ascends along a space between the outer and inner tubs  12  and  14  when centrifugal circulation water stream is formed, to be poured into the inner tub  14  along a rear surface of the outer tub cover  13 . 
     The wash water flows into the space defined between the outer and inner tubs  12  and  14  through a plurality of through holes  14   h  formed through the inner tub  14 . 
     A pulsator  16  is rotatably provided on a lower portion of the inner tub  14 . Dirt is removed from the laundry by mechanical force that is created by the rotation of the pulsator  16  and acts on the laundry. 
     A driving unit  30  may be classified into a direct driving type and an indirect driving type in accordance with a method for transferring the rotational force of the motor  22  to the pulsator  16  and the inner tub  14 . In this exemplary embodiment, the direct driving type in which the rotational shaft of the motor is coaxially engaged with the inner tub and the pulsator. However, the present invention is not limited to this type. The indirect driving type in which the rotational force is transferred through a power transmission unit such as a belt or pulley may be also possible. 
     The washing machine further includes a pump  26 . The pump  26  is designed to drain the wash water out of the outer tub  12 . In this exemplary embodiment, the pump  26  is described as a circulation/drain pump that circulates the wash water drained from the outer tub  12  through the circulation passage  18  or drains the wash water out of the washing machine W. However, the present invention is not limited to this. For example, a drain pump for draining the wash water drained from the outer tub  12  out of the washing machine W and a circulation pump for circulating the wash water drained from the outer tub  12  through the circulation passage  18  are provided. 
     The pump  26  may drain the wash water drained from the outer tub  12  out of the washing machine W through a drain passage (not shown) by proper passage change. However, in this exemplary embodiment, only a function where the wash water drained from the outer tub  12  is pumped by the pump  26  and returned to the outer tub  12  through the circulation passage  18  will be described. 
     In the washing machine of this exemplary embodiment, it is enough that the circulation passage is designed to guide the wash water drained from the outer tub  12  to be returned to the outer tub  12 . This may be realized through a variety of ways.  FIG. 2  illustrates the circulation passage according to an exemplary embodiment of the present invention. Here, the circulation passage  18  has a first end connected to a drain pipe  12   a  formed on a bottom of the outer tub  12  and extending upward through a space defined between the cabinet  1  and the outer tub  12  and a second end connected to the outer tub cover  13 . The pump  26  is provided on the circulation passage  26  to pump the wash water. 
     When the pump  26  operates, the wash water in the outer tub  12  is drained to the circulation passage  18  through the drain pipe  12   a  and directed toward the outer cover  13  along the circulation passage  18 , after which the wash water is sprayed into the outer tub  12  or the inner tub  14 . Here, a nozzle (not shown) for spraying the wash water supplied through the circulation passage  18  toward the laundry loaded in the inner tub  14  may be provided on the outer tub cover  13 . 
       FIGS. 3 a  and 3 b    are enlarged views of a portion A of  FIG. 1 , in which  FIG. 3 a    illustrates a state where a buoyancy clutch is in an upper position and  FIG. 3 b    illustrates a state where the buoyancy clutch is in a lower position.  FIG. 4  is an exploded perspective view illustrating a buoyancy clutch, a hub, and a passage guide that are illustrated in  FIGS. 3 a    and  3   b.    
     As shown in  FIGS. 3 a , 3 b   , and  4 , the driving unit  30  of the washing machine W includes a motor  22  and a power transmission shaft  33 . 
     The power transmission shaft  33  transfers the rotation force created by the motor  22  to the pulsator  16  and/or the inner tub  14 . The power transmission shaft  33  includes a first driving shaft  31  rotating the pulsator  16  and a second driving shaft  32  rotating the inner tub  14 . 
     The first and second driving shafts  31  and  32  are coaxially arranged and have different diameters. In this exemplary embodiment, the second driving shaft is a hollow shaft having an inner diameter greater than an outer diameter of the first driving shaft  31  so that the first driving shaft  31  is inserted in the second driving shaft  32 . 
     The first driving shaft  31  is coupled to the pulsator  16  to integrally rotate with the pulsator  16 . 
     Meanwhile, the inner tub  14  is supported by a hub  50  that is coupled to the second driving shaft  32 . The second driving shaft  32  passes through a coupling hole  53   h  formed through the hub  50 . A serration is formed on an outer circumference of the second driving shaft  32 . A serration that is engaged with the serration formed on the second driving shaft  32  is formed on an inner circumference of the coupling hole  53   h  of the hub  50 . Therefore, when the second driving shaft  32  rotates, the hub  50  rotates together with the second driving shaft  32 . 
     As shown in  FIGS. 3 a  and 3 b   , the hub  50  is disposed between the pulsator  16  and the outer tub  12 . The hub  50  is provided with a plurality of holes  52   h  formed along a line extending in a circumferential direction so that the wash water in the inner tub  14  can be directed into the outer tub  12 . 
     A buoyancy clutch  40  is inserted into the power transmission shaft  33  and disposed between the pulsator  16  and the hub  50 . The buoyancy clutch  40  moves up and down by buoyancy created by the wash water. When the wash water is supplied to a space under the buoyancy clutch  40 , the buoyancy clutch  40  moves upward by the buoyancy created by the wash water. When the wash water under the buoyancy clutch  40  is drained, the buoyancy is removed and thus the buoyancy clutch  40  moves downward. 
     In more detail, the hub  50  is provided with a barrier  51  that is formed along the circumference of the coupling hole  53   h  through which the power transmission shaft  33  passes. The buoyancy clutch  40  is disposed to enclose the barrier  51  and moves upward by the buoyancy creased by the wash water introduced into the barrier  51 . 
     Meanwhile, the hub  50  is provided with a through hole  54   h  that is formed through an portion surrounded by the barrier  51 . A plurality of the through holes  54   h  may be formed along the circumference of the coupling hole  53   h . When the wash water is introduced into the buoyancy clutch  40  through the through hole  54   h , the buoyancy clutch  40  moves upward. On the other hand, when the wash water is drained to the outer tub  12  through the through hole  54   h , the buoyancy acting on the buoyancy clutch  40  is released and thus the buoyancy clutch  40  moves downward. 
     The buoyancy clutch  40  includes a floater  41  that moves up and down along the power transmission shaft  33  by the buoyancy created by the wash water and a second driving shaft engaging portion  42  that is engaged with the second driving shaft  32  when the buoyancy clutch is in a lower position. In this exemplary embodiment, although the floater  41  and the second driving shaft engaging portion  42  are provided as separated parts, they may be integrally formed with each other. 
     Meanwhile, the washing machine W further includes a drain guide portion that directs the wash water under the buoyancy clutch  40  toward the drain hole  12   h  formed on the outer tub  12  such that the buoyancy clutch  40  moves downward. 
     The drain guide portion, when the pump  26  operates, generates a relatively stronger water stream from the portion under the buoyancy clutch  40  toward the outer tub  12  so that the wash water under the buoyancy clutch  40  can be quickly drained toward the circulation passage  18 . To this end, the drain guide portion forms a passage extending from a portion under the buoyancy clutch  40  toward the drain hole  12   h  of the outer tub  12 . 
     The drain guide portion includes a plurality of through holes  54   h  formed on the hub  50  and a passage guide  60  for directing the wash water drained through the through holes  54  toward the drain hole  12   h.    
     The passage guide  60  is formed between the outer tub  12  and the hub  50 . The passage guide  60  is for directing the wash water drained to the outer tub  12  through the through holes  54   h  formed on the hub  50  toward the drain pipe  12   a  formed on the outer tub  12 . The passage guide  60  is provided at a first end with an opening  61   h  through which the power transmission shaft  33  passes and at a second end with a drain pipe connecting portion  63  connected to the drain pipe  12   a  formed on the outer tub  12 . 
     An inner diameter of the opening  61   h  of the passage guide  60  is greater than an outer diameter of the power transmission shaft  33 . Accordingly, the inner circumference of the opening  61   h  is spaced apart from the power transmission shaft  33 , thereby preventing the power transmission shaft  33  from interfering with the passage guide  60  when the power transmission shaft  33  rotates. 
     The passage guide  60  is provided with a guide surface  62  extending from the opening  61   h  to the drain pipe connecting portion  63  and a rib  64  extends along an edge of the guide surface  62 . The drain pipe connecting portion  63  protrudes downward from a circumference of the drain hole  63   h  formed on the guide surface  62 . A width of the passage on the guide surface  62  is gradually reduced from the opening  61   h  toward the drain pipe connecting portion  63 . Accordingly, when the pump  26  operates, a water pressure gradient is formed between the opening  61   h  and the drain pipe connecting portion  63  and thus the tendency for directing the wash water from the opening  61   h  toward the drain pipe connecting portion  63  is intensified. 
     When the pump  26  operates, the wash water is drained from the outer tub  12  to the circulation passage  18  through the drain hole  12   h . At this point, the water stream flowing toward the drain pipe connecting portion  63  along the guide surface  62  is intensified by suction created by the pump  26 . Especially, since the rib  64  formed along the guide surface  62  further intensifies the flow of the washing water, the tendency where the washing water collected between the hub  50  and the buoyancy clutch  40  is drained to the outer tub  12  through the through hole  54   h  is intensified, the buoyancy clutch  40  may move downward even when the wash water is filled in the inner tub  14 . 
     A mechanical operation for moving the buoyancy clutch  40  upward will be described in more detail with reference to  FIG. 3   a.    
     When the wash water is supplied, the water level of the outer tub  12  gradually increases and the water is directed into the space under the buoyancy clutch  40  through the through hole  54   h  of the hub  50 . This flowing of the wash water is indicated by dotted-arrow in  FIG. 3 a   . As the wash water is supplied under the buoyancy clutch  40 , the buoyancy clutch  40  moves upward and thus the second driving shaft engaging portion  42  is separated from the second driving shaft  32 . Accordingly, when the motor  22  is driven, only the pulsator  16  rotates. 
     A mechanical operation for shifting the buoyancy clutch  40  from the upper position to the lower position will be described in more detail with reference to  FIG. 3   b.    
     When the pump operates, the wash water collected under the buoyancy clutch  40  is drained to the outer tub  12  through the through hole  54   h  of the hub  50 . The drained wash water is directed to the drain pipe  12   a  by the passage guide  60 . At this point, the guide surface  62  of the passage guide  60  and the rib  64  intensify the water stream of the wash water that is drained through the through hole  54   h  of the hub  50  and is directed to the drain pipe connecting portion  63 . Accordingly, the wash water between the buoyancy clutch  40  and the hub  50  is effectively drained to the outer tub  12  through the through hole  54   h  of the hub. This flowing of the wash water is indicated by dotted-arrow in  FIG. 3   b.    
     As the wash water collected under the buoyancy clutch  40  is drained to the outer tub  12 , the buoyancy acting on the buoyancy clutch  40  is removed and thus the buoyancy clutch  40  moves downward. As a result, the second driving shaft engaging portion  42  is engaged with the second driving shaft  32 . Therefore, the pulsator  16  and the inner tub  14  rotate together. Here, the second driving shaft engaging portion  42  is provided at an inner circumference thereof with a serration engaged with the serration of the second driving shaft  32 . 
     Meanwhile, the wash water drained through the drain pipe  12   a  flows along the circulation passage  18  and is poured into the inner tub  14 . Therefore, the inner tub  14  remains a state where the wash water is always filled in the same. 
     The washing machine W according to the exemplary embodiment of the present invention is designed, even when the wash water is filled in the inner tub  14  by forcedly lowering the buoyancy clutch  40  by operating the pump  26 . Accordingly, in wash and rinse cycles that are preformed in a state where the wash water is filled in the inner tub  14 , both a process for treating the laundry by rotating only the pulsator  16  and a process for treating the laundry by rotating both the pulsator  16  and the inner tub  14  together can be performed. 
     For example, in a state where the wash water is filled in the inner tub  14 , the controller  10  alternately drives in both directions to rotate the pulsator  16 , after which the controller  10  operates the pump  26  to drain the wash water collected under the buoyancy clutch  40  to the outer tub  12  so that the buoyancy clutch  40  is in the lower position, thereby rotating the pulsator  16  together with the inner tub  14 . Here, describing the water streams formed during the rotation of the pulsator  16  and the inner tub  14 , there are a centrifugal circulation water stream shown in  FIG. 5 a    and a pressurized water stream shown in  FIG. 5   b.    
     Referring to  FIG. 5 a   , the centrifugal circulation water stream is a stream in which the wash water between the outer and inner tubs  12  and  14  moves upward by the centrifugal force created by the rotation of the inner tub  14  and is poured into the inner tub  14 . At this point, the laundry m is adhered to an inner wall of the inner tub  14  by the centrifugal force. 
     Referring to  FIG. 5 b   , the pressurized water stream between the outer and inner tubs  12  and  14  is a stream in which the wash water moves upward by the centrifugal force created by the rotation of the inner tub  14  but does not flow over the inner tub. At this point, the laundry m is adhered to the inner wall of the inner tub  14 . 
     By the centrifugal circulation water stream or the pressurized water stream, the wash water flows into the outer tub  12  through the through hole  14   h  and passes through the laundry m. Therefore, the laundry m can sufficiently absorb the washing aid agents. Particularly, by the centrifugal circulation water stream, a tap washing effect can be attained by the wash water poured into the inner tub  14 . 
       FIG. 6  is a block diagram illustrating a control relationship between major parts of the washing machine according to an exemplary embodiment of the present invention. Referring to  FIG. 6 , the washing machine may further include a driving detecting unit  36 . 
     The driving detecting unit  36  detects the driving state of the motor  22  through an RPM, a rotation cycle, or an output current variation of the motor  22 . 
     The controller  10  determines if only the pulsator  16  rotates or the pulsator  16  rotates together with the inner tub  14  on the basis of the driving state of the motor  22  detected by the driving detecting unit  36 . 
     For example, the driving detecting unit  36  may use a hall sensor. In this case, the driving detecting unit  36  may determine if only the pulsator  16  rotates or the pulsator  16  rotates together with the inner tub  14  on the basis of the rotation cycle of the motor  22 , which is detected by the hall sensor. 
     Even if the driving of the motor is controlled with the same RPM, the loads applied to the motor  22  in a case where only the pulsator  16  rotates and in a case where the pulsator  16  rotates together with the inner tub  14  may be different from each other. Therefore, the rotation cycles in the cases are different from each other. The controller  10  may determine, on the basis of this difference, if only the pulsator  16  rotates or the pulsator  16  rotates together with the inner tub  14 . 
       FIG. 7  is a flowchart of a washing machine control method according to an exemplary embodiment of the present invention. Referring to  FIG. 7 , wash water is supplied into the inner and outer tubs  14  and  12  (S 10 ). The controller  10  opens a water supply valve  6  to supply the wash water through the water supply passage  5 . When the water level of the outer tub  12  reaches a predetermined level, the controller  10  closes the water supply valve. The wash water is supplied to a space under the buoyancy clutch  40  is supplied and thus the buoyancy clutch  40  moves to the upper position by the buoyancy created by the wash water. As a result, the buoyancy clutch  40  is separated from the second driving shaft  32 . 
     When the supply of the wash water is completed, the controller alternately rotates the motor  22  in both directions (S 20 ). Since the buoyancy clutch  40  is separated from the second driving shaft  32 , only the pulsator  16  alternately rotates in the both directions. The laundry is treated by the frictional force between the pulsator  16  and the laundry and the agitating water stream formed by the pulsator  16 . 
     After the above, the controller  10  stops the motor  22  temporarily and operates the pump  26  (S 30 ). As the pump  26  operates, the wash water collected under the buoyancy clutch  40  is drained to the outer tub  12  through the through hole  54   h  of the hub  50  to release the buoyancy acting on the buoyancy clutch  40 . Accordingly, the buoyancy clutch  40  moves downward by the gravity and thus the second driving shaft engaging portion  42  is engaged with the second driving shaft  32 . 
     Next, the controller  10  drives continuously the motor in one direction (S 40 ). In Step S 30 , if the second driving shaft engaging portion  42  is accurately engaged with the second driving shaft  32 , the pulsator  16  and the inner tub  14  rotate together with each other after Step S 40 . If not, only the pulsator  16  will still rotate. 
     In Step S 50 , it is determined if the buoyancy clutch  40  accurately operates in Step S 30  and thus the driving shaft engaging portion  42  is accurately engaged with the second driving shaft  32 . The controller  10 , as described above, determines if only the pulsator  16  rotates or the pulsator  16  rotates together with the inner tub  14  in accordance with the detecting value of the driving detecting unit  36 . 
     Here, when it is determined that the pulsator  16  and the inner tub  14  integrally rotate together with each other, it is determined that the buoyancy clutch  40  normally operates in Step S 30 . When it is determined that only the pulsator  16  rotates, Step S 60  is performed as it is case where the buoyancy clutch  40  abnormally operates in Step S 30 . 
     When the buoyancy clutch  40  abnormally operates, it can be assumed that the second driving shaft engaging portion  42  is not accurately engaged with the second driving shaft  32  and thus the buoyancy clutch  40  is not accurately lowered. 
     In Step  60 , a process for allowing the second driving shaft engaging portion  42  to be accurately engaged with the second driving shaft  32  is performed. The controller  10  alternately rotates the motor  32  in the both direction. As the buoyancy clutch  40  rotates in the both direction, the second driving shaft engaging portion  42  moves to a correct position relative to the second driving shaft  32  and thus the second driving shaft engaging portion  42  is accurately engaged with the second driving shaft  32 . 
     Then, the process is returned to Step S 40  to operate the motor  22  and repeat the Step S 50 . 
     Meanwhile, In Steps S 40 , S 50 , and the like, the pump may keep operating to maintain the lower position of the buoyancy clutch  40 . 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.