Abstract:
A work machine having: a working part; a holding part for holding the working part; and an operating part for rotating the working part around the holding part; further comprising: an angle sensing means for sensing the angle of the working part from the horizontal; and a controller for adjusting and holding the position of the working part by use of the operation means based on the angle sensing means.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a work machine, such as an agricultural machine or a construction machine, having a bucket, booms supporting the bucket, and dump cylinders for rotating the bucket around the booms. 
         [0003]    2. Description of the Relate Art 
         [0004]    In conventional work machines having a working part such as a bucket, an operator operates the bucket by use of a bucket control lever. In other words, the bucket is operated by the operator manually. Therefore, during operation, it is difficult for the operator to recognize the exact angular position of the bucket. To address this problem, patent publications, such as WO2002/040783 and U.S. Pat. No. 5,887,365 have disclosed new methods. These inventions have a control unit that controls automatic bucket operation, and the control unit recognizes the angular position of the bucket. Then, the control unit adjusts the bucket position according to a target slope angle and a target depth of excavation. 
       Problem to be Solved by the Invention 
       [0005]    During excavation work, it is sometimes necessary to keep a loaded bucket level. However, it is difficult for an unskilled operator (a driver) to do so. The two above-mentioned patent publications do not present the solution for this problem. Therefore, this invention offers a work machine, which is able to keep a loaded bucket level easily. 
       SUMMARY OF THE INVENTION 
     Means to Solve the Problem 
       [0006]    A work machine having: a working part; a holding part for holding the working part; and an operating part for rotating the working part around the holding part; further comprising: an angle sensing means for sensing the angle of the working part from the horizontal; and a controller for adjusting and holding the position of the working part by use of the operation means based on the angle sensing means. 
         [0007]    In one embodiment, the invention may offer the work machine wherein the angle sensing means is installed on the working part. 
         [0008]    In one embodiment, the invention may offer the work machine wherein the angle sensing means is installed on the back part of the working part. 
         [0009]    In one embodiment, the invention may offer the work machine wherein the angle sensing means is installed on the side part of the working part. 
         [0010]    In one embodiment, the invention may offer the work machine further comprising: a driver&#39;s seat for the operator, a working part control lever installed at the side of the driver&#39;s seat, for turning the working part around by the operator, and an angle control switch equipped on the working part control lever, for starting adjusting and holding the working part to a target angular position. 
       Advantages of the Invention 
       [0011]    The invention offers a work machine having: a working part; a holding part for holding the working part; and an operating part for rotating the working part around the holding part; 
         [0012]    further comprising: an angle sensing means for sensing the angle of the working part from the horizontal; and a controller for adjusting and holding the position of the working part by use of the operation means based on the angle sensing means. Therefore, the bucket can be easily kept level by even an unskilled operator (a driver). 
         [0013]    In one embodiment, the invention offers the work machine wherein the angle sensing means is installed on the working part. Therefore, the angle position of the working part can be easily sensed by the angle sensing means. 
         [0014]    In one embodiment, the invention offers the work machine wherein the angle sensing means is installed on the back part of the working part. Therefore, the angle position of the working part can be easily sensed by the angle sensing means. 
         [0015]    In one embodiment, the invention offers the work machine wherein the angle sensing means is installed on the side part of the working part. Therefore, the angle position of the working part can be easily sensed by the angle sensing means. 
         [0016]    In one embodiment, the invention offers the work machine further comprising: a driver&#39;s seat for the operator, a working part control lever installed at the side of the driver&#39;s seat, for turning the working part around by the operator, and an angle control switch equipped on the working part control lever, for starting adjusting and holding the working part to a target angular position. Therefore, the working part angle control can be easily operated with the working part control lever while the working part is operated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    [ FIG. 1 ] A front perspective view of a tractor (a work machine) with a front loader as a first embodiment of the invention. 
           [0018]    [ FIG. 2 ] (a) is a back view of the bucket, (b) is a side view from “B” in (a), (c) is a side view of the hitch, and (d) is a section view from “X” in (c). 
           [0019]    [ FIG. 3 ] (a) is a front view of the boom control lever and the bucket control lever, (b) is a side view of the bucket control lever from “A” in (a), (c) is a side view of the boom control lever from “A” in (a). 
           [0020]    [ FIG. 4 ] A hydraulic circuit in the first embodiment. 
           [0021]    [ FIG. 5 ] A block diagram of the main part. 
           [0022]    [ FIG. 6 ] A flow chart of the automatic bucket level control process. 
           [0023]    [ FIG. 7 ] (a) is a back view of the bucket, (b) is a side view from “C” in (a). 
           [0024]    [ FIG. 8 ] A front perspective view of the main features of a tractor (a work machine). 
           [0025]    [ FIG. 9 ] (a) is a front view of the boom-bucket control lever, (b) is a plane view of (a), (c) is a right-side view of the boom-bucket control lever, (d) is a front view of the boom-bucket control lever. 
           [0026]    [ FIG. 10 ] A hydraulic circuit. 
           [0027]    [ FIG. 11 ] A block diagram of the main part. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Best Mode of Embodying the Invention 
       [0028]    The best mode of embodying the invention is described below with reference to specific figures.  FIG. 1  shows a front perspective view of the tractor  10  (a work machine). The tractor  10  is composed of front tires  14 , rear tires  15  and a front loader  16 . In this specification, ‘front’ is defined as the direction when the tractor  10  is going forward, and ‘back’ as the direction when the tractor  10  is going backward. Furthermore, ‘left’ is defined as the left side when the tractor  10  is going forward, and ‘right’ is defined as the right side when the tractor  10  is going forward. 
         [0029]    A hood  12  is put in the front part of the tractor  10 . A tank of gas and an engine (not illustrated) are installed under the hood  12 . A frame  41  is put in the lower part of the tractor  10 , and the engine is mounted on the frame  41 . The power of the engine is transmitted to front tires  14  and rear tires  15  through a transmission (not illustrated). The power of the engine is also used for a hydraulic pump, which is described below. 
         [0030]    A driver&#39;s seat  17 , on which a driver sits when operating the tractor  10 , is at the rear part of the tractor  10 . A steering wheel  18 , a monitor (not illustrated) etc. are in front of the driver&#39;s seat  17 . The monitor is to show driving speed, fuel level, etc. and also to give an alarm when a tilt sensor (described below) is not working. A bucket control lever (a working part control lever)  19  and a boom control lever  20  are installed at the right side of the driver&#39;s seat  17 . A transmission (not illustrated) for changing gears of the tractor  10  is under the driver&#39;s seat  17 . 
         [0031]    A floor  13  is installed in front of the driver&#39;s seat  17 . The floor  13  is composed of a pair of plane parts of the floor  13   a  and a projection part of the floor  13   b.  The two plane parts of the floor  13   a  are made plane and board-like. The projection part of the floor  13   b  is installed between the two plane parts of the floor  13   a.  The projection part of the floor  13   b  and the two plane parts of the floor  13   a  are coupled with each other by bolts etc. The projection part of the floor  13   b  bends upward, and a driving shaft, which transmits power from the engine to the transmission, is located under the projection part of the floor  13   b.    
         [0032]    Fenders F are mounted on both sides of the driver&#39;s seat  17 . The bucket control lever  19  and the boom control lever  20  are at the fender F at the right side of the driver&#39;s seat  17 . A Rollover Protective Structure (ROPS) R, which is an arch-like safety protective frame, is almost vertically installed at the rear part of the fenders F. 
         [0033]    A boom holding mechanism  40  for holding the front loader  16  is installed on both sides and in the middle of the tractor  10 . 
         [0034]    The front loader  16  is detachable from the tractor  10 . The front loader  16  is composed of a pair of booms (a support part)  21  set up at both sides of the tractor  10 , a pair of lift cylinders  23  located at both sides of the main body of the tractor  10 , a bucket (working part)  26  and a pair of dump cylinders (operating part)  24 . The lift cylinders  23  are to move the boom  21  up and down. The bucket  26  is coupled with the top of the two booms  21 . The dump cylinders  24  are to turn the bucket  26  around the top of the booms  21 . The lift cylinders  23  and the dump cylinders  24  are connected with a hydraulic circuit. This hydraulic circuit is to operate the lift cylinders  23  and the dump cylinders  24  by hydraulic pumps, which work by the power from the engine. 
         [0035]    The booms  21  are shaped as an arc and stretch out toward the front of the tractor  10 . The bottom of each of the booms  21  is coupled with the upper section of the boom holding mechanism  40 , and the two booms  21  (which are located at both sides of the main body of the tractor  10 ) are rotatable around the boom holding mechanism  40 . The bucket  26 , on the other hand, is coupled with the two booms  21  by a hitch (a connection part)  25 . The hitch  25  is detachable from the bucket  26 , and is rotatable around the top of the booms  21 . A cylinder bracket  27  is welded at the middle section of each boom  21 . The cylinder bracket  27  is set upward from the boom  21  in its front part, whereas it is set downward from the boom  21  in its rear part. 
         [0036]    The bucket  26  is made of metal. It is comprised of a bottom part  26   bt,  a pair of side parts  26   s,  a rear part  26   bk,  a back part  26   b  and a top part  26   u.  The side parts  26   s  are vertically installed at both ends of the bottom part  26   bt.  The rear part  26   bk  is connected to the pair of side parts  26   s  at both of its ends. The back part  26   b  is connected to the rear part  26   bk,  and it is also connected to the pair of side parts  26   s  at both of its ends. A pair of stays  26 A is put on the rear part  26   b.  A hook bar  26 B is fixed to the pair of stays  26 A at its both ends. A tilt sensor (an angle sensing means) TS is equipped on the right side part  26   s.  Alternatively, the tilt sensor TS may be equipped on the left side part  26   s.  The tilt sensor TS senses the angle position of the bucket  26  from the horizontal; in other words, it senses the angle between an upper edge  26   su  of the side part  26   s  and the horizontal. Alternatively, the tilt sensor TS may sense the angle position of the bucket  26  from the vertical. The tilt sensor TS is connected to a controller (later described). 
         [0037]    The hitch  25  is comprised of a pair of trapezoidal metal plates. Each boom  21  is held at its top between the two metal plates and is rotatable at Pin P. Each dump cylinder  24  is also held at its top between the two metal plates and is rotatable at this point. Pin P is called a rotation center of the hitch  25  and the bucket  26 . A holding part  25 A is made at the top edge of the hitch  25 . The holding part  25 A is made by cutting the hitch  25  in a half circle shape. The holding part  25 A holds a hook part  26 B of the bucket  26 . Therefore, the bucket  26  is connected to the hitch  25  via the holding part  25 A. A pair of stays  26 A is made of a rectangular metal plate and is fixed on the rear part  26   bk  of the bucket  26 . 
         [0038]    Other attachments instead of the bucket  26  can also be attached to the hitch  25 . A harness, which connects between the tilt sensor TS and the above-mentioned controller, has a connecter. The harness can be disconnected from the bucket  26  at said connecter when the bucket  26  is detached. When another attachment is attached to the hitch  25 , the harness is to be connected to the attachment with the connecter. A swing sensor or a gyro sensor can be installed in the bucket  26  instead of the tilt sensor TS. 
         [0039]    The bottom of the lift cylinder  23  is coupled with the boom holding mechanism  40  and is rotatable at the boom holding mechanism  40 . The top of the lift cylinder  23  is, on the other hand, connected to the back part of the cylinder bracket  27  and is rotatable at the cylinder bracket  27 . Each lift cylinder  23  is installed underneath and parallel to each boom  21 . 
         [0040]    The dump cylinder  24  is connected to the front part of the cylinder bracket  27  and is rotatable at the cylinder bracket  27 . On the other hand, the top of the dump cylinder  24  is coupled with the top of the hitch  25  and is rotatable at the hitch  25 . Each dump cylinder  24  is installed over each boom  21 . 
         [0041]    Therefore, the booms  21  go up (or rotate upward) when the lift cylinders  23  stretch out, whereas the booms  21  go down (or rotate downward) when the lift cylinders  23  shrink. On the other hand, the bucket  26  turns upward when the dump cylinders  24  shrink, whereas the bucket  26  turns downward when the dump cylinders  24  stretch out. 
         [0042]    The bucket control lever  18  and the boom control lever  20  are illustrated in  FIG. 3(   a ) to  FIG. 3(   c ), and the hydraulic circuit is illustrated in  FIG. 4 . The bucket control lever  19  is to operate the dump cylinders  24  of the front loader  16 . The bucket control lever  19  consists of a grip  19   a,  a rod  19   b  and a protection part  19   c.  The grip  19   a  is made of resin and shaped by an injection molder. The lower part of the rod  19   b  is held inside the right fender F. The protection part  19   c  covers the gap between the rod  19   b  and the fender F in order to prevent dust from coming inside the fender F. The protection part  19   c  is made of a flexible material to enable it to follow the movement of the rod  19   b.  The bucket control lever  19  is connected to a selector valve  50 . The selector valve  50  changes position when the bucket control lever  19  is turned. 
         [0043]    More specifically, when the bucket control lever  19  is turned toward the D direction, the selector valve  50  moves to the D position (DOWN position). When the bucket control lever  19  is turned toward the U direction, the selector valve  50  moves to the U position (UP position). The bucket control lever  19  is always in the N position (neutral) by coil springs (not illustrated) whenever it is not operated. Operation buttons B 1 , B 2  are installed on the front side of the bucket control lever  19 . Furthermore, operation buttons B 1 , B 2  project from the front side of the bucket control lever  19 . An operation button B 3  is installed on the upper side of the bucket control lever  19 . Furthermore, the operation button B 3  projects from the upper side of the bucket control lever  19 . Each operation button B 1 , B 2 , B 3  is made of resin and is a column with a dome-like tip. The back of the operation buttons B 1 , B 2 , B 3  are flat. The actuator of each switch (the switch is a push-plunger type limit switch with the actuator) touches the back of the operation buttons B 1 , B 2  and B 3 , respectively. 
         [0044]    Three switches of the operation buttons B 1 , B 2 , B 3  are respectively connected to the controller (described later). The solenoid valve SL is moved to the position U when the operation button B 1  is pressed. On the other hand, the solenoid valve SL is moved to the position D when the operation button B 2  is pressed. The automatic bucket level control for the bucket  26  starts when the operation button (an angle control switch) B 3  is pressed (and then the switch is ON). The automatic bucket level control for the bucket  26  ends when the operation button (an angle control switch) B 3  is pressed again (and then the switch is OFF). 
         [0045]    The boom control lever  20  is to operate the lift cylinders  23 . The boom control lever  20  consists of a grip  20   a,  a rod  20   b  and a protection part  20   c.  The grip  20   a  is made of resin and shaped by an injection molder. The lower part of the rod  20   b  is held inside the right fender F. The protection part  20   c  covers the gap between the rod  20   b  and the fender F in order to prevent dust from entering the fender F. The protection part  20   c  is made of a flexible material to enable it to follow the movement of the rod  20   b.  The boom control lever  20  is connected to a selector valve  51 . The selector valve  51  changes position when the boom control lever  20  is turned. 
         [0046]    More specifically, when the boom control lever  20  is turned toward the D direction, the selector valve  51  moves to the D position (DOWN position). When the boom control lever  20  is turned toward the U direction, the selector valve  51  moves to the U position (UP position). The boom control lever  20  is always in the N position (neutral) by coil springs (not illustrated) whenever it is not operated. 
         [0047]    The hydraulic circuit is connected to the hydraulic fluid tank (not illustrated). The hydraulic fluid can move with a constant pressure between the hydraulic fluid tank and the dump cylinders  24  when the bucket control lever  19  is in operation. The hydraulic fluid can move with a constant pressure between the hydraulic fluid tank and the boom cylinders  23  when the boom control lever  20  is in operation. The dump cylinders  24  expand when the bucket control lever  19  is turned toward the D direction. On the other hand, the dump cylinders  24  shrink when the bucket control lever  19  is turned toward the U direction. The lift cylinders  23  shrink when the boom control lever  20  is turned toward the D direction. On the other hand, the lift cylinders  23  expand when the boom control lever  20  is turned toward the U direction. 
         [0048]    The procedure for operating the front loader  16  by use of the bucket control lever  19  and the boom control lever  20  is described as follows: In order to move the booms  21  upward, turn the boom control lever  20  toward the operator&#39;s side (the U direction). The selector valve  51  moves toward the U position. Then, the hydraulic circuit of the lift cylinders  23  is opened to expand the lift cylinders  23 . In order to stop moving the booms  21  upward, release your hand from the boom control lever  20 . The boom control lever  20  returns to the N position and the selector valve  51  moves toward the N position to close the hydraulic circuit of the lift cylinders  23 . 
         [0049]    In order to move the booms  21  downward, turn the boom control lever  20  frontward (the D direction). The selector valve  51  moves toward the D direction. Then, the hydraulic circuit of the lift cylinders  23  is opened to shrink the lift cylinders  23 . In order to stop moving the booms  21  downward, release your hand from the boom control lever  20 . The boom control lever  20  returns to the N position, and the selector valve  51  moves toward the N position for closing the hydraulic circuit of the lift cylinders  23 . 
         [0050]    In order to move the bucket  26  upward, turn the bucket control lever  19  backward (the U direction). The selector valve  50  moves toward the U direction. Then, the hydraulic circuit of the dump cylinders  24  is opened to shrink the dump cylinders  24 . In order to stop moving the bucket  26  upward, release your hand from the bucket control lever  19 . The bucket control lever  19  returns to the N position, and the selector valve  50  moves toward the N position for closing the hydraulic circuit of the dump cylinders  24 . 
         [0051]    In order to move the bucket  26  downward, turn the bucket control lever  19  frontward (the D direction). The selector valve  50  moves toward the D direction. Then, the hydraulic circuit of the dump cylinders  24  is opened to expand the dump cylinders  24 . In order to stop moving the bucket  26  downward, release your hand from the bucket control lever  19 . The bucket control lever  19  returns to the N position and the selector valve  50  moves toward the N position to close the hydraulic circuit of the dump cylinders  24 . 
         [0052]    The bucket  26  can also be operated upwardly or downwardly by the operation buttons B 1  and B 2 , respectively. The solenoid valve SL moves toward the U position when the operation button B 1  is pressed while the bucket control lever  19  is in the N position. The solenoid valve SL moves back to the N position when your hand is released from the operation button B 1 . The solenoid valve SL moves toward the D position when the operation button B 2  is pressed while the bucket control lever  19  is in the N position. The solenoid valve SL moves back to the N position when your hand is released from the operation button B 2 . The controller (described later) is in between the operation buttons B 1 , B 2  and the solenoid valve SL. The solenoid valve SL is moved by the controller. 
         [0053]      FIG. 5  shows the main block diagram of this embodiment (the engine is described as “E”). The controller (a control part) CT, which is inside a dashboard in front of the driver&#39;s seat  17 , is connected with the operation buttons B 1 , B 2 , B 3 . The controller CT is also connected with the tilt sensor TS. Furthermore, the controller CT is connected to the solenoid valve SL and operates it. 
         [0054]    The controller CT is able to conduct the automatic bucket level control process for the bucket  26 , which keeps the bucket  26  level.  FIG. 6  shows the automatic bucket level control program flow chart for the controller CT. The controller CT periodically senses if the tilt sensor TS is working (STEP 1 ). If the tilt sensor TS is not working, the controller CT gives an alarm on the display, which is installed in front of the driver&#39;s seat  17  (STEP 2 ). If the tilt sensor TS is working, the controller CT program proceeds to STEP 3  and the controller CT senses if the operation button (the automatic level control button) B 3  is ON. If the operation button B 3  is OFF, then, the controller CT program proceeds to STEP 4  and the controller CT ends the automatic level control program (or the controller CT does not start the program). If the operation button B 3  is ON, then the controller CT program proceeds to STEP 5  and the controller CT starts the automatic level control program. 
         [0055]    Furthermore, at STEP 6 , the controller CT periodically senses if the bucket  26  is level via the tilt sensor (an angle sensing means) TS. If the bucket  26  is level, the controller CT program goes back to STEP 3 . If the bucket  26  is not level, the controller CT program goes to STEP 7  and the controller CT senses if the upper edge  26   su  of the bucket  26  is turning more than the level (the horizontal) or, in other words, the upper edge  26   su  is turning backward (toward the driver&#39; seat  17 ) more than the level. If the upper edge  26   su  is turning more than the level, the controller CT moves the solenoid valve SL toward the DOWN position to expand the dump cylinder  24  (STEPS). If the upper edge  26   su  is not turning more than the level, the controller CT program proceeds to STEPS and the controller CT senses if the upper edge  26   su  is turning less than the level. If the upper edge  26   su  is not turning less than the level, the controller CT program goes back to STEP 1 . If the upper edge  26   su  is turning less than the level (the horizontal), the controller CT moves the solenoid valve SL toward the UP position for shrinking the dump cylinder  24  (STEP 10 ). 
         [0056]    The automatic level control precedes the bucket manual operation with the operation buttons B 1 , B 2 . 
         [0057]    The automatic level control program in this embodiment can be applied not only to the horizontal control of the bucket but also angle control of the bucket in any plane—it is called an automatic angle control program. In such cases, an angle set dial, which is connected to the operation button B 3 , should be set on the bucket control lever  19  in order to automatically control the bucket angle with respect to the level. In addition, the target angle of the bucket  26  may be input through a touch screen display. 
       Further Embodiment 1 
       [0058]      FIG. 7  shows a further embodiment of this invention. In this embodiment, a tilt sensor TS 1  is installed on the rear part  26   bk  of the bucket  26 . Other components of the tractor  10  are not described as they are the same as the previous embodiment. 
         [0059]    The bucket  26  is made of metal. It is comprised of a bottom part  26   bt,  a pair of side parts  26   s,  a rear part  26   bk,  a back part  26   b  and a top part  26   u.  The side parts  26   s  are vertically installed at both ends of the bottom part  26   bt.  The rear part  26   bk  is connected to the side parts  26   s  at both of its ends. The back part  26   b  is connected to the rear part  26   bk  and is also connected to the pair of side parts  26   s  at both of its ends. A pair of stays  26 A is put on the rear part  26   b.  A hook bar  26 B is fixed to the pair of stays  26 A at both of its ends. A tilt sensor (an angle detection means) TS 1  is equipped underneath the hook bar  26 B. Alternatively, the tilt sensor TS 1  may be equipped on the top part  26   u  or the back part. The tilt sensor TS 1  is to sense the angle position of the bucket  26  from the horizontal; however, the tilt sensor TS 1  may sense the angular position of the bucket  26  from the vertical, instead. The tilt sensor TS 1  is connected to a controller CT with a harness. 
         [0060]    The automatic bucket level control process, which gives an automatic level control for the bucket  26 , is the same as the previous embodiment shown in  FIG. 6 . If the tilt sensor is installed on the side part  26   s,  as described in the previous embodiment, the tilt sensor might be broken by sand or rocks when the tractor  10  digs. However, if the tilt sensor TS 1  is installed on the rear part  26   bk  (or the upper part  26   u,  the back part  26   b ), the risk of the tilt sensor TS 1  being broken unexpectedly as just described can be reduced. 
       Further Embodiment 2 
       [0061]    Another embodiment of this invention is shown in  FIG. 8  to  FIG. 11 . In this embodiment, a boom-bucket control lever JS is introduced instead of the bucket control lever  19  and the boom control lever  20 . In this embodiment, the boom-bucket control lever JS has two functions: a bucket control function and a boom control function. The boom-bucket control lever JS is positioned on the fender F. Other components of the tractor  10  are the same as the first embodiment. When the boom-bucket control lever JS is turned frontward or backward (BD-BU direction in  FIG. 9 ), the selector valve  51  is operated and the booms  21  are turned. On the other hand, when the boom-bucket control lever JS is turned left-sideward or right-sideward (AU-AD direction in  FIG. 9 ), the selector valve  50  is operated and the bucket  26  is turned. Operation buttons B 1 , B 2  are installed on the front side of the boom-bucket control lever JS. Furthermore, operation buttons B 1 , B 2  project from the front side of the boom-bucket control lever JS. An operation button B 3  is installed on the upper side of the boom-bucket control lever JS. Furthermore, the operation button B 3  projects from the upper side of the boom-bucket control lever JS. 
         [0062]    The hydraulic circuit is connected to the hydraulic fluid tank (not illustrated). The hydraulic fluid can move with a constant pressure between the hydraulic fluid tank and the dump cylinders  24  when the boom-bucket control lever JS is in operation. The hydraulic fluid can move with a constant pressure between the hydraulic fluid tank and the boom cylinders  23  when the boom-bucket control lever JS is in operation. The dump cylinders  24  expand when the boom-bucket control lever JS is turned toward the AD direction. On the other hand, the dump cylinders  24  shrink when the boom-bucket control lever JS is turned toward the AU direction. The lift cylinders  23  shrink when the boom-bucket control lever JS is turned toward the BD direction. On the other hand, the lift cylinders  23  expand when the boom-bucket control lever JS is turned toward the BU direction. 
         [0063]    The procedure for operating the front loader  16  by use of the boom-bucket control lever JS is described as follows: in order to move the booms  21  upward, turn the boom-bucket control lever JS backward (the BU direction). The selector valve  51  moves toward the BU position. Then, the hydraulic circuit of the lift cylinders  23  is opened to expand the lift cylinders  23 . In order to stop moving the booms  21  upward, release your hand from the boom-bucket control lever JS. The boom-bucket control lever JS returns to the N position and the selector valve  51  moves toward the N position to close the hydraulic circuit of the lift cylinders  23 . 
         [0064]    In order to move the booms  21  downward, turn the boom-bucket control lever JS forward (the BD direction). The selector valve  51  moves toward the BD direction. Then, the hydraulic circuit of the lift cylinders  23  is opened to shrink the lift cylinders  23 . In order to stop moving the booms  21  downward, release your hand from the boom-bucket control lever JS. The boom-bucket control lever JS returns to the N position, and the selector valve  51  moves toward the N position for closing the hydraulic circuit of the lift cylinders  23 . 
         [0065]    In order to move the bucket  26  upward, turn the boom-bucket control lever JS left-sideward (the AU direction). The selector valve  50  moves toward the AU direction. Then, the hydraulic circuit of the dump cylinders  24  is opened to shrink the dump cylinders  24 . In order to stop moving the bucket  26  upward, release your hand from the boom-bucket control lever JS. The boom-bucket control lever JS returns to the N position, and the selector valve  50  moves toward the N position for closing the hydraulic circuit of the dump cylinders  24 . 
         [0066]    In order to move the bucket  26  downward, turn the boom-bucket control lever JS right-sideward (the AD direction). The selector valve  50  moves toward the AD direction. Then, the hydraulic circuit of the dump cylinders  24  is opened to expand the dump cylinders  24 . In order to stop moving the bucket  26  downward, release your hand from the boom-bucket control lever JS. The boom-bucket control lever JS returns to the N position and the selector valve  50  moves toward the N position to close the hydraulic circuit of the dump cylinders  24 . 
         [0067]    As with the previous embodiments, the bucket  26  can also be operated upwardly or downwardly by the operation buttons B 1  and B 2 , respectively. The solenoid valve SL moves toward the U position when the operation button B 1  is pressed while the boom-bucket control lever JS is in the N position. The solenoid valve SL moves back to the N position when your hand is released from the operation button B 1 . The solenoid valve SL moves toward the D position when the operation button B 2  is pressed while the boom-bucket control lever JS is in the N position. The solenoid valve SL moves back to the N position when your hand is released from the operation button B 2 . The controller CT is in between the operation buttons B 1 , B 2  and the solenoid valve SL. The solenoid valve SL is moved by the controller CT. The automatic bucket level control program flow chart shown in  FIG. 6  applies to this embodiment.