Patent Publication Number: US-11655615-B1

Title: Work machine and control method for work machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U. S. C. § 119 to Japanese Patent Application No. 2022-018063, filed Feb. 8, 2022. The contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a work machine and a control method for the work machine. 
     Discussion of the Background 
     Japanese Patent No. 6919479 discloses a work machine including a hydraulic system for performing horizontal control for preventing dropping of a load by changing a tilt angle of a bucket in accordance with an arm elevation when elevating the arm. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a work machine includes a bucket including a bucket proximal end and a bucket distal end opposite to the bucket proximal end, an arm including an arm distal end that swingably supports the bucket proximal end and an arm proximal end opposite to the arm distal end, a vehicle body swingably supporting the arm proximal end, a bucket cylinder including a first chamber into which hydraulic fluid flows when the bucket distal end is tilted downward, and a second chamber into which hydraulic fluid flows when the bucket distal end is tilted upward, an arm cylinder including a third chamber into which hydraulic fluid flows when the arm distal end is raised, and a fourth chamber into which hydraulic fluid flows when the arm distal end is lowered, a first hydraulic pump configured to discharge hydraulic fluid to the bucket cylinder and the arm cylinder, a first oil passage connecting the first hydraulic pump and the first chamber of the bucket cylinder, a bucket control valve provided in the first oil passage and having a bucket control pilot port, the bucket control valve being configured to control hydraulic fluid supplied to the first chamber and the second chamber according to hydraulic pressure of the pilot oil applied to the bucket control pilot port, a second oil passage connecting the bucket control valve and the second chamber, a third oil passage connecting the first hydraulic pump and the third chamber of the arm cylinder, an arm control valve provided in the third oil passage and having an arm control pilot port, the arm control valve being configured to control hydraulic fluid supplied to the third chamber and the fourth chamber according to hydraulic pressure of the pilot oil applied to the arm control pilot port, a fourth oil passage connecting the arm control valve and the fourth chamber, a second hydraulic pump configured to discharge pilot oil to control the arm control valve and the bucket control valve, a first pilot oil passage connecting the arm control pilot port and the second hydraulic pump, an arm pilot control valve provided in the first pilot oil passage and configured to control the hydraulic pressure of the pilot oil applied to the arm control pilot port based on an operation amount of an arm input member operated to raise and lower the arm, an arm operation detection sensor configured to detect operation of the arm input member, a second pilot oil passage connecting the bucket control pilot port and the second hydraulic pump; a bucket pilot control valve provided in the second pilot oil passage and configured to control the hydraulic pressure of the pilot oil applied to the bucket control pilot port based on an operation amount of a bucket input member operated to swing the bucket, a bucket operation detection sensor to detect operation of the bucket input member, a bypass oil passage connecting the fourth oil passage to the first oil passage, a switching mechanism configured to control connection of the fourth oil passage to the first oil passage via the bypass oil passage and disconnection of the fourth oil passage to the first oil passage via the bypass oil passage; and circuitry configured to determine whether an uptilt operation to raise the arm and to tilt the bucket downward has been performed based on an output of the arm operation detection sensor and an output of the bucket operation detection sensor, control the switching mechanism to connect the fourth oil passage to the first oil passage when it is determined that the uptilt operation has been performed. 
     According to another aspect of the present invention, a control method for a work machine including: sending hydraulic fluid from a first hydraulic pump to a first chamber of a bucket cylinder via a first oil passage to tilt a bucket distal end of a bucket downward; controlling a bucket control valve provided in the first oil passage to connect a part of the first oil passage from the first hydraulic pump to a bucket control valve with a second oil passage connecting the bucket control valve and a second chamber of the bucket cylinder to send the hydraulic fluid from the first hydraulic pump to the second chamber of the bucket cylinder to tilt the bucket distal end upward; sending the hydraulic fluid from the first hydraulic pump to a third chamber of an arm cylinder via a third oil passage to raise an arm distal end of an arm that swingably supports a bucket proximal end of the bucket opposite to the bucket distal end; controlling an arm control valve provided in the third oil passage to connect a part of the third oil passage from the first hydraulic pump to the arm control valve with a fourth oil passage connecting the arm control valve and a fourth chamber of the arm cylinder to send the hydraulic fluid from the first hydraulic pump to the fourth chamber of the arm cylinder to lower the arm distal end; detecting an input of a first operation to raise the arm to an arm input member configured to be operated to raise and lower the arm; detecting an input of a second operation to tilt the bucket downward to a bucket input member configured to be operated to swing the bucket; and sending, when the first operation and the second operation are detected, the hydraulic fluid to the first chamber through the first oil passage to control a switching mechanism to connect the first oil passage with the fourth oil passage through a bypass oil passage to send hydraulic fluid in the fourth chamber to the first chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
         FIG.  1    is a side view of a work machine. 
         FIG.  2    is a top view of a work machine. 
         FIG.  3    is a schematic configuration diagram of a hydraulic system of the work machine. 
         FIG.  4    is a flowchart showing a method of controlling the work machine. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. Similar reference numerals indicate corresponding or identical components in the drawings. 
     Referring to  FIGS.  1  and  2   , a work machine  1 , for example, a compact truck loader, includes a vehicle body  2 , a pair of traveling devices  3 , and a working device  4 . The vehicle body  2  supports the traveling device  3  and the working device  4 . In the illustrated embodiment, the traveling device  3  is a crawler type traveling device. Therefore, each of the pair of traveling devices  3  includes a drive wheel  31  driven by the hydraulic motor device  30 , driven wheels  32  and  33 , and a rolling wheel  34 . However, each of the pair of traveling devices  3  is not limited to a crawler type traveling device. Each of the pair of traveling devices  3  may be, for example, a front wheel/rear wheel traveling device or a traveling device having a front wheel and a rear crawler. The working device  4  includes work equipment (bucket)  41  at the distal end of working device  4 . The bucket  41  is a bucket proximal end  41 P and a bucket distal end  41 D opposite to the bucket proximal end  41 P. A proximal end of the working device  4  is attached to a rear portion of the vehicle body  2 . The working device  4  includes a pair of arm assemblies  42  for rotatably supporting the bucket  41  via the bucket pivot shaft  43 . Each of the pair of arm assemblies  42  includes a link  44  and an arm  45 . The arm  45  includes an arm distal end  45 D for swingably supporting the bucket proximal end  41 P and an arm proximal end  45 P opposite to the arm distal end  45 D. The arm distal end  45 D swingably supports the bucket pivot shaft  43 . The vehicle body  2  swingably supports an arm proximal end  45 P. Specifically, the vehicle body  2  swingably supports the arm proximal end  45 P via the link  44 . 
     The link  44  is rotatable with respect to the vehicle body  2  around a fulcrum shaft  46 . The arm  45  is rotatable with respect to the link  44  around the joint shaft  47 . The working device  4  further includes a plurality of arm cylinders  48  and at least one bucket cylinder  49 . Each of the plurality of arm cylinders  48  is rotatably connected to the vehicle body  2  and the arm  45 , and moves the link  44 , the arm  45  and the like to move the bucket  41  up and down. At least one bucket cylinder  49  is configured to tilt the bucket  41 . A vehicle body  2  includes a cabin  5 . A cabin  5  is provided with a front window  51  which can be opened and closed freely, and an outer shape thereof is defined by a cab frame  53 . The front window  51  may be omitted. A work machine  1  includes a driver&#39;s seat  54  and an operating lever  55  in a cabin  5 . As shown in  FIG.  2   , the cab frame  53  is rotatable about rotational shafts RSL and RSR on the vehicle body  2 .  FIGS.  1  and  2    illustrate a common pivot axis AXC which is defined by the rotational axes RSL and RSR. That is, the cab frame  53  is attached to the vehicle body  2  so as to be rotatable about the pivot axis AXC. 
     In the embodiment according to the present application, the front-back direction D FB  (Forward D F /backward DB ) means a forward and backward direction (forward/backward direction) as seen from an operator sitting on the driver&#39;s seat  54  of the cabin  5 . Left direction DL, right direction D R , width direction D W  mean a left direction, a right direction, and a left/right direction, respectively, when viewed from the operator. Up direction D U , down direction D D ; height DH mean an upward direction, a downward direction and a height direction as viewed from the operator. The front/back, left/right (width), and up/down (height) directions of the work machine  1  correspond to the front/back, left/right (width), and up/down (height) directions as viewed from the operator. 
       FIG.  1    shows the left side of the work machine  1 . As shown in  FIG.  2   , the vehicle body  2  is substantially symmetrical with respect to a center plane M of the vehicle body, and has a first side surface  2 L which is a left side surface and a second side surface  2 R which is a right side surface. Among the pair of traveling devices  3 , the traveling device  3  provided on the first side surface  2 L is shown as the first traveling device  3 L, and the traveling device  3  provided on the second side surface  2 R is shown as the second traveling device  3 R. Among the pair of arm assemblies  42 , the arm assembly  42  provided on the left side with respect to the center plane M of the vehicle body is shown as a first arm assembly  42 L, and an arm assembly  42  provided on the right side with respect to the vehicle body center plane M is shown as a second arm assembly  42 R. A link  44  provided on the left side with respect to the center plane M of the vehicle body is shown as a first link  44 L. An arm  45  provided on the left side with respect to the vehicle body center plane M is shown as a first arm  45 L, and an arm  45  provided on the right side with respect to the vehicle body center plane M is shown as a second arm  45 R. The fulcrum shaft  46  located on the left side of the vehicle body center plane Mi shown as a first fulcrum shaft  46 L, a fulcrum shaft  46  provided on the right side of the vehicle body center plane M is shown as the second fulcrum shaft  46 R. A joint shaft  47  provided on the left side with respect to the vehicle body center plane M is shown as a first joint shaft  47 L, and a joint shaft  47  provided on the right side with respect to the vehicle body center plane M is shown as a second joint shaft  47 R. Among the hydraulic motor devices  30 , the hydraulic motor device  30  provided on the left side with respect to the vehicle body center plane M is shown as a first hydraulic motor device  30 L, the hydraulic motor device  30  provided on the right side with respect to the vehicle body center plane M is shown as a second hydraulic motor device  30 R. 
     Referring to  FIGS.  1  and  2   , the work machine  1  includes an engine  6  provided at a rear portion of a vehicle body  2 , and a plurality of hydraulic pumps  7  including the first traveling hydraulic pump  7 L, and a second traveling hydraulic pump  7 R. The engine  6  drives a plurality of hydraulic pumps  7 . The first hydraulic pump  7 L and the second hydraulic pump  7 R are configured to discharge hydraulic fluid to drive a hydraulic motor device  30  for driving a drive wheel  31 . The plurality of hydraulic pumps  7  other than the first hydraulic pump  7 L and the second hydraulic pump  7 R are configured to discharge hydraulic fluid to drive hydraulic actuators (a plurality of arm cylinders  48 , at least one bucket cylinder  49 , and the like) connected to the working device  4 . The engine  6  is provided between a pair of arm assemblies  42  in the width direction D W  of the work machine  1 . The work machine  1  further includes a cover  8  for covering the engine  6 . The work machine  1  further includes a bonnet cover  9  provided at the rear end of the vehicle body  2 . As the bonnet cover  9  is openable, a maintenance worker can perform maintenance work on the engine  6  and the like. 
     &lt;Hydraulic System&gt; 
     A work machine  1  is provided with a hydraulic system  100 .  FIG.  3    is a schematic configuration diagram of the hydraulic system  100  of the work machine  1 . A hydraulic system  100  includes an arm cylinder  48 , a bucket cylinder  49 , a first hydraulic pump  11 , a second hydraulic pump  12 , an arm control valve  20 , a bucket control valve  25 , an arm pilot control valve  60 , a bucket pilot control valve  65 , a first oil passage R 1 , a second oil passage R 2 , a third oil passage R 3 , a fourth oil passage R 4 , a first pilot oil passage PR 1 , and a second pilot oil passage PR 2 , a bypass oil passage BR 1 , a switching mechanism  70 , and an arm operation detection sensors AS 1  and AS 2 , a bucket operation detection sensor BS 1 , BS 2 , and a controller  15 . 
     The bucket cylinder  49  includes a first chamber CB 1  into which the hydraulic fluid flows when the bucket distal end  41 D is tilted downward and a second chamber CB 2  into which the hydraulic fluid flows when the bucket distal end  41 D is tilted upward. The arm cylinder  48  includes a third chamber CB 3  into which the hydraulic fluid flows when the arm distal end  45 D is raised and a fourth chamber CB 4  into which the hydraulic fluid flows when the arm distal end  45 D is lowered. The first hydraulic pump  11  is driven by the engine  6  and is configured to discharge the hydraulic fluid stored in the hydraulic fluid tank  10  to the bucket cylinder  49  and the arm cylinder  48 . The first hydraulic pump  11  is, for example, a variable displacement type hydraulic pump. The second hydraulic pump  12  is driven by the engine  6  and is configured to discharge pilot oil for controlling the arm control valve  20  and the bucket control valve  25 . Among the hydraulic fluid stored in the hydraulic fluid tank  10 , the hydraulic fluid discharged from the second hydraulic pump  12  and used for control may be referred to as pilot fluid, and the pressure of the pilot fluid may be referred to as pilot pressure. The second hydraulic pump  12  is, for example, a constant capacity type hydraulic pump. 
     First oil passage R 1  connects the first hydraulic pump  11  and the first chamber CB 1  of the bucket cylinder  49 . The bucket control valve  25  is provided in the first oil passage R 1 , and includes bucket control pilot ports  25 P 1  and  25 P 2  and is configured to control the hydraulic fluid supplied to the first chamber CB 1  and the second chamber CB 2  of the bucket cylinder  49  according to the hydraulic pressure of pilot oil applied to pilot ports  25 P 1  and  25 P 2 . The bucket control valve  25  switches to the bucket cylinder static control position BNP, the bucket cylinder extension control position BEP, or the bucket cylinder contraction control position BSP according to the pressure difference between the pilot pressure applied to the bucket control pilot port  25 P 1  and the pilot pressure applied to the bucket control pilot port  25 P 2 . The first oil passage R 1  includes a partial oil passage R 10  common with a third oil passage R 3  extending from the first hydraulic pump  11  to the joint J 11 , and a partial oil passage R 11  extending from the joint J 11  to the bucket control valve  25 , a partial oil passage R 12  connecting different connection ports of the bucket control valve  25 , and a partial oil passage R 13  extending from the bucket control valve  25  to the first chamber CB 1  of the bucket cylinder  49 . The second oil passage R 2  connects the bucket control valve  25  and the second chamber CB 2  of the bucket cylinder  49 . The first oil passage R 1  and the second oil passage R 2  are branched by being connected to different ports of the bucket control valve  25 . 
     More specifically, the bucket control valve  25  includes a first bucket control valve port YP 1 , a second bucket control valve  25  port YP 2 , and a third bucket control valve port YP 3 . The first bucket control valve port YP 1  is connected to the first hydraulic pump  11  via a first oil passage R 1 . The second bucket control valve port YP 2  is connected to the first chamber CB 1  of the bucket cylinder  49  through the first oil passage R 1 . The third bucket control valve port YP 3  is connected to the second oil passage R 2 . When the pilot pressure applied to the bucket control pilot port  25 P 1  is greater than a predetermined pressure than the pilot pressure applied to the bucket control pilot port  25 P 2 , the bucket control valve  25  is switched to a bucket cylinder extension control position BEP at which the first bucket control valve port YP 1  communicates with the second bucket control valve port YP 2  and the third bucket control valve port YP 3  communicates with a first drain oil passage DR 1  (described later). When the pilot pressure applied to the bucket control pilot port  25 P 2  is larger than the pilot pressure applied to the bucket control pilot port  25 P 1  than the predetermined pressure, the bucket control valve  25  communicates the first bucket control valve port YP 1  with the third bucket control valve port YP 3  and switches to the bucket cylinder contraction control position BSP at which the second bucket control valve port YP 2  communicates with DR 1  (to be described later). When the absolute value of the difference between the pilot pressure applied to the bucket control pilot port  25 P 2  and the pilot pressure applied to the bucket control pilot port  25 P 1  is equal to or less than the predetermined pressure, the bucket control valve  25  is switched to a bucket cylinder static control position BNP at which the first bucket control valve  25  port YP 1 , the second bucket control valve port YP 2 , and the third bucket control valve  25  port YP 3  are isolated. The hydraulic system  100  further includes a second pressure control valve  77  that communicates the first bucket control valve port YP 1  with the second bucket control valve port YP 2  or the third bucket control valve port YP 3  only when the pressure applied to the first bucket control valve port YP 1  is equal to or greater than the fourth threshold pressure. As a result, if the hydraulic pressure from the first hydraulic pump  11  does not reach the fourth threshold pressure, the bucket control can be prevented from being performed. 
     The third oil passage R 3  connects the first hydraulic pump  11  and the third chamber CB 3  of the arm cylinder  48 . The third oil passage R 3  includes partial oil passage R 10  common to the first oil passage R 1  extending from the first hydraulic pump  11  to the joint J 11  and a partial oil passage R 31  branched from the first oil passage R 1  at the joint J 11 . The arm control valve  20  is provided in the third oil passage R 3  and includes an arm control pilot port  20 P 1 ,  20 P 2  and is configured to control the hydraulic fluid supplied to the third chamber CB 3  and the fourth chamber CB 4  of the arm cylinder  48  according to the hydraulic pressure of the pilot oil applied to the arm control pilot ports  20 P 1  and  20 P 2 . The arm control valve  20  is configured to switch to the arm cylinder static control position ANP, the arm cylinder extension control position AEP, or the arm cylinder contraction control position ASP by the pressure difference between the pilot pressure applied to the arm control pilot port  20 P 1  and the pilot pressure applied to the arm control pilot port  20 P 2 . The third oil passage R 3  further includes a partial oil passage R 32  connecting different connection ports of the arm control valve  20  to each other and a partial oil passage R 33  extending to the third chamber CB 3  of the arm cylinder  48 . The fourth oil passage R 4  connects the arm control valve  20  and the fourth chamber CB 4  of the arm cylinder  48 . The third oil passage R 3  and the fourth oil passage R 4  are branched by being connected to different ports of the bucket control valve  25 . 
     More specifically, the arm control valve  20  further includes a first arm control valve port XP 1 , a second arm control valve  20  port XP 2 , and a third arm control valve  20  port XP 3 . The first arm control valve  20  port XP 1  is connected to the first hydraulic pump  11  via a third oil passage R 3 . The second arm control valve  20  port XP 2  is connected to the fourth oil passage R 4 . The third arm control valve  20  port XP 3  is connected to the third chamber CB 3  of the arm cylinder  48  through the third oil passage R 3 . When the pilot pressure applied to the arm control pilot port  20 P 1  is larger than the pilot pressure applied to the arm control pilot port  20 P 2  than the predetermined pressure, the arm control valve  20  is switched to an arm cylinder extension control position AEP at which the first arm control valve port XP 1  communicates with the third arm control valve port XP 3  and the second arm control valve port XP 2  communicates with the first drain oil passage DR 1  (to be described later). When the pilot pressure applied to the arm control pilot port  20 P 2  is larger than the pilot pressure applied to the arm control pilot port  20 P 1  than the predetermined pressure. the arm control valve  20  is switched to an arm cylinder contraction control position ASP at which the first arm control valve port XP 1  communicates with the second arm control valve port XP 2  and the third arm control valve port XP 3  communicates with the first drain oil passage DR 1 . When the absolute value of the difference between the pilot pressure applied to the arm control pilot port  20 P 2  and the pilot pressure applied to the arm control pilot port  20 P 1  is equal to or less than the predetermined pressure, the arm control valve  20  is switched to the arm cylinder static control position ANP at which the first arm control valve port XP 1 , the second arm control valve port XP 2  and the third arm control valve port XP 3  are isolated. In addition, the hydraulic system  100  includes a first pressure control valve  76  to connect the first arm control valve port XP 1  to the second arm control valve port XP 2  or the third arm control valve port XP 3  only when the pressure applied to the first arm control valve port XP 1  is not less than a third threshold pressure. As a result, if the hydraulic pressure from the first hydraulic pump  11  does not reach the third threshold pressure, the arm control can be prevented from being performed. 
     First pilot oil passage PR 1  connects the arm control pilot ports  20 P 1  with the second hydraulic pump  12 . Specifically, the first pilot oil passage PR 1  includes a partial oil passage PR 10  common to the second pilot oil passage PR 2  extending from the second hydraulic pump  12  to the joint J 21 , a partial oil passage PR 11  extending from the joint J 21  to the arm pilot control valve  60 , a partial oil passage PR 12  extending from the arm pilot control valve  60  to the arm control pilot port  20 P 1 , and a partial oil passage PR 13  extending from the arm pilot control valve  60  to the arm control pilot port  20 P 2 . The hydraulic system  100  further includes an additional electromagnetic valve  14  provided in the partial oil passage PR 10 . That is, the additional electromagnetic valve  14  is provided in at least one of the first pilot oil passage PR 1  between the second hydraulic pump  12  and the arm pilot control valve  60  and the second pilot oil passage PR 2  between the second hydraulic pump  12  and the bucket pilot control valve  65 . The additional electromagnetic valve  14  is an electromagnetic valve capable of changing the pressure of the hydraulic fluid output from the second hydraulic pump  12 . The pressure of the hydraulic fluid can be changed by control from the controller  15 . 
     The arm pilot control valve  60  controls the hydraulic pressure of the pilot oil applied to the arm control pilot ports  20 P 1  and  20 P 2  based on an operation amount of an arm input member  63  which is provided in the first pilot oil passage PR 1  and is operated to raise and lower the arm  45 . Specifically, the arm pilot control valve  60  includes a pressure control valve  61  connected to the partial oil passage PR 12  and a pressure control valve  62  connected to the partial oil passage PR 12 . When the arm input member  63  is tilted forward, the pressure control valve  61  is operated so that the pilot pressure is output from the pressure control valve  61  to the partial oil passage PR 12 . This pilot pressure acts on the arm control pilot port  20 P 1  of the arm control valve  20 . When the arm input member  63  is tilted rearward, the pressure control valve  62  is operated so that pilot pressure is output from the pressure control valve  62  to the partial oil passage PR 13 . This pilot pressure acts on the arm control pilot port  20 P 2  of the arm control valve  20 . 
     More specifically, when the arm input member  63  is operated to move the arm  45  upward (i. e., the arm input member  63  is moved backward), the pilot pressure applied to the arm control pilot port  20 P 1  becomes larger than the pilot pressure applied to the arm control pilot port  20 P 2  than the predetermined pressure. When the arm input member  63  is operated to lower the arm  45  (operation to lower the arm input member  63  to the front side), the pilot pressure applied to the arm control pilot port  20 P 2  becomes larger than the pilot pressure applied to the arm control pilot port  20 P 1  than the predetermined pressure. When the operation amount of the arm input member  63  is smaller than the predetermined threshold amount, the absolute value of the difference between the pilot pressure applied to the arm control pilot port  20 P 2  and the pilot pressure applied to the arm control pilot port  20 P 1  is equal to or smaller than the predetermined pressure. The arm operation detection sensors AS 1  and AS 2  are configured to detect the arm input member  63 . The arm operation detection sensor AS 1  is, for example, a pressure switch connected to the partial oil passage PR 12 . The arm operation detection sensor AS 1  outputs a predetermined electric signal to the controller  15  when the pilot pressure of the partial oil passage PR 12  exceeds a predetermined pressure. The arm operation detection sensor AS 2  outputs a predetermined electric signal to the controller  15  when the pilot pressure of the partial oil passage PR 13  exceeds a predetermined pressure. 
     The second pilot oil passage PR 2  connects the bucket control pilot ports  25 P 1  and  25 P 2  to the second hydraulic pump  12 . Specifically, the second pilot oil passage PR 2  includes a partial oil passage PR 10  common to the first pilot oil passage PR 1  extending from the second hydraulic pump  12  to the joint J 21 , a partial oil passage PR 21  extending from the joint J 21  to the bucket pilot control valve  65 , a partial oil passage PR 22  extending from the bucket pilot control valve  65  to the bucket control pilot port  25 P 1 , and a partial oil passage PR 23  extending from the bucket pilot control valve  65  to the bucket control pilot port  25 P 2 . The bucket pilot control valve  65  is provided in the second pilot oil passage PR 2 , and controls the hydraulic pressure of the pilot oil applied to the bucket control pilot ports  25 P 1  and  25 P 2  based on the operation amount of the bucket input member  68  operated for swinging the bucket  41 . Specifically, the bucket pilot control valve  65  includes a pressure control valve  66  connected to the partial oil passage PR 22  and a pressure control valve  67  connected to the partial oil passage PR 23 . 
     When the bucket input member  68  is tilted to the right, the pressure control valve  66  is operated so that the pilot pressure is output from the pressure control valve  66  to the partial oil passage PR 22 . This pilot pressure acts on the bucket control pilot port  25 P 1  of the bucket control valve  25 . When the bucket input member  68  is tilted to the left, the pressure control valve  67  is operated to output the pilot pressure to the partial oil passage PR 23  from the pressure control valve  67 . This pilot pressure acts on the bucket control pilot port  25 P 2  of the bucket control valve  25 . 
     More specifically, when the bucket input member  68  performs an operation to tilt the bucket distal end  41 D downward (an operation to tilt the bucket input member  68  to the right), the pilot pressure applied to the bucket control pilot port  25 P 1  becomes larger than the pilot pressure applied to the bucket control pilot port  25 P 2  than the predetermined pressure. When the bucket input member  68  performs an operation to tilt the bucket distal end  41 D upward (an operation to tilt the bucket input member  68  to the left), the pilot pressure applied to the bucket control pilot port  25 P 2  becomes larger than the pilot pressure applied to the bucket control pilot port  25 P 1  than the predetermined pressure. When the operation amount of the bucket input member  68  is smaller than a predetermined threshold amount, the absolute value of the difference between the pilot pressure applied to the bucket control pilot port  25 P 2  and the pilot pressure applied to the bucket control pilot port  25 P 1  is equal to or smaller than the predetermined pressure. 
     Although the arm input member  63  and the bucket input member  68  are shown as separate members in  FIG.  3   , they may be the same members. In this case, the pressure control valves  61 ,  62 ,  66  and  67  are arranged around the same member. The bucket operation detection sensor BS 1  outputs a predetermined electric signal to the controller  15  when the pilot pressure of the partial oil passage PR 22  exceeds a predetermined pressure. The bucket operation detection sensor BS 2  outputs a predetermined electric signal to the controller  15  when the pilot pressure of the partial oil passage PR 23  exceeds a predetermined pressure. 
     The bypass oil passage BR 1  connects the fourth oil passage R 4  and the first oil passage R 1 . The bypass oil passage BR 1  is connected to the fourth oil passage R 4  at the first joint J 1 . The bypass oil passage BR 1  is connected to the first oil passage R 1  at a third joint J 3 . The switching mechanism  70  is configured to control connection of the fourth oil passage R 4  to the first oil passage R 1  via the bypass oil passage BR 1  and disconnection of the fourth oil passage R 4  from the first oil passage R 1  via the bypass oil passage BR 1 . A controller  15  is configured to determine whether or not an uptilt operation to raise an arm  45  and to tilt a bucket  41  downward has been performed based on the outputs of arm operation detection sensors AS 1 , AS 2  and the outputs of bucket operation detection sensors BS 1 , BS 2 , and to control a switching mechanism  70  so as to connect a fourth oil passage R 4  and a first oil passage R 1  when it is determined that the uptilt operation has been performed. The controller  15  is, for example, a hardware controller such as an ECU. Therefore, the controller  15  includes an electronic circuit (circuitry) such as a hardware processor and a memory. 
     The switching mechanism  70  includes the electromagnetic valve  71  and the first switching valve  72 . The first switching valve  72  is provided in the fourth oil passage R 4  between the first joint J 1  and the arm control valve  20 , and includes a first pilot port PP 1 . The first switching valve  72  is configured so as to cut off the connection of the arm control valve  20  to the fourth chamber CB 4  of the arm cylinder  48  via the fourth oil passage R 4  when a pilot oil pressure equal to or greater than a first threshold pressure is applied to the first pilot port PP 1 , and to connect the arm control valve  20  and the fourth chamber CB 4  via the fourth oil passage R 4  when a pilot oil pressure smaller than the first threshold pressure is applied to the first pilot port PP 1 . The electromagnetic valve  71  is switchable between a second position VP 2  at which the first pilot port PP 1  communicates with the hydraulic fluid tank  10  and a first position VP 1  at which the first pilot port PP 1  communicates with an oil passage capable of applying a pilot pressure equal to or greater than a first threshold pressure to the first pilot port PP 1 . 
     A controller  15  is connected to a solenoid of the electromagnetic valve  71 . For example, when the pilot pressure of a partial oil passage PR 12  obtained from an arm operation detection sensor AS 1  is larger than the pilot pressure of a partial oil passage PR 13  obtained from an arm operation detection sensor AS 1  than a first threshold pressure and the pilot pressure of a partial oil passage PR 22  obtained from a bucket operation detection sensor BS 1  is larger than the pilot pressure of a partial oil passage PR 23  obtained from a bucket operation detection sensor BS 2 , the controller  15  determines that the uptilt operation has been performed and transmits to the electromagnetic valve  71  an electric signal for switching the electromagnetic valve  71  to a first position VP 1 . 
     Further, the work machine further includes an ON/OFF switch  16  connected to the controller  15 . The switch  16  is turned ON, and then, the controller  15  transmits an electric signal for switching the electromagnetic valve  71  to the first position VP 1  to the electromagnetic valve  71 . The switch  16  is turned ON when horizontal control is performed in which the tilt angle of the bucket distal end  41 D is changed corresponding to the elevation angle of the arm  45  with respect to the vehicle body  2  so that the direction from the bucket pivot shaft  43  toward the bucket distal end  41 D is maintained substantially parallel to the horizontal plane. 
     The work machine  1  further includes an additional bypass oil passage BR 2  which connects a fourth oil passage R 4  between the first switching valve  72  and the arm control valve  20  and the second oil passage R 2 . The additional bypass oil passage BR 2  is connected to the fourth oil passage R 4  at the second joint J 2  and is connected to the second oil passage R 2  at the fourth joint J 4 . The switching mechanism  70  further includes a second switching valve  73  provided in the bypass oil passage BR 1  and a third switching valve  74  provided in the additional bypass oil passage BR 2 . The bypass oil passage BR 1  includes a partial oil passage BR 10  from the first joint J 1  through the second switching valve  73 , and an oil passage BR 11  from the second switching valve  73  through the third joint J 3 . The additional bypass oil passage BR 2  includes a partial oil passage BR 20  from the second joint J 2  to the third switching valve  74  and a partial oil passage BR 21  from the third switching valve  74  to the fourth joint J 4 . The work machine  1  includes a first connection passage CR 1  connecting an additional bypass oil passage BR 2  (partial oil passage BR 20 ) between a second joint J 2  and a third switching valve  74  with f the second switching valve  73 , the second connection passage CR 2  connecting a bypass oil passage BR 1  (partial oil passage BR 11 ) between a third joint J 3  and the second switching valve  73  with a third switching valve  74 , and a third connection passage CR 3  connecting a bypass oil passage BR 1  (partial oil passage BR 11 ) between the second connection passage CR 2  and the second switching valve  73  with the second switching valve  73 . 
     The second switching valve  73  is configured to be switched to a first communication position CP 1  connected the first joint J 1  with the third joint J 3  when the first hydraulic pressure of the bypass oil passage BR 1  between the first joint J 1  and the second switching valve  73  is higher than the second hydraulic pressure of the first connection passage CR 1 , and to be switched to a first cut-off position BP 1  at which communication between the first joint J 1  and the third joint J 3  is cut off a when the second hydraulic pressure is equal to or lower than the first hydraulic pressure. When the second switching valve  73  is switched to the first cut-off position BP 1 , the second switching valve  73  is configured to connect the partial oil passage BR 10  and the first connection passage CR 1 . The second switching valve  73  is configured to cut-off the partial oil passage BR 10  and the first connection passage CR 1  when it is switched to the first communication position CP 1 . 
     The third switching valve  74  switches to any one of a second cut-off position BP 2 , the 21st communication position CP 21 , or a 22nd communication position CP 22 , based on a third hydraulic pressure of an additional bypass oil passage BR 2  (partial oil passage BR 20 ) between the second joint J 2  and the third switching valve  74 , and the fourth hydraulic pressure of the second connection passage CR 2 . The 21st communication position CP 21  and the 22nd communication position CP 22  are collectively referred to as a second communication position CP 2 . Since the check valve CKV is provided between the second connection passage CR 2  and the third joint J 3 , the hydraulic pressure in the first oil passage R 1  does not affect the hydraulic pressure in the second connection passage CR 2 . The third switching valve  74  is configured to switch to a second communication position CP 2  at which the second joint J 2  and the fourth joint J 4  communicate with each other when the third hydraulic pressure is lower than the fourth hydraulic pressure, and to switch to a second cut-off position BP 2  at which the communication between the second joint J 2  and the fourth joint J 4  is blocked when the fourth hydraulic pressure is equal to or lower than the third hydraulic pressure. When the difference between the fourth hydraulic pressure and the third hydraulic pressure is smaller than a predetermined pressure, the third switching valve  74  switches to the 21st communication position CP 21  to establish communication between the second joint J 2  and the fourth joint J 4 , and cut off the connection of the third connection passage CR 3  from the partial oil passage BR 11 . On the other hand, when the third hydraulic pressure is lower than the fourth hydraulic pressure by a predetermined pressure or more, the third switching valve  74  switches to the 21st communication position CP 21  to establish communication between the second joint J 2  and the fourth joint J 4  and to connect the third connection passage CR 3  and the bypass oil passage BR 1  (partial oil passage BR 11 ). As a result, the hydraulic pressure in the bypass oil passage BR 1  (partial oil passage BR 11 ) can be prevented from becoming too high. 
     Furthermore, the work machine  1  is further provided with a first drain oil passage DR 1  which connects the fourth chamber CB 4  of the arm cylinder  48  to the hydraulic fluid tank  10  when the arm control valve  20  is connected to the fourth chamber CB 4  of the arm cylinder  48  via the fourth oil passage R 4  by the first switching valve  72 . The switching mechanism  70  is further provided with a first counter balance valve BV 1  provided in the first drain oil passage DR 1 , a fourth switching valve  75 , a third pilot oil passage PR 3  connected to the fourth switching valve  75  and the first pilot port PP 1  of the first switching valve  72 , a fourth pilot oil passage (PR 4 ) connected to the fourth switching valve  75  and the first drain oil passage DR 1  between the first switching valve  72  and the first counter balance valve BV 1 , and a second drain oil passage DR 2  connected to the fourth switching valve  75  and the hydraulic fluid tank  10 . First counter balance valve BV 1  is a pressure control valve configured to be opened when the pressure of the joint J 8  shown in the figure becomes higher than that of the joint J 9  by a predetermined pressure or more, so that the hydraulic fluid flows from the joint J 8  to the joint J 9 . The first threshold pressure is lower than a predetermined pressure at which the first counter balance valve BV 1  opens. The similar counter balance valves BV 2  to BV 4  are connected to fourth to sixth drain oil passages DR 4  to DR 6  connected to the third oil passage R 3 , the first oil passage R 1 , and the second oil passage R 2 , respectively. The fourth to sixth drain oil passages DR 4  to DR 6  are connected to the first drain oil passage DR 1 . 
     The fourth switching valve  75  includes a first connection port ZP 1  connected to the third pilot oil passage PR 3 , a second connection port ZP 2  connected to a fourth pilot oil passage PR 4 , a third connection port ZP 3  connected to a second drain oil passage DR 2 , and a switching valve pilot port  75 P to which pilot pressure is applied by an electromagnetic valve  71 . When the pilot oil pressure equal to or higher than the second threshold pressure is applied to the switching valve pilot port  75 P, the fourth switching valve  75  is switched to a drain position DP at which the first connection port ZP 1 , the second connection port ZP 2 , and the third connection port ZP 3  communicate with each other. When a pilot oil pressure smaller than a second threshold pressure is applied to the switching valve pilot port  75 P, the fourth switching valve  75  is switched to a pressure increasing position UP where the first connection port ZP 1  and the second connection port ZP 2  communicate with each other and the third connection port ZP 3  is disconnected from the first connection port ZP 1  and the second connection port ZP 2 . 
     When the electromagnetic valve  71  is switched to the second position VP 2  by the controller  15 , a pilot oil pressure equal to or higher than the second threshold pressure is applied to the switching valve pilot port  75 P. Therefore, the fourth switching valve  75  is switched to the drain position DP, and the pilot pressure applied to the first pilot port PP 1  becomes lower than the first threshold pressure. At this time, the first switching valve  72  opens to connect the arm control valve  20  with the fourth chamber CB 4  via the fourth oil passage R 4 , and the fourth oil passage R 4  is connected to the first drain oil passage DR 1  to the first counter balance valve BV 1 . 
     At this time, when the arm input member  63  is tilted to the front side, the pilot pressure applied to the arm control pilot port  20 P 1  becomes larger than the pilot pressure applied to the arm control pilot port  20 P 2  than the predetermined pressure, so that the arm control valve  20  is switched to the arm cylinder extension control position AEP. Therefore, the hydraulic fluid having a pressure exceeding the third threshold pressure from the first hydraulic pump  11  is sent to the third chamber CB 3  of the arm cylinder  48  via the arm control valve  20 . As a result, the hydraulic fluid is output from the fourth chamber CB 4  of the arm cylinder  48  to the fourth oil passage R 4 . Since the first switching valve  72  is opened, the hydraulic fluid is sent to the second arm control valve port XP 2  of the arm control valve  20  and is discharged to the third drain oil passage DR 3  connected to the first drain oil passage DR 1 . 
     Further, when the arm input member  63  is tilted backward, the pilot pressure applied to the arm control pilot port  20 P 2  becomes larger than the pilot pressure applied to the arm control pilot port  20 P 1  than the predetermined pressure, so that the arm control valve  20  is switched to the arm cylinder contraction control position ASP. Therefore, the hydraulic fluid having a pressure exceeding the third threshold pressure from the first hydraulic pump  11  is sent to the fourth chamber CB 4  of the arm cylinder  48  via the arm control valve  20  and the first switching valve  72 . As a result, the hydraulic fluid is output from the third chamber CB 3  of the arm cylinder  48  to the third oil passage R 3 . The hydraulic fluid is sent to the third arm control valve  20  port XP 3  of the arm control valve  20  and is discharged to a third drain oil passage DR 3  connected to the first drain oil passage DR 1 . 
     When the electromagnetic valve  71  is switched to the second position VP 2  by the controller  15 , the control of the bucket  41  is performed independently of the control of the arm  45  except for the uptilt operation. In this state, when the bucket input member  68  is tilted to the right, the pilot pressure applied to the bucket control pilot port  25 P 1  becomes larger than the pilot pressure applied to the bucket control pilot port  25 P 2  than the predetermined pressure, so that the bucket control valve  25  is switched to the bucket cylinder extension control position BEP. Therefore, the hydraulic fluid having a pressure exceeding the fourth threshold pressure from the first hydraulic pump  11  is sent to the first chamber CB 1  of the bucket cylinder  49  via the bucket control valve  25 . As a result, the hydraulic fluid is output from the second chamber CB 2  of the bucket cylinder  49  to the second oil passage R 2 . The hydraulic fluid is sent to the third bucket control valve port YP 3  of the bucket control valve  25  and is discharged to the seventh drain oil passage DR 7  connected to the first drain oil passage DR 1 . 
     When the bucket input member  68  is tilted to the left, the pilot pressure applied to the bucket control pilot port  25 P 2  becomes larger than the pilot pressure applied to the bucket control pilot port  25 P 1  than the predetermined pressure, so that the bucket control valve  25  is switched to the bucket cylinder contraction control position BSP. Therefore, the hydraulic fluid having a pressure exceeding the fourth threshold pressure from the first hydraulic pump  11  is sent to the second chamber CB 2  of the bucket cylinder  49  via the bucket control valve  25 . As a result, the hydraulic fluid is output from the first chamber CB 1  of the bucket cylinder  49  to the first oil passage R 1 . The hydraulic fluid is sent to the second bucket control valve port YP 2  of the bucket control valve  25  and is discharged to the seventh drain oil passage DR 7  connected to the first drain oil passage DR 1 . 
     When the electromagnetic valve  71  is switched to the first position VP 1  by the controller  15  after the electromagnetic valve  71  is switched to the second position VP 2  by the controller  15 , a pilot oil pressure smaller than the second threshold pressure is applied to the switching valve pilot port  75 P, and the fourth switching valve  75  is switched to the pressure increasing position UP. Although the pilot pressure applied to the first pilot port PP 1  is lower than the first threshold pressure at the beginning of the switching, the pilot pressure rises immediately due to the pressure of the hydraulic fluid flowing from the fourth oil passage R 4 . Since the predetermined pressure at which the first counter balance valve opens is higher than the first threshold pressure, the pilot pressure exceeds the first threshold pressure, and the first switching valve  72  is closed. Therefore, the connection of the arm control valve  20  to the fourth chamber CB 4  of the arm cylinder  48  via the fourth oil passage R 4  is cut off. 
     When the first switching valve  72  is closed, the arm control valve  20  is switched to the arm cylinder extension control position AEP, and the bucket control valve  25  is switched to the bucket cylinder static control position BNP when the arm  45  is being operated to raise (the arm input member  63  is tilted to the rear side) and the bucket  41  is not being operated at all (the bucket input member  68  is not tilted to any place). When the arm input member  63  and the bucket input member  68  are the same as each other, the same input member is moved backward without moving left and right. 
     At this time, as described above, the hydraulic fluid flows out from the fourth chamber CB 4  of the arm cylinder  48 , but flows from the first joint J 1  to the second switching valve  73  because the first switching valve  72  is closed. Further, since the additional bypass oil passage BR 2  and the first connection passage CR 1  connected thereto are connected to the third drain oil passage DR 3 , the first hydraulic pressure of the bypass oil passage BR 1  (partial oil passage BR 10 ) between the first joint J 1  and the second switching valve  73  is greater than the second hydraulic pressure. 
     As a result, the second switching valve  73  is switched to the first communication position CP 1 . At this time, the hydraulic fluid is sent from the second switching valve  73  to the first chamber CB 1  of the bucket cylinder  49  via the bypass oil passage BR 1  (partial oil passage BR 11 ) and the first oil passage R 1  (partial oil passage R 13 ). At this time, the hydraulic fluid is output from the second chamber CB 2  of the bucket cylinder  49  to the second oil passage R 2  and the additional bypass oil passage BR 2  (partial oil passage BR 21 ). Since the third switching valve  74  is located at the second cut-off position BP 2 , the hydraulic fluid cannot flow and the hydraulic pressure in the second connection passage CR 2  rises. When the difference between the fourth hydraulic pressure in the second connection passage CR 2  and the third hydraulic pressure in the additional bypass oil passage BR 2  (partial oil passage BR 20 ) exceeds the connection threshold pressure, switching to either the 21st communication position CP 21  or the 22nd communication position CP 22  is performed based on the difference between the third hydraulic pressure in the additional bypass oil passage BR 2  (partial oil passage BR 20 ) and the fourth hydraulic pressure in the second connection passage CR 2 . When the difference between the fourth hydraulic pressure and the third hydraulic pressure is smaller than a predetermined switching threshold value, the third switching valve  74  is switched to the 21st communication position CP 21 . When the difference between the fourth hydraulic pressure and the third hydraulic pressure is equal to or greater than a predetermined switching threshold value, the third switching valve  74  is switched to the 22nd communication position CP 22 . When the third switching valve  74  is switched to the 21st communication position CP 21  or the 22nd communication position CP 22 , the hydraulic fluid output from the second chamber CB 2  of the bucket cylinder  49  is discharged to the hydraulic fluid tank  10  via the additional bypass oil passage BR 2  and the third drain oil passage DR 3 . 
     However, when the hydraulic fluid is discharged from the fourth chamber CB 4  of the arm cylinder  48 , the second hydraulic pressure of the first connection passage CR 1  rise, and the second switching valve  73  is switched to the first cut-off position BP 1 , the hydraulic fluid flowing out from the fourth chamber CB 4  is discharged to the hydraulic fluid tank  10  via the first connection passage CR 1 , the additional bypass oil passage BR 2 , and the third drain oil passage DR 3 . Therefore a portion of the oil discharged from the fourth chamber CB 4  of the arm cylinder  48  flows into the first chamber CB 1  of the bucket cylinder  49  by flowing to the third chamber CB 3  of the arm cylinder  48 , and the remainder is discharged into the hydraulic fluid tank  10 . The amount discharged into the hydraulic fluid tank  10  is adjusted by empirically adjusting the size of the throttle of the first cut-off position BP 1  of the second switching valve  73  so that the change in the elevation angle of the arm  45  with respect to the vehicle body  2  per unit amount of the hydraulic fluid is substantially equal to the change in the swing angle with respect to the arm  45 . Therefore, the swing angle of the bucket distal end  41 D with respect to the arm  45  is changed in accordance with the elevation angle of the arm to control so that the direction from the bucket pivot shaft  43  toward the bucket distal end is directed generally in the horizontal direction. Such control is referred to as horizontal control. 
     When the first switching valve  72  is closed, the arm control valve  20  is switched to the arm cylinder contraction control position ASP and the bucket control valve  25  is switched to the bucket cylinder static control position BNP when the arm  45  is being lowered (the arm input member  63  is being tilted forward) and the bucket  41  is not being operated (the bucket input member  68  is not being tilted anywhere). At this time, the hydraulic fluid having a pressure exceeding the third threshold pressure from the first hydraulic pump  11  is output from the second arm control valve  20  port XP 2 . Since the third threshold pressure is larger than the first threshold pressure and the threshold pressure for opening the first counter balance valve BV 1 , the hydraulic fluid is sent to the fourth chamber CB 4  of the arm cylinder  48  through the fourth oil passage R 4  when the first switching valve  72  is pushed to open. At this time, the hydraulic fluid is discharged from the third chamber CB 3  of the arm cylinder  48  to the hydraulic fluid tank  10  through the third oil passage R 3  and the third drain oil passage DR 3 . 
     When the electromagnetic valve  71  is switched to the first position VP 1  by the controller  15 , the control of the bucket  41  is performed independently of the control of the arm  45  except for the uptilt operation. These methods have been described above and will be omitted. Next, the uptilt operation will be described. In the uptilt operation, the arm  45  is raised (the arm input member  63  is lowered to the front side) and the bucket  41  is tilted downward (the bucket input member  68  is lowered to the right side), which is referred to as the uptilt operation. When the arm input member  63  and the bucket input member  68  are the same, the same input member is tilted to the right and front side. This operation can be detected from the outputs of the arm operation detection sensors AS 1  and AS 2  and the outputs of the bucket operation detection sensors BS 1  and BS 2 . 
     For example, it can be determined that the uptilt operation has been performed when the pilot pressure of the partial oil passage PR 12  detected by the arm operation detection sensor AS 1  is larger than a predetermined pressure (the arm operation detection sensor AS 1  transmits a predetermined signal), and the pilot pressure of the partial oil passage PR 12  detected by the arm operation detection sensor AS 2  is smaller than the predetermined pressure (the arm operation detection sensor AS 2  does not transmit the predetermined signal), and the pilot pressure of the partial oil passage PR 22  detected by the bucket operation detection sensor BS 1  is larger than the predetermined pressure (the bucket operation detection sensor BS 1  transmits a predetermined signal), and the pilot pressure of the partial oil passage PR 13  detected by the bucket operation detection sensor BS 2  is smaller than the predetermined pressure (the bucket operation detection sensor BS 1  does not transmit the predetermined signal). The controller  15  acquires the outputs of the arm operation detection sensors AS 1 , AS 2  and the outputs of the bucket operation detection sensors BS 1 , BS 2 , and determines whether or not the uptilt operation has been performed. When it is determined that the uptilt operation has been performed, the controller  15  transmits to the electromagnetic valve  71  an electric signal for switching the electromagnetic valve  71  to the first position VP 1  regardless of the setting of the ON/OFF switch  16 . 
     When the uptilt operation is performed, the arm control valve  20  is switched to the arm cylinder extension control position AEP, and the bucket control valve  25  is switched to the bucket cylinder extension control position BEP. At this time, as in the case of the horizontal control, the hydraulic fluid flowing out from the fourth chamber CB 4  of the arm cylinder  48  is sent to the first chamber CB 1  of the bucket cylinder  49 . Further, the hydraulic fluid having a pressure exceeding the fourth threshold pressure from the first hydraulic pump  11  is sent to the first chamber CB 1  of the bucket cylinder  49  via the bucket control valve  25 . Therefore, the tilt speed of the bucket  41  is increased, and the response of the uptilt operation is improved. The hydraulic fluid output from the second chamber CB 2  of the bucket cylinder  49  is discharged to the seventh drain oil passage DR 7  as described above. 
     &lt;Control Method of Work machine  1 &gt; 
     Next, a method of controlling the work machine  1  according to the present embodiment will be described.  FIG.  4    is a flowchart showing a method of controlling the work machine  1 . In step S 1 , the controller  15  of the work machine  1  detects the operation of the arm input member  63  which is operated to raise and lower the arm  45 . Specifically, based on the outputs from the arm operation detection sensors AS 1  and AS 2 , the controller  15  detects an operation to raise the arm  45  (first operation), an operation of lowering the arm  45 , and an operation of bringing the arm  45  to rest. For example, the controller  15  receives a predetermined signal from the arm operation detection sensor AS 1  and does not receive the predetermined signal from the arm operation detection sensor AS 2 , whereby the controller  15  can detect an operation to raise the arm  45 . When the controller  15  receives a predetermined signal from the arm operation detection sensor AS 2  and does not receive the predetermined signal from the arm operation detection sensor AS 1 , the controller  15  can detect the operation of lowering the arm  45 . When the controller  15  does not receive the predetermined signal from the arm operation detection sensor AS 1  and does not receive the predetermined signal from the arm operation detection sensor AS 2 , the controller  15  can detect the operation of stopping the arm  45 . 
     When the operation to raise the arm  45  is detected in step S 1 , in step S 2 , the work machine  1  sends the hydraulic fluid from the first hydraulic pump  11  to the third chamber CB 3  of the arm cylinder  48  via the third oil passage R 3  to raise (move upward) the arm distal end  45 D of the arm  45  which swingably supports the bucket proximal end  41 P opposite to the bucket distal end  41 D of the bucket  41 . When an operation to lower an arm  45  is detected in step S 1 , in step S 3 , the work machine  1  uses an arm control valve  20  provided in a third oil passage R 3  to establish communication between a third oil passage R 3  from a first hydraulic pump  11  to the arm control valve  20  and a fourth oil passage R 4  connecting the arm control valve  20  and a fourth chamber CB 4  of an arm cylinder  48 , send hydraulic fluid from the first hydraulic pump  11  to the fourth chamber CB 4  of the arm cylinder  48 , and lower (move downward) an arm distal end  45 D. When an operation for bringing the arm  45  to a standstill is detected in step S 1 , in step S 4 , the work machine  1  switches the arm control valve  20  to the arm cylinder static control position ANP so as to stabilize the arm  45 . 
     In step S 5 , the controller  15  of the work machine  1  detects the operation of the bucket input member  68  to be operated for swinging the bucket  41 . Specifically, based on the outputs from the bucket operation detection sensors BS 1  and BS 2 , the controller  15  detects an operation to tilt the bucket  41  upward and to tilt the bucket  41  downward (second operation) and an operation not to tilt the bucket  41  upward or downward (third operation). For example, when the controller  15  receives a predetermined signal from the bucket operation detection sensor BS 2  and does not receive the predetermined signal from the bucket operation detection sensor BS 1 , the controller  15  detects an operation to tilt the bucket  41  upward. When the controller  15  receives a predetermined signal from the bucket operation detection sensor BS 1  and does not receive the predetermined signal from the bucket operation detection sensor BS 2 , the controller  15  can detect an operation (second operation) to tilt the bucket  41  downward. When the controller  15  does not receive a predetermined signal from the bucket operation detection sensor BS 1  and does not receive the predetermined signal from the bucket operation detection sensor BS 2 , the controller  15  can detect an operation (third operation) in which the bucket  41  is not tilted upward or downward. 
     When an operation to tilt the bucket  41  upward is detected in step S 5 , in step S 6 , the work machine  1  uses the bucket control valve  25  provided in the first oil passage R 1  to communicate the first oil passage R 1  from the first hydraulic pump  11  to the bucket control valve  25  with the second oil passage R 2  that connects the bucket control valve  25  and the second chamber CB 2  of the bucket cylinder  49 , and sends hydraulic fluid from the first hydraulic pump  11  to the second chamber CB 2  of the bucket cylinder  49  to tilt the bucket distal end  41 D upward. When an operation (second operation) to tilt the bucket  41  downward is detected in step S 5 , in step S 7 , the controller  15  determines whether or not the first operation is detected in step S 1 . If it is determined that the first operation has not been detected (NO in step S 7 ), in step S 8 , the work machine  1  sends hydraulic fluid from the first hydraulic pump  11  to the first chamber CB 1  of the bucket cylinder  49  via the first oil passage R 1  tilt the bucket distal end  41 D of the bucket  41  downward. When the first operation and the second operation are detected (YES in step S 7 ), in step S 9 , the work machine  1  sends the hydraulic fluid to the first chamber CB 1  via the first oil passage R 1 , and connects the fourth oil passage R 4  with the first oil passage R 1  via the bypass oil passage BR 1  by the switching mechanism  70  to send the hydraulic fluid in the fourth oil passage R 4  to the first oil passage R 1 . 
     If the third operation is detected in step S 5 , then in step S 10 , the work machine  1  detects whether or not the ON/OFF switch  16  is set to ON to detect whether or not the horizontal control is instructed. Further, the work machine  1  determines whether or not the first operation is detected in step S 1 . When the horizontal control instruction is not detected, or the first operation is not detected (No in step S 10 ), in step S 11 , the work machine  1  switches the bucket control valve  25  to the bucket cylinder static control position BNP so as to stabilize the bucket  41 . When the horizontal control is instructed and the first operation and the third operation are detected (YES in step S 10 ), in step S 12 , the work machine  1  cuts the first oil passage R 1  by the bucket control valve  25 , connects the fourth oil passage R 4  and the first oil passage R 1  by the switching mechanism  70 , and sends the hydraulic fluid in the fourth chamber CB 4  to the first chamber CB 1 . 
     Effect of the Embodiment 
     A controller  15  of a work machine  1  according to this embodiment is configured to determine whether or not the uptilt operation to raise an arm  45  and to tilt a bucket  41  downward has been performed based on the outputs of arm operation detection sensors AS 1 , AS 2  and the outputs of bucket operation detection sensors BS 1 , BS 2 , and to control a switching mechanism  70  so as to connect a fourth oil passage R 4  and a first oil passage R 1  when it is determined that the uptilt operation has been performed. When a first operation to raise an arm  45  and a second operation to tilt a bucket  41  downward are detected, a method for controlling a work machine  1  sends hydraulic fluid to a first chamber CB 1  via a first oil passage R 1 , connects a fourth oil passage R 4  and the first oil passage R 1  by a switching mechanism  70 , and sends hydraulic fluid from the fourth chamber CB 4  to the first chamber CB 1 . As a result, it is possible to provide the work machine  1  and the method of controlling the work machine  1  which can realize the same degree of operability in the uptilt operation regardless of whether the horizontal control is enabled or not. 
     As used herein, “comprising” and its derivatives are non-limiting terms that describe the presence of a component, and do not exclude the presence of other components not described. This also applies to “having”, “including” and their derivatives. 
     The terms “member,” “part,” “element,” “body,” and “structure” may have multiple meanings, such as a single part or multiple parts. 
     Ordinal numbers such as “first” and “second” are simply terms used to identify configurations and do not have other meanings (e.g., a particular order). For example, the presence of the “first element” does not imply the presence of the “second element”, and the presence of the “second element” does not imply the presence of the “first element”. 
     Terms such as “substantially”, “about”, and “approximately” indicating degrees can mean reasonable deviations such that the final result is not significantly altered, unless otherwise stated in the embodiments. All numerical values described herein may be interpreted to include words such as “substantially,” “about,” and “approximately.” 
     In the present application, the phrase “at least one of A and B” should be interpreted to include only A, only B, and both A and B. 
     In view of the above disclosure, it will be apparent that various changes and modifications of the present invention are possible. Therefore, the present invention may be carried out by a method different from the specific disclosure of the present application without departing from the spirit of the present invention.