Patent Publication Number: US-2023145883-A1

Title: Work machine and method for controlling work machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2021/023146, filed on Jun. 18, 2021. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2020-118864, filed in Japan on Jul. 10, 2020, the entire contents of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a work machine and a method for controlling the work machine. 
     BACKGROUND ART 
     Among work machines, a so-called articulated vehicle is known in which a front frame and a rear frame are turnably connected to each other as described in U.S. Ser. No. 10/215,119. Such a work machine includes a steering cylinder, a hydraulic pump, and an engine. The steering cylinder is connected to the front frame and the rear frame. The hydraulic pump is driven by the engine to discharge hydraulic fluid. Hydraulic fluid discharged from the hydraulic pump is supplied to the steering cylinder. The steering cylinder extends and contracts thereby turning the front frame with respect to the rear frame. Consequently, the front frame bends with respect to the rear frame and the work machine turns. 
     SUMMARY 
     In the above-mentioned work machine of the prior art, the operator operates an accelerator pedal whereby the rotation speed of the engine is controlled. However, the flow rate of the hydraulic fluid supplied from the hydraulic pump to the steering cylinder changes in accordance with the rotation speed of the engine. The flow rate of hydraulic fluid supplied to the steering cylinder signifies the volume of hydraulic fluid supplied to the steering cylinder per unit of time. 
     When the rotation speed of the engine is low, the flow rate of hydraulic fluid supplied to the steering cylinder decreases. As a result, the displacement of the hydraulic pump is designed to take into consideration a situation when the rotation speed of the engine is lowest in order to assure the minimum necessary flow rate of hydraulic fluid for the work machine to turn. Therefore, the size of the hydraulic pump is set to be on the large side with room for margin. However, in this case, when the rotation speed of the engine is high, the hydraulic pump may discharge hydraulic fluid at a flow rate that is greater than necessary. As a result, there is a problem that fuel consumption deteriorates. 
     In addition, when the engine rotation speed is low, there is a problem that the followability of the bending motion of the work machine with respect to the steering operation by the operator is poor. For example, when the operator has quickly performed a steering operation, the phenomenon that the bending motion is slow is reflected in the operational feel of the operator because the discharge flow rate of the hydraulic pump is insufficient. 
     An object of the present invention is to improve the followability of a bending motion of a work machine with respect to a steering operation. 
     A work machine according to a first aspect includes a first frame, a second frame, a steering cylinder, a hydraulic pump, an engine, a steering operating member, a steering operation sensor, and a controller. The second frame is turnably connected to the first frame. The steering cylinder is connected to the second frame and the first frame. The steering cylinder causes the second frame to turn with respect to the first frame. The hydraulic pump supplies hydraulic fluid to the steering cylinder. The engine drives the hydraulic pump. The steering operating member is operable by an operator. The steering operation sensor outputs a steering command signal corresponding to the operation of the steering operating member. The controller controls the flow rate of hydraulic fluid supplied from the hydraulic pump to the steering cylinder by controlling the rotation speed of the engine in accordance with the steering command signal. 
     A method according to a second aspect is a method for controlling a work machine, the work machine including a first frame, a second frame, a steering cylinder, a hydraulic pump, and an engine. The second frame is turnably connected to the first frame. The steering cylinder is connected to the second frame and the first frame. The steering cylinder causes the second frame to turn with respect to the first frame. The hydraulic pump supplies hydraulic fluid to the steering cylinder. The engine drives the hydraulic pump. The method according to the present aspect includes the following processes. A first process is acquiring a steering command signal corresponding to a steering operating member that is operable by an operator. A second process is controlling the flow rate of hydraulic fluid supplied from the hydraulic pump to the steering cylinder by controlling the rotation speed of the engine in accordance with the steering command signal. 
     According to the present disclosure, the rotation speed of the engine is controlled in response to a steering command signal corresponding to the operation of the steering operating member. As a result, the flow rate of hydraulic fluid supplied from the hydraulic pump to the steering cylinder is controlled in accordance with the operation of the steering operating member. Consequently, the followability of a bending motion of the work machine with respect to a steering operation can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a side view of a work machine according to an embodiment. 
         FIG.  2    is a schematic view of a control system of the work machine. 
         FIG.  3    is a diagram illustrating the articulate angle of the work machine. 
         FIG.  4    is a flow chart illustrating processing executed by a controller. 
         FIG.  5    is a diagram illustrating an example of target articulate data. 
         FIG.  6    is a diagram illustrating an example of required flow rate data. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following is a description of a work machine according to an embodiment with reference to the drawings.  FIG.  1    is a side view of a work machine  1  according to the embodiment. The work machine  1  according to the present embodiment is a wheel loader. The wheel loader  1  includes a vehicle body frame  2 , a work implement  3 , a pair of front tires  4 , a cab  5 , an engine compartment  6 , and a pair of rear tires  7 . In the following explanations, “front,” “rear,” “right,” “left,” “up,” and “down” indicate directions relative to a state of looking forward from an operator&#39;s seat within the cab  5 . 
     The vehicle body frame  2  includes a front frame  11 , a rear frame  12 , and a pivot joint  13 . The front frame  11  is disposed in front of the rear frame  12 . The pivot joint  13  is disposed in the center in the left-right direction of the work machine  1 . The pivot joint  13  turnably couples the front frame  11  and the rear frame  12 . The pair of front tires  4  are attached to the front frame  11 . The pair of rear tires  7  are attached to the rear frame  12 . 
     The work implement  3  includes a boom  14 , a bucket  15 , a lift cylinder  16 , and a bucket cylinder  17 . The boom  14  is mounted to the front frame  11 . The bucket  15  is attached to the tip of the boom  14 . 
     The lift cylinder  16  and the bucket cylinder  17  are hydraulic cylinders. One end of the lift cylinder  16  is attached to the front frame  11  and the other end of the lift cylinder  16  is attached to the boom  14 . The boom  14  swings up and down due to the extension and contraction of the lift cylinder  16 . One end of the bucket cylinder  17  is attached to the front frame  11  and the other end of the bucket cylinder  17  is attached to the bucket  15  via a bell crank  18 . The bucket  15  swings up and down due to the extension and contraction of the bucket cylinder  17 . 
     The cab  5  is disposed on the rear frame  12 . The engine compartment  6  is disposed behind the cab  5 . The engine compartment  6  is disposed on the rear frame  12 . 
       FIG.  2    is a schematic view of a control system of the work machine  1 . As illustrated in  FIG.  2   , the work machine  1  includes an engine  20 , a transmission  21 , and a work implement pump  22 . The engine  20  is an internal combustion engine. The engine  20  is disposed in the engine compartment  6 . 
     The transmission  21  is connected to the engine  20 . The transmission  21  causes the tires  4  and  7  to rotate with the driving power of the engine  20 . The transmission  21  may be, for example, a mechanical transmission including a plurality of speed change gears. Alternatively, the transmission  21  may be another type of transmission, such as a hydrostatic transmission (HST), a hydro-mechanical type transmission (HMT), or an electro-mechanical type transmission (EMT). 
     The work implement pump  22  is connected to the engine  20 . The work implement pump  22  is driven by the engine  20  and discharges hydraulic fluid. The work implement pump  22  is a variable displacement hydraulic pump. The work implement pump  22  has a swash plate  22   a . The displacement of the work implement pump  22  is changed by changing the angle of the swash plate  22   a . The displacement of the pump is the maximum discharge displacement of hydraulic fluid per one rotation of the pump. The work implement pump  22  is connected to a displacement control device  22   b . The displacement control device  22   b  changes the displacement of the work implement pump  22  by changing the angle of the swash plate  22   a.    
     The work implement pump  22  is connected to the lift cylinder  16  and the bucket cylinder  17  via a hydraulic circuit  23 . The hydraulic fluid discharged from the work implement pump  22  is supplied to the lift cylinder  16  and the bucket cylinder  17 . Consequently, the work implement  3  moves. A control valve  24  is disposed in the hydraulic circuit  23 . The control valve  24  controls the flow rate of hydraulic fluid supplied from the work implement pump  22  to the lift cylinder  16  and the bucket cylinder  17 . 
     The work machine  1  includes a steering pump  25 , steering cylinders  26  and  27 , a steering operating member  28 , and a steering valve  29 . The steering pump  25  is a variable displacement hydraulic pump. The steering pump  25  is connected to the engine  20 . The steering pump  25  is driven by the engine  20  and discharges hydraulic fluid. 
     The steering pump  25  has a swash plate  25   a . The displacement of the steering pump  25  is changed by changing the angle of the swash plate  25   a . The steering pump  25  is connected to a displacement control device  25   b . The displacement control device  25   b  changes the displacement of the steering pump  25  by changing the angle of the swash plate  25   a . For example, the displacement control device  25   b  includes a piston and a valve. The piston is connected to the swash plate  25   a . The valve controls the hydraulic pressure to the piston. 
     The steering cylinders  26  and  27  are hydraulic cylinders. The steering cylinders  26  and  27  include a left steering cylinder  26  and a right steering cylinder  27 . The left steering cylinder  26  is disposed leftward of the pivot joint  13 . One end of the left steering cylinder  26  is attached to the front frame  11  and the other end is attached to the rear frame  12 . The right steering cylinder  27  is disposed rightward of the pivot joint  13 . One end of the right steering cylinder  27  is attached to the front frame  11  and the other end is attached to the rear frame  12 . 
     The steering cylinders  26  and  27  expand and contract whereby the articulate angle of the work machine  1  changes. As illustrated in  FIG.  3   , an articulate angle θ is the angle between the front frame  11  and the rear frame  12 . The traveling direction of the work machine  1  is changed by changing the articulate angle. 
     The cylinder chamber of the left steering cylinder  26  is divided by a piston into an extension chamber  26   a  and a contraction chamber  26   b . When hydraulic fluid is supplied to the extension chamber  26   a , the piston moves and the left steering cylinder  26  extends, and when hydraulic fluid is supplied to the contraction chamber  26   b , the piston moves and the left steering cylinder  26  contracts. 
     The cylinder chamber of the right steering cylinder  27  is divided by a piston into an extension chamber  27   a  and a contraction chamber  27   b . When hydraulic fluid is supplied to the extension chamber  27   a , the piston moves and the right steering cylinder  27  extends, and when hydraulic fluid is supplied to the contraction chamber  27   b , the piston moves and the right steering cylinder  27  contracts. 
     When the left steering cylinder  26  extends and the right steering cylinder  27  contracts, the front frame  11  bends clockwise with respect to the rear frame  12  and the articulate angle is changed. Consequently, the work machine  1  bends to the right (see R in  FIG.  2   ). When the left steering cylinder  26  contracts and the right steering cylinder  27  extends, the front frame  11  bends counterclockwise with respect to the rear frame  12  and the articulate angle is changed. Consequently, the work machine  1  bends to the left (see L in  FIG.  2   ). 
     The steering operating member  28  is disposed in the cab  5 . The steering operating member  28  is, for example, a steering lever. However, the steering operating member  28  may be another member, such as a steering wheel or a switch. The steering operating member  28  is operable by an operator. The steering operating member  28  is rotatable about a center axis of the steering operating member  28 . The steering operating member  28  is rotatable to the left and right from a neutral position. The steering operating member  28  is connected to an input shaft  28   a.    
     The input shaft  28   a  is connected to the steering valve  29 . The steering valve  29  supplies hydraulic fluid to the steering cylinders  26  and  27  in accordance with an operation of the steering operating member  28 . The steering valve  29  is, for example, a hydraulic pilot type of valve. The steering valve  29  is controlled by changing the pilot hydraulic pressure to the steering valve  29  in response to the operation of the steering operating member  28 . Alternatively, the steering valve  29  may be a solenoid valve that is controlled electrically. 
     The steering valve  29  has ports P 1  to P 4 . The port P 1  is connected to the steering pump  25  through a pipe  31 . The hydraulic fluid discharged from the steering pump  25  is supplied to the steering valve  29  through the pipe  31 . The port P 2  is connected to a tank  30  through a pipe  32 . The tank  30  stores hydraulic fluid. The hydraulic fluid drained from the steering cylinders  26  and  27  is drained from the port P 2  to the tank  30 . 
     The port P 3  is connected to a first supply path  33 . The first supply path  33  is connected to the extension chamber  26   a  of the left steering cylinder  26  and the contraction chamber  27   b  of the right steering cylinder  27 . The port P 4  is connected to a second supply path  34 . The second supply path  34  is connected to the contraction chamber  26   b  of the left steering cylinder  26  and the extension chamber  27   a  of the right steering cylinder  27 . 
     The steering valve  29  switches the connections to the ports P 1  to P 4  in accordance with the operating direction of the steering operating member  28 . The steering valve  29  changes the valve opening degree of the steering valve  29  in accordance with the operating amount of the steering operating member  28 . The operating amount of the steering operating member  28  is the operating angle from the neutral position of the steering operating member  28 . 
     When the steering operating member  28  is positioned in the neutral position, the steering valve  29  closes the ports P 1  to P 4 . When the steering operating member  28  is rotated to the right, the steering valve  29  connects the port P 1  and the port P 3  and connects the port P 2  and the port P 4 . Consequently, hydraulic fluid discharged from the steering pump  25  is supplied through the pipe  31  and the first supply path  33  to the extension chamber  26   a  and the contraction chamber  27   b . Moreover, the hydraulic fluid in the contraction chamber  26   b  and the extension chamber  27   a  is drained to the tank  30  through the second supply path  34  and the pipe  32 . Consequently, the front frame  11  turns around the pivot joint  13  to the right with respect to the rear frame  12 . 
     When the steering operating member  28  is rotated to the left, the steering valve  29  connects the port P 1  and the port P 4  and connects the port P 2  and the port P 3 . Consequently, hydraulic fluid discharged from the steering pump  25  is supplied through the pipe  31  and the second supply path  34  to the contraction chamber  26   b  and the extension chamber  27   a . Moreover, the hydraulic fluid in the extension chamber  26   a  and the contraction chamber  27   b  is drained to the tank  30  through the first supply path  33  and the pipe  32 . Consequently, the front frame  11  turns around the pivot joint  13  to the left with respect to the rear frame  12 . 
     The work machine  1  includes a controller  40 . The controller  40  controls travel of the work machine  1  and work by the work implement  3 . The controller  40  includes a processor  40   a  and a storage device  40   b . The processor  40   a  may be, for example, a central processing unit (CPU). Alternatively, the processor may be a processor different from a CPU. The processor  40   a  executes processing for controlling the work machine  1  in accordance with a program. 
     The storage device  40   b  includes a non-volatile memory, such as a read-only memory (ROM), and a volatile memory, such as a random access memory (RAM). The storage device  40   b  may include an auxiliary storage device, such as a hard disk or a solid state drive (SSD). The storage device  40   b  is an example of a non-transitory computer-readable recording medium. The storage device  40   b  stores programs and data for controlling the work machine  1 . 
     The work machine  1  includes an accelerator operating member  41 , an accelerator operation sensor  42 , and an engine rotation speed sensor  43 . The accelerator operating member  41  is operable by the operator. The accelerator operating member is disposed in the cab  5 . The accelerator operating member  41  is, for example, a pedal. However, the accelerator operating member  41  may be another member, such as a lever or a switch. 
     The accelerator operation sensor  42  detects an operating amount (referred to below as “accelerator operating amount”) of the accelerator operating member  41 . The accelerator operation sensor  42  outputs an accelerator command signal that indicates the accelerator operating amount. The accelerator command signal is input to the controller  40 . The engine rotation speed sensor  43  detects the rotation speed of the engine  20 . The engine rotation speed sensor  43  outputs an engine rotation speed signal that indicates the rotation speed of the engine  20 . The engine rotation speed signal is input to the controller  40 . 
     The controller  40  controls the output of the engine  20  and the transmission  21  in accordance with the accelerator command signal. Consequently, the work machine  1  travels at a speed corresponding to the accelerator operating amount. For example, the controller  40  determines a target engine rotation speed that corresponds to the accelerator operating amount. The controller  40  determines a throttle command to the engine  20  so that the actual engine rotation speed indicated by the engine rotation speed signal matches the target engine rotation speed. The controller  40  controls a fuel injection amount of the engine  20  in response to the throttle command. Alternatively, the controller  40  may determine a target tractive force that corresponds to the accelerator operating amount. The controller  40  may determine the throttle command to the engine  20  so that the target tractive force is achieved. 
     The work machine  1  includes a work operating member  44  and a work operation sensor  45 . The work operating member  44  is operable by the operator. The work operating member  44  is disposed in the cab  5 . The work operating member  44  is, for example, a lever. However, the work operating member  44  may be another member, such as a switch. The work operation sensor  45  detects the operating amount (referred to below as “work operating amount”) of the work operating member  44 . The work operation sensor  45  outputs a work command signal that indicates the work operating amount. The work command signal is input to the controller  40 . 
     The controller  40  controls the control valve  24  in accordance with the work command signal. The controller  40  controls the flow rate of hydraulic fluid supplied to the lift cylinder  16  and the bucket cylinder  17  by controlling the control valve  24 . Consequently, the work implement  3  is operated in accordance with the work operating amount. The control valve  24  may be controlled electrically by the controller  40 . Alternatively, the control valve  24  may be controlled with pilot hydraulic pressure from the work operating member  44 . 
     The work machine  1  includes a steering pump pressure sensor  46 , articulate angle sensors  47  and  48 , and a steering operation sensor  49 . The steering pump pressure sensor  46  detects the discharge pressure of the steering pump  25 . The steering pump pressure sensor  46  outputs a pump pressure signal that indicates the discharge pressure of the steering pump  25 . The pump pressure signal is input to the controller  40 . 
     The articulate angle sensors  47  and  48  detect articulate angles. The articulate angle sensors  47  and  48  output articulate angle signals that indicate the articulate angles. The articulate angle signals are input to the controller  40 . The articulate angle sensors  47  and  48  are potentiometers, for example, and detect the articulate angles directly. Alternatively, the articulate angle sensor  47  may detect the stroke length of the left steering cylinder  26 . The articulate angle sensor  48  may detect the stroke length of the right steering cylinder  27 . The controller  40  may calculate the articulate angle from the stroke lengths of the steering cylinders  26  and  27 . 
     The steering operation sensor  49  detects an operating amount (referred to below as “steering operating amount”) of the steering operating member  28 . The steering operation sensor  49  outputs a steering command signal that corresponds to the steering operating amount. The steering operation sensor  49  is, for example, a potentiometer. The steering command signal is input to the controller  40 . The controller  40  acquires the operating direction and the steering operating amount of the steering operating member  28  from the steering command signal. 
     The displacement control device  25   b  controls the displacement of the steering pump  25  in accordance with the pressure differential between the load pressure of hydraulic fluid to the steering pump  25  and the discharge pressure of the steering pump  25 . Alternatively, the controller  40  may control the displacement of the steering pump  25  by controlling the displacement control device  25   b  in accordance with the steering operating amount. 
     The controller  40  controls the flow rate of hydraulic fluid supplied from the steering pump  25  to the steering cylinders  26  and  27  by controlling the rotation speed of the engine  20  in accordance with the articulate angle signals and the steering command signal during a steering operation. The control of the engine  20  during a steering operation will be explained below.  FIG.  4    is a flow chart illustrating processing executed by the controller  40 . 
     In step S 101  as illustrated in  FIG.  4   , the controller  40  acquires the steering operating speed. The steering operating speed is the operating speed of the steering operating member  28 . The steering operating speed is represented by the angular speed of the steering operating member  28 . The controller  40  calculates the angular speed of the steering operating member  28  from the steering command signal. 
     In step S 102 , the controller  40  acquires the actual articulate angular speed. The controller  40  calculates the actual articulate angular speed from the articulate command signals. 
     In step S 103 , the controller  40  determines a target articulate angular speed. The controller  40  refers to target articulate data and determines the target articulate angular speed from the steering operating speed.  FIG.  5    is a diagram illustrating an example of the target articulate data. The target articulate data defines the relationship between the steering operating speed and the target articulate angular speed. The target articulate data is saved in the storage device  40   b.    
     As illustrated in  FIG.  5   , the target articulate data defines the target articulate angular speed that increases in accordance with an increase in the steering operating speed. The rate of change of the target articulate angular speed when the steering operating speed is equal to or greater than a predetermined value w 1 , is greater than the rate of change of the target articulate angular speed when the steering operating speed is less than the predetermined value w 1 . 
     The controller  40  corrects the target articulate angular speed with feedback control from the target articulate angular speed determined from the steering operating speed and the actual articulate angular speed. For example, the controller  40  increases the target articulate angular speed so as to reduce a delay of the bending motion of the work machine  1  when the actual bending motion of the work machine  1  is delayed in comparison to the target articulate angular speed. 
     In step S 104 , the controller  40  determines the required flow rates of the steering cylinders  26  and  27 . The controller  40  refers to required flow rate data and determines the required flow rates of the steering cylinders  26  and  27  from the target articulate angular speed.  FIG.  6    is a diagram illustrating an example of required flow rate data. The required flow rate data defines the relationship between the target articulate angular speed and the required flow rates of the steering cylinders  26  and  27 . The required flow rate data is saved in the storage device  40   b . As illustrated in  FIG.  6   , the required flow rate data defines the required flow rates of the steering cylinders  26  and  27  that increase in accordance with an increase in the target articulate angular speed. 
     In step S 105 , the controller  40  determines a required engine rotation speed. The controller  40  calculates the required engine rotation speed from the required flow rates of the steering cylinders  26  and  27 . For example, the controller  40  calculates the required engine rotation speed using the following equation (1). 
     
       
         
           
             
               
                 
                   Nd 
                   = 
                   
                     
                       Qd 
                       × 
                       1000 
                     
                     
                       Q 
                       ⁢ 
                       a 
                       × 
                       E 
                       ⁢ 
                       v 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Nd is the required engine rotation speed (rpm). Qd is the required flow rate (L/min) of each of the steering cylinders  26  and  27 . Qa is the maximum displacement (cc/rev) of the steering pump  25 . Ev is the volume efficiency of the steering pump  25 . 
     In step S 106 , the controller  40  determines the required torque of the steering pump  25 . The controller  40  calculates the required torque of the steering pump  25  from the discharge pressure of the steering pump  25  and the required flow rate of each of the steering cylinders  26  and  27 . For example, the controller  40  calculates the required torque of the steering pump  25  with the following equations (2) and (3). 
     
       
         
           
             
               
                 
                   Td 
                   = 
                   
                     
                       
                         qd 
                         × 
                         P 
                       
                       
                         2 
                         ⁢ 
                         π 
                       
                     
                     × 
                     
                       1 
                       
                         E 
                         ⁢ 
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   qd 
                   = 
                   
                     
                       Qd 
                       × 
                       1000 
                     
                     
                       N 
                       ⁢ 
                       a 
                       × 
                       E 
                       ⁢ 
                       v 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Td is the required torque (Nm) of the steering pump  25 . P is the discharge pressure (Mpa) of the steering pump  25 . qd is the required displacement (cc/rev) of the steering pump  25 . Na is the actual engine rotation speed. 
     In step S 107 , the controller  40  determines a required engine output. The controller  40  calculates the required engine output from the required torque of the steering pump  25  and the required engine rotation speed. The controller  40  calculates the required engine output using the following equation (4). 
     
       
         
           
             
               
                 
                   W 
                   = 
                   
                     
                       2 
                       ⁢ 
                       π 
                       × 
                       Td 
                       × 
                       Nd 
                     
                     
                       6 
                       ⁢ 
                       0 
                       × 
                       1 
                       ⁢ 
                       0 
                       ⁢ 
                       0 
                       ⁢ 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     W is the required engine output (kW). The required engine output in this case is the required output of the engine  20  for achieving the above required flow rate in the steering cylinders  26  and  27  and for achieving the above required torque in the steering pump  25 . 
     In step S 108 , the controller  40  determines the throttle command for the engine  20 . The controller  40  determines the throttle command for the engine  20  based on the actual engine rotation speed, the surplus ratio of the output of the engine  20 , and the required engine output determined in step S 107 . 
     For example, when the output of the engine  20  based on the current throttle command is insufficient with respect to the required engine output determined in step S 107 , the controller  40  increases the throttle command for the engine  20  in comparison to the current throttle command in consideration of the required engine output determined in step S 107 . The current throttle command is determined, for example, in accordance with the accelerator operating amount. Alternatively, the current throttle command may be determined in accordance with the accelerator operating amount and the operating amount of the work implement  3 . When the output of the engine  20  based on the current throttle command sufficiently covers the required engine output determined in step S 107 , the controller  40  maintains the current throttle command. 
     In the work machine  1  according to the embodiment discussed above, the rotation speed of the engine  20  is controlled in accordance with the steering command signal corresponding to the operation of the steering operating member  28 . As a result, the flow rate of hydraulic fluid supplied from the steering pump  25  to the steering cylinders  26  and  27  is controlled in accordance with the operation of the steering operating member  28 . Consequently, the followability of the bending motion of the work machine  1  with respect to a steering operation can be improved. In addition, fuel consumption can be improved because hydraulic fluid can be supplied to the steering cylinders  26  and  27  at a flow rate that is required in accordance with the steering operation. 
     The controller  40  controls the rotation speed of the engine  20  in accordance with the steering operating speed. If the steering operating speed is high, the required flow rates of the steering cylinders are increased and the required engine rotation speed is also increased. Consequently, the followability of the bending motion of the work machine  1  with respect to a steering operation can be improved. 
     The controller  40  determines the required torque of the steering pump  25  based on the discharge pressure of the steering pump  25  and the required flow rates of the steering cylinders  26  and  27 . The controller  40  then determines the required engine output based on the required torque and the required engine rotation speed. Consequently, the driving torque of the steering pump  25  required for the bending motion in accordance with the operation of the steering operating member  28  can be assured. 
     The controller  40  increases the rotation speed of the engine  20  when the actual articulate angular speed is slower than the target articulate angular speed. Consequently, the followability of a bending motion of the work machine  1  with respect to a steering operation can be improved. 
     Although an embodiment of the present invention has been described so far, the present invention is not limited to the above embodiment and various modifications may be made within the scope of the invention. 
     The work machine  1  is not limited to a wheel loader and may be another machine such as an articulated dump truck or motor grader, etc. The configuration of the work machine  1  is not limited to the above embodiment and may be modified. For example, the work machine  1  is not limited to tires and may travel using another travel device, such as crawler belts. The configuration of the work implement  3  is not limited to the above embodiment and may be modified. 
     The structure for the bending motion of the work machine  1 , such as the pivot joint  13  and the steering cylinders  26  and  27 , may be changed. The work machine  1  may be remotely operated. In this case, the accelerator operating member  41 , the work operating member  44 , and the steering operating member  28  may be disposed outside of the work machine  1 . The controller  40  may also be configured by a plurality of controllers. 
     The processing executed by the controller  40  may be distributed and executed among the plurality of controllers  40 . The processing by the controller  40  is not limited to that of the above embodiment and may be changed. For example, the controller  40  may determine the throttle command for the engine  20  from a total of the required engine output corresponding to the operation of the steering operating member  28 , the required engine output corresponding to the operation of the accelerator operating member  41 , and the required engine output corresponding to the operation of the work operating member  44 . 
     In the present embodiment, the steering valve  29  changes the valve opening degree of the steering valve  29  in accordance with the operating amount of the steering operating member  28 . However, the valve opening degree may be determined based on deviation between the target articulate angle and the actual articulate angle. In this case, the ports P 1  to P 4  may be closed when target articulate angle and the actual articulate angle match. 
     According to the present disclosure, the followability of the bending motion of the work machine with respect to a steering operation can be improved and fuel consumption can be improved.