Patent Description:
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 <CIT>. 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.

<CIT> discloses a work vehicle and a control method that inhibits a reduction in ease of operation and that can improve the effect of reduced fuel consumption. Hereto the lower limit values of these engine rotation speeds are set to a value at which it is difficult for the engine rotation speed to decrease by a large amount, even in the case that a large load is suddenly imposed in the low-load conditions.

In the abovementioned 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. The controller is further configured to acquire an operating speed of the steering operating member from the steering command signal, and control the rotation speed of the engine in accordance with the operating speed.

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. A third process is acquiring an operating speed of the steering operating member from the steering command signal. A fourth process is controlling the rotation speed of the engine in accordance with the operating speed.

According to the present invention, 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.

The following is a description of a work machine according to an embodiment with reference to the drawings. <FIG> is a side view of a work machine <NUM> according to the embodiment. The work machine <NUM> according to the present embodiment is a wheel loader. The wheel loader <NUM> includes a vehicle body frame <NUM>, a work implement <NUM>, a pair of front tires <NUM>, a cab <NUM>, an engine compartment <NUM>, and a pair of rear tires <NUM>. In the following explanations, "front," "rear," "right," "left," "up," and "down" indicate directions relative to a state of looking forward from an operator's seat within the cab <NUM>.

The vehicle body frame <NUM> includes a front frame <NUM>, a rear frame <NUM>, and a pivot joint <NUM>. The front frame <NUM> is disposed in front of the rear frame <NUM>. The pivot joint <NUM> is disposed in the center in the left-right direction of the work machine <NUM>. The pivot joint <NUM> turnably couples the front frame <NUM> and the rear frame <NUM>. The pair of front tires <NUM> are attached to the front frame <NUM>. The pair of rear tires <NUM> are attached to the rear frame <NUM>.

The work implement <NUM> includes a boom <NUM>, a bucket <NUM>, a lift cylinder <NUM>, and a bucket cylinder <NUM>. The boom <NUM> is mounted to the front frame <NUM>. The bucket <NUM> is attached to the tip of the boom <NUM>.

The lift cylinder <NUM> and the bucket cylinder <NUM> are hydraulic cylinders. One end of the lift cylinder <NUM> is attached to the front frame <NUM> and the other end of the lift cylinder <NUM> is attached to the boom <NUM>. The boom <NUM> swings up and down due to the extension and contraction of the lift cylinder <NUM>. One end of the bucket cylinder <NUM> is attached to the front frame <NUM> and the other end of the bucket cylinder <NUM> is attached to the bucket <NUM> via a bell crank <NUM>. The bucket <NUM> swings up and down due to the extension and contraction of the bucket cylinder <NUM>.

The cab <NUM> is disposed on the rear frame <NUM>. The engine compartment <NUM> is disposed behind the cab <NUM>. The engine compartment <NUM> is disposed on the rear frame <NUM>.

<FIG> is a schematic view of a control system of the work machine <NUM>. As illustrated in <FIG>, the work machine <NUM> includes an engine <NUM>, a transmission <NUM>, and a work implement pump <NUM>. The engine <NUM> is an internal combustion engine. The engine <NUM> is disposed in the engine compartment <NUM>.

The transmission <NUM> is connected to the engine <NUM>. The transmission <NUM> causes the tires <NUM> and <NUM> to rotate with the driving power of the engine <NUM>. The transmission <NUM> may be, for example, a mechanical transmission including a plurality of speed change gears. Alternatively, the transmission <NUM> 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 <NUM> is connected to the engine <NUM>. The work implement pump <NUM> is driven by the engine <NUM> and discharges hydraulic fluid. The work implement pump <NUM> is a variable displacement hydraulic pump. The work implement pump <NUM> has a swash plate 22a. The displacement of the work implement pump <NUM> is changed by changing the angle of the swash plate 22a. The displacement of the pump is the maximum discharge displacement of hydraulic fluid per one rotation of the pump. The work implement pump <NUM> is connected to a displacement control device 22b. The displacement control device 22b changes the displacement of the work implement pump <NUM> by changing the angle of the swash plate 22a.

The work implement pump <NUM> is connected to the lift cylinder <NUM> and the bucket cylinder <NUM> via a hydraulic circuit <NUM>. The hydraulic fluid discharged from the work implement pump <NUM> is supplied to the lift cylinder <NUM> and the bucket cylinder <NUM>. Consequently, the work implement <NUM> moves. A control valve <NUM> is disposed in the hydraulic circuit <NUM>. The control valve <NUM> controls the flow rate of hydraulic fluid supplied from the work implement pump <NUM> to the lift cylinder <NUM> and the bucket cylinder <NUM>.

The work machine <NUM> includes a steering pump <NUM>, steering cylinders <NUM> and <NUM>, a steering operating member <NUM>, and a steering valve <NUM>. The steering pump <NUM> is a variable displacement hydraulic pump. The steering pump <NUM> is connected to the engine <NUM>. The steering pump <NUM> is driven by the engine <NUM> and discharges hydraulic fluid.

The steering pump <NUM> has a swash plate 25a. The displacement of the steering pump <NUM> is changed by changing the angle of the swash plate 25a. The steering pump <NUM> is connected to a displacement control device 25b. The displacement control device 25b changes the displacement of the steering pump <NUM> by changing the angle of the swash plate 25a. For example, the displacement control device 25b includes a piston and a valve. The piston is connected to the swash plate 25a. The valve controls the hydraulic pressure to the piston.

The steering cylinders <NUM> and <NUM> are hydraulic cylinders. The steering cylinders <NUM> and <NUM> include a left steering cylinder <NUM> and a right steering cylinder <NUM>. The left steering cylinder <NUM> is disposed leftward of the pivot joint <NUM>. One end of the left steering cylinder <NUM> is attached to the front frame <NUM> and the other end is attached to the rear frame <NUM>. The right steering cylinder <NUM> is disposed rightward of the pivot joint <NUM>. One end of the right steering cylinder <NUM> is attached to the front frame <NUM> and the other end is attached to the rear frame <NUM>.

The steering cylinders <NUM> and <NUM> expand and contract whereby the articulate angle of the work machine <NUM> changes. As illustrated in <FIG>, an articulate angle θ is the angle between the front frame <NUM> and the rear frame <NUM>. The traveling direction of the work machine <NUM> is changed by changing the articulate angle.

The cylinder chamber of the left steering cylinder <NUM> is divided by a piston into an extension chamber 26a and a contraction chamber 26b. When hydraulic fluid is supplied to the extension chamber 26a, the piston moves and the left steering cylinder <NUM> extends, and when hydraulic fluid is supplied to the contraction chamber 26b, the piston moves and the left steering cylinder <NUM> contracts.

The cylinder chamber of the right steering cylinder <NUM> is divided by a piston into an extension chamber 27a and a contraction chamber 27b. When hydraulic fluid is supplied to the extension chamber 27a, the piston moves and the right steering cylinder <NUM> extends, and when hydraulic fluid is supplied to the contraction chamber 27b, the piston moves and the right steering cylinder <NUM> contracts.

When the left steering cylinder <NUM> extends and the right steering cylinder <NUM> contracts, the front frame <NUM> bends clockwise with respect to the rear frame <NUM> and the articulate angle is changed. Consequently, the work machine <NUM> bends to the right (see R in <FIG>). When the left steering cylinder <NUM> contracts and the right steering cylinder <NUM> extends, the front frame <NUM> bends counterclockwise with respect to the rear frame <NUM> and the articulate angle is changed. Consequently, the work machine <NUM> bends to the left (see L in <FIG>).

The steering operating member <NUM> is disposed in the cab <NUM>. The steering operating member <NUM> is, for example, a steering lever. However, the steering operating member <NUM> may be another member such as a steering wheel or a switch. The steering operating member <NUM> is operable by an operator. The steering operating member <NUM> is rotatable about a center axis of the steering operating member <NUM>. The steering operating member <NUM> is rotatable to the left and right from a neutral position. The steering operating member <NUM> is connected to an input shaft 28a.

The input shaft 28a is connected to the steering valve <NUM>. The steering valve <NUM> supplies hydraulic fluid to the steering cylinders <NUM> and <NUM> in accordance with an operation of the steering operating member <NUM>. The steering valve <NUM> is, for example, a hydraulic pilot type of valve. The steering valve <NUM> is controlled by changing the pilot hydraulic pressure to the steering valve <NUM> in response to the operation of the steering operating member <NUM>. Alternatively, the steering valve <NUM> may be a solenoid valve that is controlled electrically.

The steering valve <NUM> has ports P1 to P4. The port P1 is connected to the steering pump <NUM> through a pipe <NUM>. The hydraulic fluid discharged from the steering pump <NUM> is supplied to the steering valve <NUM> through the pipe <NUM>. The port P2 is connected to a tank <NUM> through a pipe <NUM>. The tank <NUM> stores hydraulic fluid. The hydraulic fluid drained from the steering cylinders <NUM> and <NUM> is drained from the port P2 to the tank <NUM>.

The port P3 is connected to a first supply path <NUM>. The first supply path <NUM> is connected to the extension chamber 26a of the left steering cylinder <NUM> and the contraction chamber 27b of the right steering cylinder <NUM>. The port P4 is connected to a second supply path <NUM>. The second supply path <NUM> is connected to the contraction chamber 26b of the left steering cylinder <NUM> and the extension chamber 27a of the right steering cylinder <NUM>.

The steering valve <NUM> switches the connections to the ports P1 to P4 in accordance with the operating direction of the steering operating member <NUM>. The steering valve <NUM> changes the valve opening degree of the steering valve <NUM> in accordance with the operating amount of the steering operating member <NUM>. The operating amount of the steering operating member <NUM> is the operating angle from the neutral position of the steering operating member <NUM>.

When the steering operating member <NUM> is positioned in the neutral position, the steering valve <NUM> closes the ports P1 to P4. When the steering operating member <NUM> is rotated to the right, the steering valve <NUM> connects the port P1 and the port P3 and connects the port P2 and the port P4. Consequently, hydraulic fluid discharged from the steering pump <NUM> is supplied through the pipe <NUM> and the first supply path <NUM> to the extension chamber 26a and the contraction chamber 27b. Moreover, the hydraulic fluid in the contraction chamber 26b and the extension chamber 27a is drained to the tank <NUM> through the second supply path <NUM> and the pipe <NUM>. Consequently, the front frame <NUM> turns around the pivot joint <NUM> to the right with respect to the rear frame <NUM>.

When the steering operating member <NUM> is rotated to the left, the steering valve <NUM> connects the port P1 and the port P4 and connects the port P2 and the port P3. Consequently, hydraulic fluid discharged from the steering pump <NUM> is supplied through the pipe <NUM> and the second supply path <NUM> to the contraction chamber 26b and the extension chamber 27a. Moreover, the hydraulic fluid in the extension chamber 26a and the contraction chamber 27b is drained to the tank <NUM> through the first supply path <NUM> and the pipe <NUM>. Consequently, the front frame <NUM> turns around the pivot joint <NUM> to the left with respect to the rear frame <NUM>.

The work machine <NUM> includes a controller <NUM>. The controller <NUM> controls travel of the work machine <NUM> and work by the work implement <NUM>. The controller <NUM> includes a processor 40a and a storage device 40b. The processor 40a may be, for example, a central processing unit (CPU). Alternatively, the processor may be a processor different from a CPU. The processor 40a executes processing for controlling the work machine <NUM> in accordance with a program.

The storage device 40b 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 40b may include an auxiliary storage device such as a hard disk or a solid state drive (SSD). The storage device 40b is an example of a non-transitory computer-readable recording medium. The storage device 40b stores programs and data for controlling the work machine <NUM>.

The work machine <NUM> includes an accelerator operating member <NUM>, an accelerator operation sensor <NUM>, and an engine rotation speed sensor <NUM>. The accelerator operating member <NUM> is operable by the operator. The accelerator operating member is disposed in the cab <NUM>. The accelerator operating member <NUM> is, for example, a pedal. However, the accelerator operating member <NUM> may be another member such as a lever or a switch.

The accelerator operation sensor <NUM> detects an operating amount (referred to below as "accelerator operating amount") of the accelerator operating member <NUM>. The accelerator operation sensor <NUM> outputs an accelerator command signal that indicates the accelerator operating amount. The accelerator command signal is input to the controller <NUM>. The engine rotation speed sensor <NUM> detects the rotation speed of the engine <NUM>. The engine rotation speed sensor <NUM> outputs an engine rotation speed signal that indicates the rotation speed of the engine <NUM>. The engine rotation speed signal is input to the controller <NUM>.

The controller <NUM> controls the output of the engine <NUM> and the transmission <NUM> in accordance with the accelerator command signal. Consequently, the work machine <NUM> travels at a speed corresponding to the accelerator operating amount. For example, the controller <NUM> determines a target engine rotation speed that corresponds to the accelerator operating amount. The controller <NUM> determines a throttle command to the engine <NUM> so that the actual engine rotation speed indicated by the engine rotation speed signal matches the target engine rotation speed. The controller <NUM> controls a fuel injection amount of the engine <NUM> in response to the throttle command. Alternatively, the controller <NUM> may determine a target tractive force that corresponds to the accelerator operating amount. The controller <NUM> may determine the throttle command to the engine <NUM> so that the target tractive force is achieved.

The work machine <NUM> includes a work operating member <NUM> and a work operation sensor <NUM>. The work operating member <NUM> is operable by the operator. The work operating member <NUM> is disposed in the cab <NUM>. The work operating member <NUM> is, for example, a lever. However, the work operating member <NUM> may be another member such as a switch. The work operation sensor <NUM> detects the operating amount (referred to below as "work operating amount") of the work operating member <NUM>. The work operation sensor <NUM> outputs a work command signal that indicates the work operating amount. The work command signal is input to the controller <NUM>.

The controller <NUM> controls the control valve <NUM> in accordance with the work command signal. The controller <NUM> controls the flow rate of hydraulic fluid supplied to the lift cylinder <NUM> and the bucket cylinder <NUM> by controlling the control valve <NUM>. Consequently, the work implement <NUM> is operated in accordance with the work operating amount. The control valve <NUM> may be controlled electrically by the controller <NUM>. Alternatively, the control valve <NUM> may be controlled with pilot hydraulic pressure from the work operating member <NUM>.

The work machine <NUM> includes a steering pump pressure sensor <NUM>, articulate angle sensors <NUM> and <NUM>, and a steering operation sensor <NUM>. The steering pump pressure sensor <NUM> detects the discharge pressure of the steering pump <NUM>. The steering pump pressure sensor <NUM> outputs a pump pressure signal that indicates the discharge pressure of the steering pump <NUM>. The pump pressure signal is input to the controller <NUM>.

The articulate angle sensors <NUM> and <NUM> detect articulate angles. The articulate angle sensors <NUM> and <NUM> output articulate angle signals that indicate the articulate angles. The articulate angle signals are input to the controller <NUM>. The articulate angle sensors <NUM> and <NUM> are potentiometers, for example, and detect the articulate angles directly. Alternatively, the articulate angle sensor <NUM> may detect the stroke length of the left steering cylinder <NUM>. The articulate angle sensor <NUM> may detect the stroke length of the right steering cylinder <NUM>. The controller <NUM> may calculate the articulate angle from the stroke lengths of the steering cylinders <NUM> and <NUM>.

The steering operation sensor <NUM> detects an operating amount (referred to below as "steering operating amount") of the steering operating member <NUM>. The steering operation sensor <NUM> outputs a steering command signal that corresponds to the steering operating amount. The steering operation sensor <NUM> is, for example, a potentiometer. The steering command signal is input to the controller <NUM>. The controller <NUM> acquires the operating direction and the steering operating amount of the steering operating member <NUM> from the steering command signal.

The displacement control device 25b controls the displacement of the steering pump <NUM> in accordance with the pressure differential between the load pressure of hydraulic fluid to the steering pump <NUM> and the discharge pressure of the steering pump <NUM>. Alternatively, the controller <NUM> may control the displacement of the steering pump <NUM> by controlling the displacement control device 25b in accordance with the steering operating amount.

The controller <NUM> controls the flow rate of hydraulic fluid supplied from the steering pump <NUM> to the steering cylinders <NUM> and <NUM> by controlling the rotation speed of the engine <NUM> in accordance with the articulate angle signals and the steering command signal during a steering operation. The control of the engine <NUM> during a steering operation will be explained below. <FIG> is a flow chart illustrating processing executed by the controller <NUM>.

In step S101 as illustrated in <FIG>, the controller <NUM> acquires the steering operating speed. The steering operating speed is the operating speed of the steering operating member <NUM>. The steering operating speed is represented by the angular speed of the steering operating member <NUM>. The controller <NUM> calculates the angular speed of the steering operating member <NUM> from the steering command signal.

In step S102, the controller <NUM> acquires the actual articulate angular speed. The controller <NUM> calculates the actual articulate angular speed from the articulate command signals.

In step S103, the controller <NUM> determines a target articulate angular speed. The controller <NUM> refers to target articulate data and determines the target articulate angular speed from the steering operating speed. <FIG> 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 40b.

As illustrated in <FIG>, 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 w1, is greater than the rate of change of the target articulate angular speed when the steering operating speed is less than the predetermined value w1.

The controller <NUM> 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 <NUM> increases the target articulate angular speed so as to reduce a delay of the bending motion of the work machine <NUM> when the actual bending motion of the work machine <NUM> is delayed in comparison to the target articulate angular speed.

In step S104, the controller <NUM> determines the required flow rates of the steering cylinders <NUM> and <NUM>. The controller <NUM> refers to required flow rate data and determines the required flow rates of the steering cylinders <NUM> and <NUM> from the target articulate angular speed. <FIG> 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 <NUM> and <NUM>. The required flow rate data is saved in the storage device 40b. As illustrated in <FIG>, the required flow rate data defines the required flow rates of the steering cylinders <NUM> and <NUM> that increase in accordance with an increase in the target articulate angular speed.

In step S105, the controller <NUM> determines a required engine rotation speed. The controller <NUM> calculates the required engine rotation speed from the required flow rates of the steering cylinders <NUM> and <NUM>. For example, the controller <NUM> calculates the required engine rotation speed using the following equation (<NUM>).

Nd is the required engine rotation speed (rpm). Qd is the required flow rate (L/min) of each of the steering cylinders <NUM> and <NUM>. Qa is the maximum displacement (cc/rev) of the steering pump <NUM>. Ev is the volume efficiency of the steering pump <NUM>.

In step S106, the controller <NUM> determines the required torque of the steering pump <NUM>. The controller <NUM> calculates the required torque of the steering pump <NUM> from the discharge pressure of the steering pump <NUM> and the required flow rate of each of the steering cylinders <NUM> and <NUM>. For example, the controller <NUM> calculates the required torque of the steering pump <NUM> with the following equations (<NUM>) and (<NUM>). <MAT> <MAT>.

Td is the required torque (Nm) of the steering pump <NUM>. P is the discharge pressure (Mpa) of the steering pump <NUM>. qd is the required displacement (cc/rev) of the steering pump <NUM>. Na is the actual engine rotation speed.

In step S107, the controller <NUM> determines a required engine output. The controller <NUM> calculates the required engine output from the required torque of the steering pump <NUM> and the required engine rotation speed. The controller <NUM> calculates the required engine output using the following equation (<NUM>).

W is the required engine output (kW). The required engine output in this case is the required output of the engine <NUM> for achieving the above required flow rate in the steering cylinders <NUM> and <NUM> and for achieving the above required torque in the steering pump <NUM>.

In step S108, the controller <NUM> determines the throttle command for the engine <NUM>. The controller <NUM> determines the throttle command for the engine <NUM> based on the actual engine rotation speed, the surplus ratio of the output of the engine <NUM>, and the required engine output determined in step S107.

For example, when the output of the engine <NUM> based on the current throttle command is insufficient with respect to the required engine output determined in step S107, the controller <NUM> increases the throttle command for the engine <NUM> in comparison to the current throttle command in consideration of the required engine output determined in step S107. 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 <NUM>. When the output of the engine <NUM> based on the current throttle command sufficiently covers the required engine output determined in step S107, the controller <NUM> maintains the current throttle command.

In the work machine <NUM> according to the embodiment discussed above, the rotation speed of the engine <NUM> is controlled in accordance with the steering command signal corresponding to the operation of the steering operating member <NUM>. As a result, the flow rate of hydraulic fluid supplied from the steering pump <NUM> to the steering cylinders <NUM> and <NUM> is controlled in accordance with the operation of the steering operating member <NUM>. Consequently, the followability of the bending motion of the work machine <NUM> 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 <NUM> and <NUM> at a flow rate that is required in accordance with the steering operation.

The controller <NUM> controls the rotation speed of the engine <NUM> 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 <NUM> with respect to a steering operation can be improved.

The controller <NUM> determines the required torque of the steering pump <NUM> based on the discharge pressure of the steering pump <NUM> and the required flow rates of the steering cylinders <NUM> and <NUM>. The controller <NUM> 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 <NUM> required for the bending motion in accordance with the operation of the steering operating member <NUM> can be assured.

The controller <NUM> increases the rotation speed of the engine <NUM> 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 <NUM> 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 <NUM> 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 <NUM> is not limited to the above embodiment and may be modified. For example, the work machine <NUM> is not limited to tires and may travel using another travel device such as crawler belts. The configuration of the work implement <NUM> is not limited to the above embodiment and may be modified.

The structure for the bending motion of the work machine <NUM> such as the pivot joint <NUM> and the steering cylinders <NUM> and <NUM> may be changed. The work machine <NUM> may be remotely operated. In this case, the accelerator operating member <NUM>, the work operating member <NUM>, and the steering operating member <NUM> may be disposed outside of the work machine <NUM>. The controller <NUM> may also be configured by a plurality of controllers.

The processing executed by the controller <NUM> may be distributed and executed among the plurality of controllers <NUM>. The processing by the controller <NUM> is not limited to that of the above embodiment and may be changed. For example, the controller <NUM> may determine the throttle command for the engine <NUM> from a total of the required engine output corresponding to the operation of the steering operating member <NUM>, the required engine output corresponding to the operation of the accelerator operating member <NUM>, and the required engine output corresponding to the operation of the work operating member <NUM>.

In the present embodiment, the steering valve <NUM> changes the valve opening degree of the steering valve <NUM> in accordance with the operating amount of the steering operating member <NUM>. 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 P1 to P4 may be closed when target articulate angle and the actual articulate angle match.

Claim 1:
A work machine (<NUM>) comprising:
a first frame (<NUM>);
a second frame (<NUM>) turnably connected to the first frame (<NUM>);
a steering cylinder (<NUM>, <NUM>) connected to the second frame (<NUM>) and the first frame (<NUM>) and causing the second frame (<NUM>) to turn with respect to the first frame (<NUM>);
a hydraulic pump (<NUM>) that supplies hydraulic fluid to the steering cylinder (<NUM>, <NUM>);
an engine (<NUM>) that drives the hydraulic pump (<NUM>);
a steering operating member (<NUM>) that is operable by an operator;
a steering operation sensor (<NUM>) that outputs a steering command signal corresponding to an operation of the steering operating member (<NUM>); and
a controller (<NUM>) configured to control a flow rate of hydraulic fluid supplied from the hydraulic pump (<NUM>) to the steering cylinder (<NUM>, <NUM>) by controlling a rotation speed of the engine (<NUM>) in accordance with the steering command signal;
characterised in that the controller (<NUM>) is further configured to
acquire an operating speed of the steering operating member (<NUM>) from the steering command signal, and
control the rotation speed of the engine (<NUM>) in accordance with the operating speed.