Work machine and automatic control method for blade of work machine

A work machine includes a vehicle body, a blade supported by the vehicle body, a pair of first hydraulic cylinders, a pair of second hydraulic cylinders, a pair of lift stroke sensors and a control unit. The first hydraulic cylinders are configured to lower and raise the blade. The second hydraulic cylinders are configured to tilt the blade forward and backward and left and right. The lift stroke sensors are configured to detect stroke amounts from the pair of first hydraulic cylinders, respectively. The control unit is configured to start actuating the pair of second hydraulic cylinders when the stroke amounts of the pair of first hydraulic cylinders match. The control unit is configured to stop actuating the pair of second hydraulic cylinders based on a magnitude relation between a prescribed threshold and a difference in the stroke amounts of the pair of first hydraulic cylinders.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of International Application No. PCT/JP2013/066214, filed on Jun. 12, 2013. This U.S. National stage application claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2012-174436, filed in Japan on Aug. 6, 2012, the entire contents of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a work machine provided with a blade and an automatic control method for the blade of the same.

BACKGROUND INFORMATION

A work machine such as a bulldozer or a motor grader is equipped with a blade as work implement for excavating earth.

US Patent Application Publication No. 2005/0065689 discloses a work machine equipped with a pair of lift cylinders for lowering and raising the blade, and a pair of pitch/tilt cylinders for pitching the blade forward and backward. The work machine of US Patent Application Publication No. 2005/0065689 is able to recognize the blade attitude on the basis of detection values from four stroke sensors for detecting stroke amounts of the lift cylinders and the pitch/tilt cylinders.

The blade can be tilted forward and backward and left and right by actuating one or both of the pair of pitch/tilt cylinders in the work machine of US 2005/0065689.

SUMMARY

Since stroke sensors are generally expensive, there is a desire to recognize the blade attitude with, for example, only two stroke sensors for detecting the stroke amount of the pair of lift cylinders.

However, the degree of the tilt of the blade forward, backward, left or right cannot be recognized on the basis of only the stroke amount of the pair of lift cylinders. As a result, excavation work cannot be conducted effectively if excavation work is conducted without controlling the attitude of the blade.

Specifically, even if the blade is lifted to a standard height, the cutting edge of the blade may dig into the ground surface and excavate too much earth if the blade is pitched forward too much. Conversely, even when the blade is lifted to the standard height, the cutting edge may be too far removed from the ground surface so that not enough earth is excavated if the blade is pitched backward too much.

In consideration of the above condition, an object of the present invention is to provide a work machine that allows automatic control for reference attitudes of the blade attitude on the basis of the stroke amount of a pair of lift cylinders, and an automatic control method for a blade on the work machine.

A work machine according to a first embodiment is equipped with a vehicle body, a blade supported by the vehicle body, a pair of first hydraulic cylinders, a pair of second hydraulic cylinders, a pair of lift stroke sensors, and a control unit. The pair of first hydraulic cylinders is configured to lower and raise the blade. The pair of second hydraulic cylinders is configured to tilt the blade forward and backward and left and right. The pair of lift stroke sensors is configured to detect stroke amounts from each of the pair of first hydraulic cylinders. The control unit is configured to start actuating the pair of second hydraulic cylinders when the stroke amounts of the pair of first hydraulic cylinders match. The control unit is configured to stop actuating the pair of second hydraulic cylinders on the basis of a magnitude relation between a prescribed threshold and a difference in the stroke amounts of the pair of first hydraulic cylinders.

Based on the work machine according to the first embodiment, a reference attitude of the blade can be restored by actuating the pair of second hydraulic cylinders using only the pair of lift stroke sensors. As a result, the edge of the blade excessively digging into the ground surface or the edge of the blade being too far removed from the ground surface can be suppressed. Consequently, excavation work can be executed effectively.

The work machine according to a second embodiment is related to the first embodiment. The pair of second hydraulic cylinders includes a pitch cylinder configured to pitch the blade forward and backward, and a pitch/tilt cylinder configured to tilt the blade forward and backward and left and right. The maximum stroke length of the pitch cylinder is shorter than the maximum stroke length of the pitch/tilt cylinder. The control unit stops actuating the pair of second hydraulic cylinders when the control unit has determined that the difference between the stroke amounts of the pair of first hydraulic cylinders has become larger than the prescribed threshold while the pair of second hydraulic cylinders are being actuated at a same speed.

The work machine according to a third embodiment is related to the first embodiment. The pair of second hydraulic cylinders includes a pitch cylinder configured to pitch the blade forward and backward, and a pitch/tilt cylinder configured to tilt the blade forward and backward and left and right. The maximum stroke length of the pitch cylinder is the same as the maximum stroke length of the pitch/tilt cylinder. The control unit stops actuating the pair of second hydraulic cylinders when the control unit has determined that the difference between the stroke amounts of the pair of first hydraulic cylinders has become smaller than the prescribed threshold while the pair of second hydraulic cylinders are being actuated at different speeds.

The blade is lowered and raised by the pair of first hydraulic cylinders in an automatic control method for the blade on a work machine according to a fourth embodiment. The automatic control method of the blade includes detecting positions of the pair of first hydraulic cylinders, comparing the detected positions of the pair of first hydraulic cylinders, tilting the blade until the pair of first hydraulic cylinders becomes parallel, and pitching the blade until the pair of first hydraulic cylinders becomes non-parallel after the blade has been tilted until the pair of first hydraulic cylinders becomes parallel.

Based on the automatic control method of the blade on the work machine according to the fourth embodiment, the blade is tilted (tilting action in left-right direction) until the positions of the pair of first hydraulic cylinders are parallel, that is until the blade reaches a horizontal position in the right-left direction. Next, the pitch (tilting action in the front-back direction) of the blade is changed until the positions of the pair of first hydraulic cylinders are shifted. In other words, the limit of the allowable pitch action is determined by the positions of the pair of first hydraulic cylinders, and the pitch is changed up to that limit. According to this action, the blade can be moved horizontally in the left-right direction to the allowable limit of the pitch action by comparing the positions of the pair of first hydraulic cylinders.

The blade is lowered and raised by the pair of first hydraulic cylinders and tilted forward and backward and left and right by the pair of second hydraulic cylinders in an automatic control method of the blade on a work machine according to a fifth embodiment. The automatic control method for the blade includes detecting positions of the pair of first hydraulic cylinders, comparing the detected positions of the pair of first hydraulic cylinders, tilting the blade until the pair of first hydraulic cylinders becomes parallel, and stopping the pair of second hydraulic cylinders when the pair of first hydraulic cylinders has become parallel again while the pair of second hydraulic cylinders are being actuated at different speeds after the blade has been tilted until the pair of first hydraulic cylinders becomes parallel.

Based on the automatic control method of the blade on the work machine according to the fifth embodiment, blade is tilted the (tilting action in left-right direction) to a point where the positions of the pair of first hydraulic cylinders are parallel, that is a point where the blade reaches a horizontal position in the right-left direction. Next, after actuating the pair of second hydraulic cylinders at different speeds, the actuation of the second hydraulic cylinders is terminated when the positions of the first hydraulic cylinders are the same again. According to this action, the blade can be moved horizontally in the left-right direction to the allowable limit position of the second hydraulic cylinder action by comparing the positions of the pair of first hydraulic cylinders.

According to the present invention, a work machine that allows automatic control for reference attitudes of the blade attitude on the basis of the stroke amount of the pair of first hydraulic cylinders, and an automatic control method for the blade on the work machine are provided.

DETAILED DESCRIPTION OF EMBODIMENTS

The following is a discussion of a bulldozer as an example of a “work machine” with reference to the drawings. In the following description, “up,” “down,” “front,” “back,” “left,” and “right” are terms used on the basis of an operator sitting in the driver's seat.

First Embodiment

Configuration of Bulldozer100

FIG. 1is a front perspective view of a configuration of a bulldozer100.FIG. 2is a rear perspective view of a configuration of an actuation system for a blade50.

The bulldozer100is equipped with a vehicle body10, a cab20, a travel device30, a pair of lift frames40, the blade50, a pair of lift cylinders (first hydraulic cylinders)60, and a pair of pitch/tilt cylinders (second hydraulic cylinders)70. The bulldozer100includes a blade control system200(seeFIG. 4) for automatically controlling an attitude of the blade50. The blade control system200will be discussed below.

The vehicle body10supports the cab20. The vehicle body10is supported by the travel device30. The cab20includes a driver's seat for an operator to sit in, and pedals and levers for operating the travel device30and the blade50. In particular, a blade attitude restore button210(seeFIG. 4) for restoring the attitude of the blade50to a reference attitude is provided in the cab20. The operator presses the blade attitude restore button220for restoring the blade50to the reference attitude while reversing the bulldozer100to a starting position after completing one pass of excavation or grading by the blade50while advancing the bulldozer100. The reference attitude of the blade50signifies an attitude of the blade50having a certain inclination forward or backward but not having any inclination to the left or right at a certain height above the ground surface. In the present embodiment, the inclination in the front-back direction of the blade50that is the reference attitude is an inclination in which the blade50is inclined furthest toward the rear.

The travel device30supports the vehicle body10. The travel device30has a pair of crawlers31and a pair of sprocket wheels32. The pair of crawlers31are rotated by the pair of sprocket wheels32.

The pair of lift frames40are disposed on both outer sides of the pair of crawlers31so that the vehicle body10is disposed between the pair of lift frames40. The pair of lift frames40includes a right lift frame41and a left lift frame42as illustrated inFIG. 2. The rear end parts of the pair of lift frames40are attached in a rotatable manner on both outer sides of the travel device30. The front end parts of the pair of lift frames40are coupled to the blade50.

The blade50is disposed in front of the vehicle body10. The blade50is supported by the pair of lift frames40, the pair of lift cylinders60, and the pair of pitch/tilt cylinders70. The blade50is lowered and raised by the pair of lift cylinders60. The blade50is tilted forward and backward and left and right by the pair of pitch/tilt cylinders70. Cutting edges51are attached at the lower end part of the blade50for digging into the ground when excavating or grading.

The pair of lift cylinders60is coupled to the vehicle body10and the blade50. The pair of lift cylinders60include a right lift cylinder61and a left lift cylinder62as illustrated inFIG. 2. The blade50is lowered or raised due to simultaneous extension or retraction of the right lift cylinder61and the left lift cylinder62actuated by operating fluid.

As illustrated inFIG. 1, a pair of lift stroke sensors63is attached to the pair of lift cylinders60. The pair of lift stroke sensors63each has a rotating roller for detecting the position of the cylinder rod, and a magnetic sensor for returning the cylinder rod to a home position. The pair of lift stroke sensors63detects the stroke amount of the right lift cylinder61(referred to as “right lift stroke amount61S”) and the stroke amount of the left lift cylinder62(referred to as “left lift stroke amount62S”). The stroke amount herein refers to a movement amount of the cylinder rod from the state where the cylinder rod is contracted the most. The end parts of the lift cylinder60are coupled to the vehicle body10and the blade50, and the position of the lift cylinder60is detected by detecting the stroke amount.

The pair of pitch/tilt cylinders70is coupled to the pair of lift frames40and the blade50. The pair of pitch/tilt cylinders70includes a right pitch cylinder71and a left pitch/tilt cylinder72as illustrated inFIG. 2. The blade50is tilted forward and backward due to the right pitch cylinder71and the left pitch/tilt cylinder72being simultaneously extended and retracted at the same speed. The tilting action of a blade in the front-back direction is called a pitch action. Specifically, the blade50is tilted forward when the right pitch cylinder71and the left pitch/tilt cylinder72are both extended, and the blade50is tilted backward when the right pitch cylinder71and the left pitch/tilt cylinder72are both retracted.

The left side of the blade50is moved substantially up or down due to the extension or retraction of only the left pitch/tilt cylinder72while the right pitch cylinder71is not extended or retracted. Specifically, the blade50is tilted to the right when only the left pitch/tilt cylinder72is extended, and the blade50is tilted to the left when only the left pitch/tilt cylinder72is retracted. The action of tilting the blade50in the right-left direction is referred to as a tilting action. When the blade50is tilted in the right-left direction due to the tilting action, a difference in the stroke amounts between the right lift cylinder61and the left lift cylinder62is generated and the cylinder positions are no longer parallel. The stroke amounts of the right lift cylinder61and the left lift cylinder62become equal and the cylinder positions become parallel when the blade50is in a position of not being tilted to the right or left.

When the cylinder positions are parallel, the right lift cylinder61and the left lift cylinder62lie in the same plane and the shaft centers of the right lift cylinder61and the left lift cylinder62do not cross each other. Conversely, when the cylinder positions are non-parallel, the right lift cylinder61and the left lift cylinder62do not lie in the same plane and the shaft centers of the right lift cylinder61and the left lift cylinder62do not intersect.

FIGS. 3A to 3Dare schematic views of configurations of the right lift cylinder61, the left lift cylinder62, the right pitch cylinder71, and the left pitch/tilt cylinder72. The states of each cylinder when the blade50is moved to the reference attitude are illustrated inFIGS. 3A to 3D. In the following explanation, the positions of the cylinders when the blade50is in the reference attitude are referred to as “reference positions”.

As illustrated inFIGS. 3A and 3B, the right lift cylinder61and the left lift cylinder62have the same configurations. Specifically, a cylinder body61aand a rod61bof the right lift cylinder61are similar to a cylinder body62aand a rod62bof the left lift cylinder62. The reference position of the right lift cylinder61is set to be near the center of the cylinder body61aas illustrated inFIG. 3A. The reference position of the left lift cylinder62is set to be near the center of the cylinder body62aas illustrated inFIG. 3B.

In contrast, the right pitch cylinder71and the left pitch/tilt cylinder72have different configurations as illustrated inFIGS. 3C and 3D. Specifically, a cylinder body71aof the right pitch cylinder71is shorter than a cylinder body72aof the left pitch/tilt cylinder72. As a result, the maximum stroke length in the cylinder body71ais approximately half of the maximum stroke length in the cylinder body72a. A rod71bof the right pitch cylinder71is similar to a rod72bof the left pitch/tilt cylinder72. The reference position of the rod71bof the right pitch cylinder71is set to be at the proximal end of the cylinder body61aas illustrated inFIG. 3C. The reference position of the rod72bof the left pitch/tilt cylinder72is set to be near the center of the cylinder body72ain line with the right pitch cylinder71as illustrated inFIG. 3D.

The cylinder body71ais shorter than the cylinder body72ain order to restrict the fore-and-aft pitch angle so that the lower edge of the blade50does not dig into the ground surface while maintaining a sufficient stroke amount of the left pitch/tilt cylinder72to allow for tilting of the blade50to the right or left.

(Configuration of Blade Control System200)

The following is a discussion of the configuration of the blade control system200installed in the bulldozer100with reference to the drawings.FIG. 4is a block diagram illustrating the configuration of the blade control system200.FIGS. 5A to 5Care schematic views illustrating states of the actuations of the pair of lift cylinders60and the pair of pitch/tilt cylinders70. The dashed lines inFIGS. 5A to 5Crepresent the reference positions of the hydraulic cylinders.

As illustrated inFIG. 4, the blade control system200is provided with the pair of lift cylinders60, the pair of lift stroke sensors63, the pair of pitch/tilt cylinders70, the blade attitude restore button210, a control device220, a hydraulic pump230, and main valves240. The pair of lift cylinders60includes the right lift cylinder61and the left lift cylinder62. The pair of pitch/tilt cylinders70includes the right pitch cylinder71and the left pitch/tilt cylinder72.

The blade attitude restore button210transmits a blade attitude restore signal to the control device220when pressed by the operator.

The control device220is able to individually supply operating fluid from the hydraulic pump230to the right lift cylinder61, the left lift cylinder62, the right pitch cylinder71, and the left pitch/tilt cylinder72according to the transmission of control signals to the main valves240. Specifically, the control device220is able to actuate the cylinders individually. The control device220executes “blade attitude restore control” to restore the blade50to the reference attitude in response to the blade attitude restore signal from the blade attitude restore button210.

First, the control device220detects the right lift stroke amount61S and the left lift stroke amount62S on the basis of detection values from the pair of lift stroke sensors63. Next, when the right lift stroke amount61S and the left lift stroke amount62S are different as illustrated inFIG. 5A, the control device220actuates only the left pitch/tilt cylinder72so that the two stroke amounts match. The control device220temporarily stops actuating the left pitch/tilt cylinder72at the point in time that the right lift stroke amount61S and the left lift stroke amount62S match as illustrated inFIG. 5B. The pair of lift cylinders60enters a parallel positional relationship when the left and right lift stroke amounts match. While the actuation of the left pitch/tilt cylinder72is temporarily stopped at this point in time in the present embodiment, the process may advance to the next step without stopping.

Next, the control device220starts actuating the right pitch cylinder71and the left pitch/tilt cylinder72at the same speed when the right lift stroke amount61S and the left lift stroke amount62S match. At this time, the control device220determines the magnitude relation between a prescribed threshold TH1 (e.g., 3 mm) and the difference in the right lift stroke amount61S and the left lift stroke amount62S (herein referred to as “stroke difference ΔS”). The control device220stops the right pitch cylinder71and the left pitch/tilt cylinder72when it is determined that the stroke difference ΔS is larger than the threshold TH1.

Operation of Control Device220

The following is an explanation of the blade attitude restore control performed by the control device220with reference to the drawings.FIG. 6is a flow chart for explaining the blade attitude restore control performed by the control device220. The blade attitude restore control is activated in response to the operator pressing the blade attitude restore button210.

In step S101, the control device220obtains the detection values of the pair of lift stroke sensors63.

In step S201, the control device220determines the magnitude relation between the right lift stroke amount61S and the left lift stroke amount62S on the basis of the detection values from the pair of lift stroke sensors63.

If it is determined in step S102that the right lift stroke amount61S is larger than the left lift stroke amount62S, the control device220retracts the left pitch/tilt cylinder72in step S103. At this time, the left lift stroke amount62S gradually becomes larger due to the retraction of the left pitch/tilt cylinder72(seeFIGS. 5A and 5B).

In step S104, the control device220determines whether the right lift stroke amount61S and the left lift stroke amount62S match while the left pitch/tilt cylinder72is being retracted. The control device220repeats steps S103and S104if the stroke amounts do not match, and the process advances to step S107when both stroke amounts match. At the point in time that both stroke amounts match, the stroke amount of the right pitch cylinder71and the stroke amount of the left pitch/tilt cylinder72match (seeFIG. 5B).

If it is determined in step S102that the left lift stroke amount62S is larger than the right lift stroke amount61S, the control device220extends the left pitch/tilt cylinder72in step S105. At this time, the left lift stroke amount62S gradually becomes smaller due to the extension of the left pitch/tilt cylinder72(seeFIGS. 5A and 5B).

In step S106, the control device220determines whether the right lift stroke amount61S and the left lift stroke amount62S match while the left pitch/tilt cylinder72is being extended. The control device220repeats steps S105and S106if the stroke amounts do not match, and the process advances to step S107when both stroke amounts match. At the point in time that both stroke amounts match, the stroke amount of the right pitch cylinder71and the stroke amount of the left pitch/tilt cylinder72match (seeFIG. 5B) in the same way as in step S104.

In step S107, the control device220starts contracting the right pitch cylinder71and the left pitch/tilt cylinder72at the same speed when the right lift stroke amount61S and the left lift stroke amount62S match in steps S102, S104, and S106. When the right pitch cylinder71and the left pitch/tilt cylinder72are retracted at the same speed, the right lift stroke amount61S and the left lift stroke amount62S gradually become larger while maintaining the state of matching (seeFIGS. 5B and 5C).

In step S108, the control device220determines whether the stroke difference ΔS of the right lift stroke amount61S and the left lift stroke amount62S is larger than the threshold TH1 (e.g., 3 mm). The control device220repeats the process in step S107if the stroke difference ΔS is not larger than the threshold TH1, and the process advances to step S109if the stroke difference ΔS is larger than the threshold TH1.

The stroke difference ΔS becomes larger than the threshold TH1 since the left lift stroke amount62S becomes longer than the right lift stroke amount61S due to the left pitch/tilt cylinder72continuing to retract even after the retraction of the right pitch cylinder71has stopped (seeFIG. 5C). The retraction of the left pitch/tilt cylinder72is continued even after the retraction of the right pitch cylinder71has stopped since the maximum stroke length of the right pitch cylinder71is shorter than the maximum stroke length of the left pitch/tilt cylinder72as described above (seeFIGS. 3C and 3D).

In step S109, the control device220stops the retraction of the right pitch cylinder71and the left pitch/tilt cylinder72when it is determined in step S108that the stroke difference ΔS is larger than the threshold TH1. As a result, the blade50enters the state of having a certain pitch in the front-back direction without being substantially tilted in the right-left direction. Since the length of the lift cylinder60is several meters and thus much longer than the threshold TH1 (e.g., 3 mm), the pair of lift cylinders60are deemed to have a parallel positional relationship even if there is a stroke difference ΔS of about the same amount as the threshold TH1. In this way, “the pair of lift cylinders60are parallel” in the present embodiment is a concept that includes the stroke difference ΔS being about the same as the threshold TH1 and does not only include the stroke difference ΔS being “0”.

In step S110, the control device220positions the rod61band the rod62bat the reference position by actuating the right lift cylinder61and the left lift cylinder62at the same speed. As a result, the blade50is moved to a certain height above the ground surface.

Accordingly, the blade attitude restore control for restoring the blade50to the reference position is complete.

Characteristics

The control device220according to the first embodiment actuates the right pitch cylinder71and the left pitch/tilt cylinder72at the same speed when the right lift stroke amount61S and the left lift stroke amount62S match each other. Since the maximum stroke length of the right pitch cylinder71is larger than the maximum stroke length of the left pitch/tilt cylinder72, the actuation of the left pitch/tilt cylinder72is able to continue even after the right pitch cylinder71has been stopped and thus the stroke difference ΔS becomes larger when only the left pitch/tilt cylinder72is actuated. The control device220stops the right pitch cylinder71and the left pitch/tilt cylinder72when it is determined that the stroke difference ΔS is larger than the threshold TH1.

Therefore, the blade50can be automatically returned to the reference attitude by actuating the pair of pitch/tilt cylinders70using only the pair of lift stroke sensors63. As a result, cutting edges51of the blade50excessively digging into the ground surface or being positioned too far away from the ground surface can be suppressed. Consequently, excavation work can be executed effectively.

Second Embodiment

The following is an explanation of a bulldozer according to a second embodiment. The differences between the second embodiment and the first embodiment lie in the configuration of the pair of pitch/tilt cylinders and in the blade attitude restoration method. Therefore, the differences between the first and second embodiments will be mainly discussed below.

Configuration of Pair of Pitch/Tilt Cylinders70A

A pair of pitch/tilt cylinders70A according to the second embodiment includes a right pitch cylinder73and a left pitch/tilt cylinder74.FIG. 7Ais a schematic view of a configuration of the right pitch cylinder73.FIG. 7Bis a schematic view of a configuration of the left pitch/tilt cylinder74. The states of each cylinder when the blade50is moved to the reference attitude are illustrated inFIGS. 7A and 7B.

As illustrated inFIGS. 7A and 7B, the right pitch cylinder73and the left pitch/tilt cylinder74have the same configurations. Specifically, a cylinder body73aof the right pitch cylinder73has the same length as a cylinder body74aof the left pitch/tilt cylinder74. As a result, the maximum stroke length in the cylinder body73ais the same as the maximum stroke length in the cylinder body74a.

A rod73bof the right pitch cylinder73has the same configuration as a rod74bof the left pitch/tilt cylinder74. The reference position of the rod73bof the right pitch cylinder73is set to be at the proximal end of the cylinder body73a. The reference position of the rod74bof the left pitch/tilt cylinder74is set to be at the proximal end of the cylinder body74ain line with the right pitch cylinder73.

The configuration of the left pitch/tilt cylinder74is the same as the configuration of the left pitch/tilt cylinder72according to the first embodiment. The only difference between both configurations is the reference position.

Operation of Control Device220A

The following is an explanation of the blade attitude restore control performed by the control device220A with reference to the drawings.FIG. 8is a flow chart for explaining the blade attitude restore control performed by the control device220A.FIGS. 9A to 9Dare schematic views illustrating the states of actuation of the pair of lift cylinders60and the pair of pitch/tilt cylinders70A.

In step S201, the control device220A obtains the detection values of the pair of lift stroke sensors63.

In step S202, the control device220A determines the magnitude relation between the right lift stroke amount61S and the left lift stroke amount62S on the basis of the detection values from the pair of lift stroke sensors63.

If it is determined in step S202that the right lift stroke amount61S is larger than the left lift stroke amount62S, the control device220retracts the left pitch/tilt cylinder74in step S203. At this time, the left lift stroke amount62S gradually becomes larger due to the retraction of the left pitch/tilt cylinder74(seeFIGS. 9A and 9B).

In step S204, the control device220A determines whether the right lift stroke amount61S and the left lift stroke amount62S match while the left pitch/tilt cylinder74is being retracted. The control device220A repeats steps S203and S204if the stroke amounts do not match, and the process advances to step S207when both stroke amounts match. At the point in time that both stroke amounts match, the stroke amount of the right pitch cylinder73and the stroke amount of the left pitch/tilt cylinder74match (seeFIG. 9B).

If it is determined in step S202that the left lift stroke amount62S is larger than the right lift stroke amount61S, the control device220A extends the left pitch/tilt cylinder74in step S205. At this time, the left lift stroke amount62S gradually becomes smaller due to the extension of the left pitch/tilt cylinder74.

In step S206, the control device220A determines whether the right lift stroke amount61S and the left lift stroke amount62S match while the left pitch/tilt cylinder74is being extended. The control device220A repeats steps S205and S206if the stroke amounts do not match, and the process advances to step S207when both stroke amounts match. At the point in time that both stroke amounts match, the stroke amount of the right pitch cylinder73and the stroke amount of the left pitch/tilt cylinder74match in the same way as in step S204(seeFIG. 9B).

In step S207, the control device220A contracts the right pitch cylinder73and the left pitch/tilt cylinder74at different speeds when the right lift stroke amount61S and the left lift stroke amount62S match in steps S202, S204, and S206. When the right pitch cylinder73and the left pitch/tilt cylinder74are retracted at different speeds, the difference between the matching right lift stroke amount61S and the left lift stroke amount62S gradually becomes larger (seeFIG. 9C).

In step S208, the control device220A determines whether the stroke difference ΔS between the right lift stroke amount61S and the left lift stroke amount62S is smaller than a threshold TH2 (e.g., 3 mm). The control device220repeats the process in step S207if the stroke difference ΔS is not smaller than the threshold TH2, and the process advances to step S209if the stroke difference ΔS is smaller than the threshold TH2.

The stroke difference ΔS becomes smaller than the threshold TH2 since the left lift stroke amount62S approaches the right lift stroke amount61S as illustrated inFIG. 9Cdue to the left pitch/tilt cylinder74continuing to retract even after the retraction of the right pitch cylinder73has stopped (seeFIG. 9D). The retraction of the left pitch/tilt cylinder74is continued even after the retraction of the right pitch cylinder73has stopped since the left pitch/tilt cylinder74is retracted more slowly than the right pitch cylinder73as described above.

In step S209, the control device220A stops the retraction of the right pitch cylinder73and the left pitch/tilt cylinder74when it is determined in step S208that the stroke difference ΔS is smaller than the threshold TH2. As a result, the blade50enters the state of having a certain pitch in the front-back direction without being substantially tilted in the right-left direction. Since the length of the lift cylinder60is several meters and thus much longer than the threshold TH2 (e.g., 3 mm) in the present embodiment, the pair of lift cylinders60are deemed to have a parallel positional relationship even if there is a stroke difference ΔS of about the same amount as the threshold TH2. In this way, “the pair of lift cylinders60are parallel” in the present embodiment is a concept that includes the stroke difference ΔS being about the same as the threshold TH2 and does not only include the stroke difference ΔS being “0”.

In step S210, the control device220A positions the rod61band the rod62bat the reference position by actuating the right lift cylinder61and the left lift cylinder62at the same speed. As a result, the blade50is raised to a certain height above the ground surface.

Accordingly, the blade attitude restore control for restoring the blade50to the reference position is complete.

Characteristics

The control device220A according to the second embodiment actuates the right pitch cylinder73and the left pitch/tilt cylinder74at different speeds when the right lift stroke amount61S and the left lift stroke amount62S match each other. Since the maximum stroke length of the right pitch cylinder73is the same as the maximum stroke length of the left pitch/tilt cylinder74, the stroke difference ΔS becomes smaller by actuating only the left pitch/tilt cylinder74after the right pitch cylinder73is stopped. The control device220A stops the right pitch cylinder73and the left pitch/tilt cylinder74when it is determined that the stroke difference ΔS is smaller than the threshold TH2.

Therefore, the blade50can be returned to the reference attitude by actuating the pair of pitch/tilt cylinders70A using only the pair of lift stroke sensors63. As a result, cutting edges51of the blade50excessively digging into the ground surface or being positioned too far away from the ground surface can be suppressed. Consequently, excavation work can be executed effectively.

Other Embodiments

While the present invention has been described with the embodiments provided above, the description and drawings form a portion of the disclosure and are not to be understood as limiting the invention. Various substitutions, embodiments, and operation techniques will be apparent to those skilled in the art.

(A) While the reference positions of the right pitch cylinders71and73in the above embodiments are set to the respective proximal ends of the cylinder bodies71aand73a, the reference positions are not limited as such. The reference positions of the right pitch cylinders71and73may be set to the distal ends of the respective cylinder bodies71aand73a. Specifically, the reference positions of the right pitch cylinders71and73may be set to either end of the respective cylinder bodies71aand73a. In this case, the retraction in steps S103, S105and S107inFIG. 6is replaced by extension, and the retraction in steps S203, S205and S207inFIG. 8is replaced by extension.

(B) While the reference positions of the right pitch cylinders71and73in the above embodiments are set to the respective proximal ends of the cylinder bodies71aand73a, the reference positions are not limited as such. The reference positions of the right pitch cylinders71and73may be set to a certain position between the distal ends and the proximal ends of the respective cylinder bodies71aand73a. In this case, a pitch action of a certain amount in the direction opposite that of the pitch action described in the flow charts may be performed after the completion of the flows of the above embodiments.

(C) While the bulldozer100has been exemplified as a work machine in the above embodiments, a motor grader and the like may be exemplified as the work machine.

(D) While the right pitch cylinder73and the left pitch/tilt cylinder74are retracted at different speeds in the above second embodiment, the second embodiment is not limited as such. The right pitch cylinder73and the left pitch/tilt cylinder74may be contracted at the same speed even if the maximum stroke lengths of the right pitch cylinder73and the left pitch/tilt cylinder74are the same. In this case, a pressure sensor may be provided for detecting the relief pressures of the right pitch cylinder73and the left pitch/tilt cylinder74so that the restoration of the right pitch cylinder73and the left pitch/tilt cylinder74to the reference position can be detected using the generation of relief pressure in both cylinders. Therefore, the control device220A in this case does not need to detect whether the stroke difference ΔS is smaller than the threshold TH2.