Road machine

A road machine includes a travel switch to cause the road machine to start traveling, a hydrostatic transmission including a traveling hydraulic pump and a traveling hydraulic motor, a pump regulator to control the discharge quantity of the traveling hydraulic pump; and a controller to control the pump regulator using a pump command current. The pump regulator increases the discharge quantity of the traveling hydraulic pump as the pump command current increases, and the controller increases the pump command current to a predetermined value smaller than a maximum current value in response to the travel switch being turned ON. Alternatively, the pump regulator increases the discharge quantity of the traveling hydraulic pump as the pump command current decreases, and the controller decreases the pump command current to a predetermined value greater than a minimum current value in response to the travel switch being turned ON.

BACKGROUND

Technical Field

The present invention relates to road machines with a traveling hydraulic circuit in which a hydrostatic transmission (HST) is adopted.

Description of Related Art

An asphalt finisher with a hill backward rolling prevention control circuit for preventing backward rolling at a hill start on a steep hill is known.

This asphalt finisher includes an HST formed of a single hydraulic pump and two hydraulic motors, an angle sensor configured to detect the inclination of the vehicle body in its front-rear direction, and a controller. Traveling wheels are connected one to each hydraulic motor through a speed reducer. The controller controls the discharge quantity of the hydraulic pump through a tilt regulator, controls the absorption quantity of the hydraulic motors through other tilt regulators, and controls the reduction ratio of the speed reducers through solenoid-operated directional control valves. In response to determining that the asphalt finisher is positioned on a steep uphill at the time of not working but moving and traveling, the controller increases the absorption quantity of the hydraulic motors and increases the reduction ratio of the speed reducers and thereafter gradually increases the discharge quantity of the hydraulic pump to gradually increase the rotational speed of the traveling wheels.

SUMMARY

According to an aspect of the present invention, a road machine includes a travel switch configured to cause the road machine to start traveling, a hydrostatic transmission including a traveling hydraulic pump and a traveling hydraulic motor, a pump regulator configured to control the discharge quantity of the traveling hydraulic pump, and a controller configured to control the pump regulator using a pump command current. The pump regulator is configured to increase the discharge quantity of the traveling hydraulic pump as the current value of the pump command current increases, and the controller is configured to increase the current value of the pump command current to a predetermined value smaller than a maximum current value in response to the travel switch being turned ON. Alternatively, the pump regulator is configured to increase the discharge quantity of the traveling hydraulic pump as the current value of the pump command current decreases, and the controller is configured to decrease the current value of the pump command current to a predetermined value greater than a minimum current value in response to the travel switch being turned ON.

DETAILED DESCRIPTION

Related art, however, is silent with respect to the temporal relationship between the timing of releasing the brake on the traveling wheels and the timing of starting the above-described gradual increase in the discharge quantity of the hydraulic pump. Therefore, in the case where the brake is released when the discharge quantity of the hydraulic pump is small, it may be impossible to prevent backward rolling at a hill start even when the absorption quantity of the hydraulic motors and the reduction ratio of the speed reducers are increased.

In view of the above, it is desired to provide a road machine that more reliably prevents backward rolling at a hill start.

According to an aspect of the present invention, a road machine that more reliably prevents backward rolling at a hill start is provided.

FIG. 1is a side view of an asphalt finisher100that is an example of a road machine according to an embodiment of the present invention. The asphalt finisher100includes a tractor1, a hopper2, and a screed unit3as main components.

The tractor1is a mechanism for causing the asphalt finisher100to travel. According to this embodiment, the tractor1rotates front wheels and rear wheels using traveling hydraulic motors to move the asphalt finisher100. The traveling hydraulic motors are supplied with hydraulic oil from a hydraulic source to rotate.

The hopper2is a mechanism for receiving a paving material. According to this embodiment, the hopper2is configured to be openable and closable in the directions of the vehicle width through hopper cylinders2a. The asphalt finisher100normally fully opens the hopper2to receive a paving material (such as an asphalt mixture) from the bed of a dump truck. The paving material received into the hopper2is fed to the screed unit3, using a conveyor and a screw.

The screed unit3is a mechanism for evenly spreading the paving material. According to this embodiment, the screed unit3is a floating screed unit towed by the tractor1, and is coupled to the tractor1through leveling arms3a.

Next, a hydraulic system installed in the asphalt finisher100ofFIG. 1is described with reference toFIG. 2.FIG. 2is a hydraulic circuit diagram illustrating a configuration of a hydraulic system installed in the asphalt finisher100ofFIG. 1.

The hydraulic system mainly includes a hydraulic source14, a rear wheel driving part F1, and a conveyor and screw driving part F2.

The hydraulic source14is a functional element to supply hydraulic oil for operating various hydraulic driving parts including the rear wheel driving part F1and the conveyor and screw driving part F2. According to this embodiment, the hydraulic source14mainly includes an engine14E, a rear wheel traveling pump14R, a charge pump14C, and a conveyor and screw pump14S.

The engine14E is a drive source to drive the rear wheel traveling pump14R, the charge pump14C, and the conveyor and screw pump14S.

The rear wheel traveling pump14R serving as a traveling hydraulic pump is a variable displacement hydraulic pump to supply the rear wheel driving part F1with hydraulic oil for driving. According to this embodiment, the rear wheel traveling pump14R is a swash-plate variable displacement bidirectional hydraulic pump used in a closed circuit (HST), and the discharge quantity of the rear wheel traveling pump14R is controlled by a pump regulator15. Technically, the discharge quantity is the quantity of discharge per pump revolution, and is also referred to as a geometric displacement.

The pump regulator15is a device to control the discharge quantity of the rear wheel traveling pump14R. According to this embodiment, the pump regulator15controls the discharge quantity of the rear wheel traveling pump14R in accordance with a pump command current from a controller30. For example, the pump regulator15increases the discharge quantity of the rear wheel traveling pump14R as the current value of the pump command current increases.

The charge pump14C is a fixed displacement hydraulic pump to supply the rear wheel driving part F1with hydraulic oil for controlling.

The conveyor and screw pump14S is a variable displacement hydraulic pump to supply the conveyor and screw driving part F2with hydraulic oil. According to this embodiment, the conveyor and screw pump14S is a swash-plate variable displacement hydraulic pump.

The rear wheel driving part F1is a functional element to drive rear wheels5L and5R. According to this embodiment, the rear wheel driving part F1includes a left rear wheel traveling motor20L, a right rear wheel traveling motor20R, check valves20La and20Ra, relief valves20Lb and20Rb, reduction ratio controllers21L and21R, and brake controllers22L and22R.

The left rear wheel traveling motor20L is a hydraulic motor to drive the left rear wheel5L. The right rear wheel traveling motor20R is a hydraulic motor to drive the right rear wheel5R. According to this embodiment, the left rear wheel traveling motor20L and the right rear wheel traveling motor20R are variable displacement hydraulic motors, and form a closed circuit (HST) together with the rear wheel traveling pump14R. The left rear wheel traveling motor20L and the right rear wheel traveling motor20R may alternatively be fixed displacement hydraulic motors.

The check valve20La maintains the pressure of hydraulic oil inside a conduit C1at or above a predetermined pressure, the conduit C1connecting a first port of the rear wheel traveling pump14R and respective second ports of the left rear wheel traveling motor20L and the right rear wheel traveling motor20R. Specifically, when the pressure of hydraulic oil inside the conduit C1falls below the discharge pressure of the charge pump14C, the check valve20La causes the hydraulic oil discharged by the charge pump14C to flow into the conduit C1. In the drawing, parenthesized numbers indicate port numbers. Likewise, the check valve20Ra maintains the pressure of hydraulic oil inside a conduit C2at or above a predetermined pressure, the conduit C2connecting a second port of the rear wheel traveling pump14R and respective first ports of the left rear wheel traveling motor20L and the right rear wheel traveling motor20R. Specifically, when the pressure of hydraulic oil inside the conduit C2falls below the discharge pressure of the charge pump14C, the check valve20Ra causes the hydraulic oil discharged by the charge pump14C to flow into the conduit C2.

The relief valve20Lb maintains the pressure of hydraulic oil inside the conduit C1below a predetermined relief pressure. Specifically, when the pressure of hydraulic oil inside the conduit C1exceeds the relief pressure, the relief valve20Lb causes the hydraulic oil inside the conduit C1to flow out of the closed circuit. Likewise, the relief valve20Rb maintains the pressure of hydraulic oil inside the conduit C2below a predetermined relief pressure. Specifically, when the pressure of hydraulic oil inside the conduit C2exceeds the relief pressure, the relief valve20Rb causes the hydraulic oil inside the conduit C2to flow out of the closed circuit.

The reduction ratio controller21L is a device to control the reduction ratio of a speed reducer coupled to the left rear wheel traveling motor20L. According to this embodiment, the reduction ratio controller21L controls the reduction ratio of the speed reducer coupled to the left rear wheel traveling motor20L, using the hydraulic oil discharged by the charge pump14C, in response to a control command from the controller30. The same is the case with the reduction ratio controller21R that controls the reduction ratio of a speed reducer coupled to the right rear wheel traveling motor20R.

The brake controller22L is a device to control the braking force of a left rear wheel brake that brakes the left rear wheel5L of the asphalt finisher100. According to this embodiment, the brake controller22L controls the braking force of the left rear wheel brake, using the hydraulic oil discharged by the charge pump14C, in response to a control command from the controller30. The same is the case with the brake controller22R that controls the braking force of a right rear wheel brake.

The conveyor and screw driving part F2is a functional element to drive a conveyor and a screw. According to this embodiment, the conveyor and screw driving part F2mainly includes a left screw motor42SL, a right screw motor42SR, a left conveyor motor42CL, a right conveyor motor42CR, and a conveyor and screw valve41.

Each of the left screw motor42SL, the right screw motor42SR, the left conveyor motor42CL, and the right conveyor motor42CR is a variable displacement hydraulic motor that forms an open circuit.

The conveyor and screw valve41includes a conveyor control valve and a screw control valve. The conveyor control valve switches in response to a control command from the controller30, and causes the hydraulic oil discharged by the conveyor and screw pump14S to flow into the intake port of at least one of the left conveyor motor42CL and the right conveyor motor42CR. Furthermore, the conveyor control valve discharges hydraulic oil flowing out of the discharge port of at least one of the left conveyor motor42CL and the right conveyor motor42CR to a hydraulic oil tank T. Likewise, the screw control valve switches in response to a control command from the controller30, and causes the hydraulic oil discharged by the conveyor and screw pump14S to flow into the intake port of at least one of the left screw motor42SL and the right screw motor42SR. Furthermore, the screw control valve discharges hydraulic oil flowing out of the discharge port of at least one of the left screw motor42SL and the right screw motor42SR to the hydraulic oil tank T.

Next, a travel control system50installed in the asphalt finisher100is described with reference toFIG. 3.

The travel control system50mainly includes a travel switch10, a velocity dial11, the controller30, the pump regulator15, the reduction ratio controllers21L and21R, and the brake controllers22L and22R.

The travel switch10is a switch for causing the asphalt finisher100to start traveling. According to this embodiment, the travel switch10is attached where the travel switch10is operable by an operator sitting in a cab installed on top of the tractor1.

The velocity dial11is a dial for setting a target travel velocity for the asphalt finisher100. According to this embodiment, like the travel switch10, the velocity dial11is attached where the velocity dial11is operable by the operator sitting in the cab installed on top of the tractor1.

The controller30is a control device to control the asphalt finisher100. According to this embodiment, the controller30is composed of a processing unit including a CPU and an internal memory. The CPU executes a program stored in the internal memory to implement various functions of the controller30.

The asphalt finisher100positioned on a slope moves backward when a forward rotation torque to rotate the rear wheel traveling motors20L and20R forward is smaller than a reverse rotation torque when the rear wheel brakes are released. The forward rotation torque is generated based on the discharge pressure of the rear wheel traveling pump14R, and the reverse rotation torque is generated based on the own weight of the asphalt finisher100. Therefore, the asphalt finisher100is likely to move backward if the discharge quantity of the rear wheel traveling pump14R is small when the asphalt finisher100starts to move.

Therefore, the controller30prevents the backward movement of the asphalt finisher100by increasing the discharge quantity of the rear wheel traveling pump14R when the asphalt finisher100starts to move.

An excessive increase in the discharge quantity of the rear wheel traveling pump14R at the start of movement, however, may cause a sudden start of the asphalt finisher100. Furthermore, if there is a delay in releasing the brakes, the forward rotation torque by the rear wheel traveling pump14R may act to cancel the braking torque by the brakes to damage the brakes. Accordingly, it is necessary to appropriately determine the size of an increase in the discharge quantity of the rear wheel traveling pump14R at the start of movement and the timing of releasing the brakes.

Specifically, the controller30executes various operations based on the outputs of the travel switch10and the velocity dial11, and outputs various control commands based on the results of the operations to the pump regulator15, the reduction ratio controllers21L and21R, and the brake controllers22L and22R.

When the travel switch10is turned ON, the controller30obtains a target travel velocity Vt set by the velocity dial11, and determines various parameters used for controlling the pump regulator15. The various parameters include a movement start current value Am, a movement start time D1, a current value increase rate α, a target current value At, etc.

The movement start current value Am is a current value at the time when the asphalt finisher100starts to move forward in the case of gradually increasing the current value of the pump command current to move the asphalt finisher100forward. For example, in the case where the asphalt finisher100positioned on a slope temporarily moves backward when starting to move, the movement start current value Am corresponds to a current value at the time when the asphalt finisher100returns to the initial position through a subsequent forward movement. The movement start current value Am may alternatively be a current value at the time when the backward movement stops.

The movement start time D1is a time taken before the current value of the pump command current reaches the movement start current value Am after starting to gradually increase to move the asphalt finisher100positioned on a slope forward.

The movement start current value Am and the movement start time D1, which are values determined in accordance with the gradient of a slope, the type of a road machine, etc., may be prestored in the internal memory, input through an input device that is not depicted, or dynamically calculated based on the output of an angle sensor attached to the tractor1to detect the inclination of the vehicle body in its front-rear direction.

The current value increase rate α is the value obtained by dividing the movement start current value Am by the movement start time D1, and corresponds to the traveling acceleration of the asphalt finisher100. That is, as the current value increase rate α increases, the traveling acceleration of the asphalt finisher100at the start of movement increases.

The target current value At is the current value of the pump command current corresponding to the target travel velocity Vt of the asphalt finisher100set by the velocity dial11. Furthermore, the target current value At means a maximum current value employable as the pump command current unless the setting of the velocity dial11is changed. Furthermore, the movement start current value Am is smaller than the target current value At serving as a maximum current value. That is, the maximum current value is a current value corresponding to a maximum horsepower, and the movement start current value Am is a current value corresponding to a horsepower smaller than the maximum horsepower.

Next, a process of controlling the traveling of the asphalt finisher100positioned on an uphill by the controller30(hereinafter referred to “travel control process”) is described with reference toFIG. 4.FIG. 4is a flowchart illustrating a travel control process. The controller30repeatedly executes this travel control process in a predetermined control cycle while the asphalt finisher100is in operation.

First, the controller30determines whether the travel switch10is turned ON (step ST1).

In response to determining that the travel switch10is not turned ON (NO at step ST1), the controller30ends a current travel control process.

In response to determining that the travel switch10is turned ON (YES at step ST1), the controller30outputs a brake release command to the brake controllers22L and22R, and at the same time, sets the current value of the pump command current to output to the pump regulator15to a predetermined value (the movement start current value Am) (step ST2).

Furthermore, the controller30may start driving the left screw motor42SL, the right screw motor42SR, the left conveyor motor42CL, and the right conveyor motor42CR in response to the travel switch10being turned ON.

Thereafter, the controller30determines whether a predetermined time (the movement start time D1) has passed since the travel switch10is turned ON (step ST3).

In response to determining that the movement start time D1has not passed (NO at step ST3), the controller30repeats the determination of step ST3until the movement start time D1passes.

In response to determining that the movement start time D1has passed (YES at step ST3), the controller30increases the current value of the pump command current (step ST4). According to this embodiment, the controller30increases the current value of the pump command current to a predetermined maximum current value (the target current value At) at a predetermined increase rate (the current value increase rate α). The current value increase rate α is a value determined by the movement start current value Am and the movement start time D1, and the target current value At is a value determined by the target travel velocity Vt set by the velocity dial11.

Next, a temporal transition of various physical quantities related to the travel control process is described with reference toFIGS. 5A through 5D. The various physical quantities are the output state of the travel switch10illustrated inFIG. 5A, the output state of the brake release command illustrated inFIG. 5B, the current value of the pump command current illustrated inFIG. 5C, and the travel velocity of the asphalt finisher100illustrated inFIG. 5D. The solid line ofFIGS. 5A through 5Dindicates a temporal transition in the case where the travel control process is executed to increase the pump command current stepwise to the movement start current value Am. The one-dot chain line ofFIGS. 5A through 5Dindicates a temporal transition in the case where the travel control process is not executed to gradually increase the pump command current.

Specifically, as illustrated inFIG. 5A, when the travel switch10is turned ON at time t0, the output state of the travel switch10becomes ON level. Furthermore, as illustrated inFIG. 5B, the output state of the brake release command as well becomes ON level.

Furthermore, as illustrated inFIG. 5C, the pump command current increases stepwise to the movement start current value Am at time t0. This makes it possible to sharply increase the discharge horsepower of the rear wheel traveling pump14R to a predetermined value. As a result, as illustrated inFIG. 5D, the travel speed of the asphalt finisher100starts to increase at time t1, and reaches the target travel velocity Vt at time t2. That is, the asphalt finisher100can move forward to move up a hill without moving backward at the start of movement.

In contrast, in the case where the travel control process is not executed, for example, as indicated by the one-dot chain line ofFIG. 5C, the pump command current starts to gradually increase at the current value increase rate α at time t0and reaches the movement start current value Am at time t1. As a result, as indicated by the one-dot chain line ofFIG. 5D, the travel velocity of the asphalt finisher100starts to fall to the negative side (backward movement side) at time t0. Then, after temporarily swinging to the negative side, the travel velocity of the asphalt finisher100starts to increase to the positive side (forward movement side) to return to a value 0 at time t1, and reaches the target travel velocity Vt at time t2. That is, the asphalt finisher100temporarily moves backward when starting to move, and thereafter, starts to move forward. The time between time t0and time t1corresponds to the time required for the pump command current to reach the movement start current value Am after starting to move from the value 0 (the movement start time D1).

Thus, the controller30increases the discharge quantity of the rear wheel traveling pump14R at the start of movement earlier. Therefore, the controller30increases the discharge pressure of the rear wheel traveling pump14R and thus a forward rotation torque to rotate the rear wheel traveling motors20L and20R forward at an earlier point, and causes the forward rotation torque to be greater than a reverse rotation torque due to the own weight of the asphalt finisher100. As a result, it is possible to prevent the asphalt finisher100positioned on an uphill from moving backward when starting the asphalt finisher100moving forward. Furthermore, it is possible to cause the asphalt finisher100to smoothly start traveling to prevent the degradation of the quality of a work surface.

A preferred embodiment of the present invention is described above. The present invention, however, is not limited to the above-described embodiment, and various modifications and replacements may be made to the above-described embodiment without departing from the scope of the present invention.

For example, according to the above-describe embodiment, the timing of increasing the current value of the pump command current stepwise to the movement start current value Am and the timing of outputting the brake release command are simultaneous. The timing of outputting the brake release command, however, may be later than the timing of increasing the pump command current. This is for more reliably causing the intake-side pressure of hydraulic oil of the rear wheel traveling motors20L and20R to be greater than or equal to a predetermined pressure before the brakes are completely released, namely, for more reliably causing a forward rotation torque by the rear wheel traveling pump14R to be greater than a reverse rotation torque before the brakes are completely released. There is a time difference between the timing of the output of the brake release command and the timing of the actual complete release of braking by the brakes. Therefore, the temporal relationship between the timing of increasing the pump command current and the timing of outputting the brake release command may be determined in consideration of this time difference. For example, if the time difference is significant, the timing of outputting the brake release command may be earlier than the timing of increasing the pump command current.

Furthermore,FIG. 5Cillustrates an example of control in the case of using the pump regulator15structured to increase the discharge quantity of the rear wheel traveling pump14R as the current value of the pump command current increases. The present invention, however, is not limited to this configuration. For example, a pump regulator structured to increase the discharge quantity of the rear wheel traveling pump14R as the current value of the pump command current decreases may alternatively be used. In this case, the controller30increases the pump command current toward a maximum current value corresponding to a minimum horsepower when it is desired to decrease the discharge quantity, and decreases the pump command current toward a minimum current value corresponding to a maximum horsepower when it is desired to increase the discharge quantity. Then, the controller30decreases the current value of the pump command current stepwise to the movement start current value Am (a predetermined value greater than the minimum current value) in response to the travel switch10being turned ON. That is, the minimum current value is a current value corresponding to the maximum horsepower, and the movement start current value Am is a current value corresponding to a horsepower smaller than the maximum horsepower.