Hydraulic drive device for cargo handling vehicle

A hydraulic drive device for cargo handling vehicle has: a tank; a pump for drawing in a hydraulic oil from the tank and supplying the hydraulic oil to hydraulic cylinders; an electric motor for driving the pump; a solenoid-controlled proportional valve disposed between an inlet port of the pump and a bottom chamber of an up-and-down hydraulic cylinder and configured to open with a valve travel depending upon a control input of a descent control of an up-and-down control member; a pressure compensation valve disposed between a branch portion located between the pump and the solenoid-controlled proportional valve and the tank and configured to open with a valve travel depending upon a pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve; and a valve disposed between the pressure compensation valve and the tank and configured to be switched between an open position and a close position.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a hydraulic drive device for cargo handling vehicle.

Related Background Art

A battery-powered forklift, which is one of cargo handling vehicles, is provided, for example, with a lift cylinder for moving a fork up and down, a tilt cylinder for tilting a mast, a hydraulic pump for supplying hydraulic oil to the lift cylinder and the tilt cylinder, and an electric motor for driving the hydraulic pump. The battery-powered forklift is sometimes subjected to cargo-handling regeneration as described below. During an operation of lowering a cargo with the fork, the hydraulic oil is returned from the lift cylinder to the hydraulic pump by making use of the weight of the cargo. This causes the hydraulic pump to drive the electric motor, whereby the electric motor comes to generate electric power. When the pressure of the return oil to the hydraulic pump is low, e.g., in a state in which there is no cargo on the fork (light load state), the hydraulic pump needs to be rotated by the electric motor in order to achieve a desired descent speed. In this case, the electric motor consumes power.

There is the hydraulic system described in Japanese Patent Application Laid-Open Publication No. 2-231398, which offers the technology for controlling the consumption of power. The hydraulic system described in the Publication No. 2-231398 is provided with a lift control valve arranged between a bottom chamber of the lift cylinder and the hydraulic pump, and a pilot-controlled directional control valve disposed on a branch path between the lift control valve and a tank-side passage of the hydraulic pump and configured to be switched between an open position and a close position. When the fork descends with a heavy load, the pressure of the return oil from the lift cylinder exceeds a pilot setting pressure. For this reason, the pilot-controlled directional control valve is switched to the close position, so as to forcibly feed the return oil to the hydraulic pump. When the fork descends in a state without load or with an extremely low load, the pressure of the return oil from the lift cylinder does not reach the pilot setting pressure. For this reason, the pilot-controlled directional control valve is kept at the open position and the low-pressure return oil does not flow to the hydraulic pump. The electric motor is held at a standstill.

SUMMARY OF THE INVENTION

Technical Problem

However, the above-described conventional technology has the following problem. Namely, during a single descent operation of the fork (object), even if the fork is loaded with such a cargo that the pressure of the return oil from the lift cylinder to the hydraulic pump is one enough to implement the regeneration operation of the hydraulic pump (electric motor) in a low descent speed state of the object, the pressure of the return oil will be a pressure that cannot implement the regeneration operation of the hydraulic pump (electric motor), in a high descent speed state of the object. In this situation as well, the pilot-controlled directional control valve is not opened and thus the return oil from the lift cylinder flows to the hydraulic pump. Therefore, the electric motor is driven with supply of power to achieve a desired descent speed, resulting in consumption of power.

An object of the present invention is to provide a hydraulic drive device for cargo handling vehicle capable of efficiently performing the cargo-handling regeneration in a high load state and ensuring a necessary descent speed with low power consumption in a low load state, in the single descent operation of the object.

One aspect of the present invention is a hydraulic drive device for cargo handling vehicle with a plurality of hydraulic cylinders including an up-and-down hydraulic cylinder for moving an up and down with supply and discharge of hydraulic oil, and a plurality of manual control means including an up-and-down control means for actuating the up-and-down hydraulic cylinder, the hydraulic drive device comprising: a tank reserving the hydraulic oil; a hydraulic pump for drawing in the hydraulic oil from the tank and supplying the hydraulic oil to the hydraulic cylinders; an electric motor for driving the hydraulic pump; a solenoid-controlled proportional valve disposed between an inlet port of the hydraulic pump and a bottom chamber of the up-and-down hydraulic cylinder and configured to open with a valve travel depending upon a control input of a descent control of the up-and-down control means; a pressure compensation valve disposed between a branch portion located between the hydraulic pump and the solenoid-controlled proportional valve and the tank and configured to open with a valve travel depending upon a pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve; and valve means disposed between the pressure compensation valve and the tank and configured to be switched between an open position and a close position.

In the foregoing aspect of the present invention, the solenoid-controlled proportional valve opens with the valve travel depending upon the control input of the up-and-down control means when the descent control of the up-and-down control means is performed, in a single descent operation of the object. At this time, the pressure difference between the pressures upstream and downstream of the solenoid-controlled proportional valve becomes large, for example, in a high load state in which there is a cargo on the object, and, therefore, the pressure compensation valve is switched toward the close position. If the valve means is switched to the close position here, there is no hydraulic oil returning from the up-and-down hydraulic cylinder to the tank and the whole hydraulic oil from the up-and-down hydraulic cylinder flows to the hydraulic pump. This enables efficient implementation of the cargo-handling regeneration. Since the pressure difference between the pressures upstream and downstream of the solenoid-controlled proportional valve becomes small, for example, in a low load state in which there is no cargo on the object, the pressure compensation valve is switched toward the open position. When the valve means is switched to the open position here, most of the hydraulic oil from the up-and-down hydraulic cylinder returns to the tank. This ensures a necessary descent speed of the object. Since there is little hydraulic oil flowing to the hydraulic pump, the electric motor is prevented from being driven with supply of power, which can decrease power consumption.

In the foregoing aspect of the present invention, when the descent operation of the object and another cargo handling operation are performed simultaneously, the valve means is switched to the open position. At this time, a flow rate of the hydraulic oil flowing to the hydraulic pump varies depending upon the cargo weight or load condition on the object. With a decrease of the flow rate of the hydraulic oil flowing to the hydraulic pump, the valve travel of the pressure compensation valve increases so as to increase the hydraulic oil returning to the tank by the decrease of the flow rate. With an increase of the flow rate of the hydraulic oil flowing to the hydraulic pump, the valve travel of the pressure compensation valve decreases so as to decrease the hydraulic oil returning to the tank by the increase of the flow rate. Through these operations, the flow rate of the hydraulic oil flowing from the bottom chamber of the up-and-down hydraulic cylinder is kept substantially constant. Therefore, the descent speed of the object can be kept substantially constant. Since it is possible to perform another cargo handling operation except for the descent of the object by making use of regenerative energy, the power consumption can be reduced.

In the foregoing aspect of the present invention, the hydraulic drive device may be configured as follows: the hydraulic drive device further comprises: setting means for setting a command rotation rate of the electric motor; rotation rate detecting means for detecting an actual rotation rate of the electric motor; electric motor controlling means for controlling the electric motor, based on the command rotation rate set by the setting means and the actual rotation rate detected by the rotation rate detecting means; and determining means for determining whether the descent control of the up-and-down control means is performed singly or controls of the plurality of manual control means including the descent control of the up-and-down control means are performed simultaneously; when the determining means determines that the descent control of the up-and-down control means is performed singly, the setting means sets the command rotation rate depending upon the control input of the descent control of the up-and-down control means; when the determining means determines that the controls of the plurality of manual control means including the descent control of the up-and-down control means are performed simultaneously, the setting means sets the command rotation rate depending upon a control input of the manual control means other than the up-and-down control means.

When the descent operation of the object and another cargo handling operation are carried out simultaneously, the setting means sets the command rotation rate depending upon the control input of the manual control means other than the up-and-down control means. This prevents the hydraulic oil from being supplied more than necessary to the hydraulic cylinder other than the up-and-down hydraulic cylinder, even in a control such that the up-and-down control means requires the rotation rate of the electric motor higher than the other manual control means does. For this reason, increase of loss can be inhibited.

In the foregoing aspect of the present invention, the hydraulic drive device may be configured as follows: the hydraulic drive device further comprises valve opening and closing controlling means for controlling the valve means in the following manner: when the determining means determines that the descent control of the up-and-down control means is performed singly, the valve opening and closing controlling means controls the valve means so as to be switched to the open position if a difference between the command rotation rate set by the setting means and the actual rotation rate detected by the rotation rate detecting means is not less than a predetermined value; when the determining means determines that the controls of the plurality of manual control means including the descent control of the up-and-down control means are performed simultaneously, the valve opening and closing controlling means controls the valve means so as to be switched to the open position.

Since in the single descent operation of the object the determination on switching of opening and closing of the valve means is made depending upon the difference between the command rotation rate of the electric motor and the actual rotation rate of the electric motor, there is no need for a pressure sensor or the like for control of the valve means. In simultaneous execution of the descent operation of the object and another cargo handling operation, the valve means is switched to the open position, whereby the hydraulic oil from the up-and-down hydraulic cylinder returns to the tank. Therefore, variation in the descent speed of the object can be suppressed even in the case where the other cargo handling operation is carried out during the descent operation of the object.

In the foregoing aspect of the present invention, the hydraulic drive device may be configured as follows: the hydraulic drive device further comprises torque limit controlling means operating in the following manner: when the determining means determines that the descent control of the up-and-down control means is performed singly, the torque limit controlling means imposes a limit on a power drive torque of the electric motor; when the determining means determines that the controls of the plurality of manual control means including the descent control of the up-and-down control means are performed simultaneously, the torque limit controlling means removes the limit on the power drive torque of the electric motor.

In the single descent operation of the object the power drive torque of the electric motor is limited, which can prevent power consumption more than necessary. In simultaneous execution of the descent operation of the object and another cargo handling operation, the limit on the power drive torque of the electric motor is removed, which can surely achieve the rotation rate of the electric motor necessary for the other cargo handling operation.

In the foregoing aspect of the present invention, the hydraulic drive device may be configured as follows: the hydraulic drive device further comprises running direction detecting means for detecting a running direction of the cargo handling vehicle; the torque limit controlling means has: first power drive torque limit value setting means for setting a power drive torque limit value of the electric motor to a predetermined value when the determining means determines that the descent control of the up-and-down control means is performed singly, in a state in which the running direction detecting means detects the running direction of the cargo handling vehicle being neutral; and second power drive torque limit value setting means for setting the power drive torque limit value of the electric motor, based on the actual rotation rate detected by the rotation rate detecting means and based on an idle rotation rate of the hydraulic pump or a target rotation rate corresponding to a rotation rate higher than the idle rotation rate, when the determining means determines that the descent control of the up-and-down control means is performed singly, in a state in which the running direction detecting means detects the running direction of the cargo handling vehicle being forward or backward.

For example, in the case of the cargo handling vehicle equipped with hydraulic power steering, the hydraulic pump needs to be kept rotating at a rotation rate not less than the idle rotation rate for power steering, during a forward or backward moving state. When the single descent operation of the object is performed in the state in which the running direction of the cargo handling vehicle is forward or backward, the power drive torque limit value of the electric motor is set based on the actual rotation rate of the electric motor and the target rotation rate. For example, the power drive torque limit value of the electric motor is set so that the rotation rate of the hydraulic pump becomes not less than the idle rotation rate. By this setting, the hydraulic pump rotates at the rotation rate not less than the idle rotation rate in the state in which the running direction of the cargo handling vehicle is forward or backward, which ensures smooth steering during the single descent operation of the object. When the rotation rate of the hydraulic pump is the idle rotation rate, the hydraulic pump rotates at the necessary minimum rotation rate, which reduces power consumption.

In the foregoing aspect of the present invention, the hydraulic drive device may be configured as follows: the second power drive torque limit value setting means sets the power drive torque limit value of the electric motor in the following manner: when the actual rotation rate is not less than the target rotation rate, the second power drive torque limit value setting means sets the power drive torque limit value of the electric motor to a first setting value; when the actual rotation rate is less than the target rotation rate, the second power drive torque limit value setting means sets the power drive torque limit value of the electric motor to a second setting value larger than the first setting value.

In this case, the actual rotation rate of the electric motor becomes closer to the target rotation rate. For this reason, the hydraulic pump can be made to rotate at the rotation rate not less than the idle rotation rate, while simplifying a processing configuration of the second power drive torque limit value setting means.

In the foregoing aspect of the present invention, the hydraulic drive device may be configured as follows: the second power drive torque limit value setting means calculates a rotation rate deviation between the target rotation rate and the actual rotation rate and sets the power drive torque limit value of the electric motor so as to make the rotation rate deviation zero.

In this case, there occurs little pulsation of the actual rotation rate of the electric motor and thus the actual rotation rate of the electric motor becomes substantially coincident with the target rotation rate. For this reason, the hydraulic pump can be made to rotate certainly at the rotation rate not less than the idle rotation rate.

Another aspect of the present invention is a hydraulic drive device for cargo handling vehicle provided with a plurality of hydraulic cylinders including an up-and-down hydraulic cylinder for moving an object up and down with supply and discharge of hydraulic oil, and a plurality of manual control members including an up-and-down control lever configured so as to actuate the up-and-down hydraulic cylinder, the hydraulic drive device comprising: a tank reserving the hydraulic oil; a hydraulic pump having an inlet port for drawing in the hydraulic oil and an outlet port for discharging the hydraulic oil: an electric motor for driving the hydraulic pump; a first oil passage connecting the tank and the inlet port; a second oil passage connecting the outlet port and the plurality of hydraulic cylinders; a third oil passage branching off from the first oil passage and connecting the first oil passage and a bottom chamber of the up-and-down hydraulic cylinder; a solenoid-controlled proportional valve disposed on the third oil passage and configured to open with a valve travel depending upon a control input of a descent control of the up-and-down control lever; a pressure compensation valve disposed between a branch portion of the third oil passage and the tank on the first oil passage and configured to open with a valve travel depending upon a pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve on the third oil passage; a directional control valve disposed between the pressure compensation valve and the tank on the first oil passage and configured to be switched between an open position and a close position; and a controller configured to control the solenoid-controlled proportional valve and the directional control valve.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below in detail with reference to the drawings. In the drawings identical or equivalent elements will be denoted by the same reference signs, without redundant description.

FIG. 1is a side view showing a cargo handling vehicle with a hydraulic drive device according to the first embodiment. In the same drawing, the cargo handling vehicle1according to the present embodiment is a battery-powered forklift. The forklift1is provided with a body frame2and a mast3arranged in the front part of the body frame2. The mast3consists of a right and left pair of outer masts3aand inner masts3b. Each outer mast3ais supported on the body frame2so as to be tiltable. Each inner mast3bis arranged inside the outer mast3aand is movable up and down relative to the outer mast3a.

A lift cylinder4as an up-and-down hydraulic cylinder is arranged on the back of the mast3. The top end of a piston rod4pof the lift cylinder4is coupled to the upper part of the inner masts3b.

A lift bracket5is supported on the inner masts3bso as to be movable up and down. A fork (up-and-down object)6to carry a cargo is attached to the lift bracket5. A chain wheel7is provided in the upper part of the inner masts3band a chain8is hooked on the chain wheel7. One end of the chain8is coupled to the lift cylinder4and the other end of the chain8to the lift bracket5. With extension and contraction of the lift cylinder4, the fork6moves up and down along with the lift bracket5through the chain8.

Tilt cylinders9as tilt hydraulic cylinders are supported on the right and left sides of the body frame2, respectively. The tip of a piston rod9bof each tilt cylinder9is rotatably coupled to a nearly central part in the height direction of the outer mast3a. With extension and contraction of the tilt cylinders9, the mast3tilts.

A driver's cabin10is provided on the top of the body frame2. In the front part of the driver's cabin10the vehicle is provided with a lift control lever11for actuating the lift cylinder4to move the fork6up and down, and a tilt control lever12for actuating the tilt cylinders9to tilt the mast3.

A steering mechanism13for steering is provided in the front part of the driver's cabin10. The steering mechanism13is a hydraulic power steering mechanism. Namely, the steering mechanism13can assist driver's steering action by a PS (Power Steering) cylinder14(cf.FIG. 2) as a hydraulic cylinder for power steering (PS).

The forklift1has an attachment cylinder15(cf.FIG. 2) as an attachment hydraulic cylinder to operate an attachment (not shown). The attachment is, for example, a mechanism for laterally moving, inclining, or rotating the fork6, or the like. The driver's cabin10is provided with an attachment control lever (not shown) for actuating the attachment cylinder15to operate the attachment.

The driver's cabin10is provided with a direction switch for switching a running direction of the forklift1(forward, backward, or neutral), which is not shown in particular.

FIG. 2is a hydraulic circuit diagram showing the hydraulic drive device according to the first embodiment. In the same drawing, the hydraulic drive device16of the present embodiment drives the lift cylinder4, tilt cylinders9, attachment cylinder15, and PS cylinder14.

The hydraulic drive device16has a single hydraulic pump motor17and a single electric motor18for driving the hydraulic pump motor17. The hydraulic pump motor17has an inlet port17afor drawing in the hydraulic oil and an outlet port17bfor discharging the hydraulic oil. The hydraulic pump motor17is configured so as to be rotatable in one direction.

The electric motor18functions as an electric motor or as an electric generator. Specifically, when the hydraulic pump motor17operates as a hydraulic pump, the electric motor18functions as an electric motor; when the hydraulic pump motor17operates as a hydraulic motor, the electric motor18functions as an electric generator. While the electric motor18functions as an electric generator, electricity generated by the electric motor18is stored in a battery (not shown). Namely, regeneration operation is carried out.

A tank19reserving the hydraulic oil is connected through a hydraulic pipe20to the inlet port17aof the hydraulic pump motor17. The hydraulic pipe20is provided with a check valve21for allowing the hydraulic oil to flow only in the direction from the tank19to the hydraulic pump motor17.

The outlet port17bof the hydraulic pump motor17and a bottom chamber4bof the lift cylinder4are connected through a hydraulic pipe22. A solenoid-controlled proportional valve23for lift ascent is disposed on the hydraulic pipe22. The solenoid-controlled proportional valve23is switched between an open position23aand a close position23b. At the open position23a, the solenoid-controlled proportional valve23allows the hydraulic oil to flow from the hydraulic pump motor17to the bottom chamber4bof the lift cylinder4. At the close position23b, the solenoid-controlled proportional valve23interrupts the flow of the hydraulic oil from the hydraulic pump motor17to the bottom chamber4bof the lift cylinder4.

The solenoid-controlled proportional valve23is normally at the close position23b(as shown). When a control signal (solenoid current command value for lift ascent depending upon a control input of an ascent control of the lift control lever11) is fed to a solenoid control unit23c, the solenoid-controlled proportional valve23is switched to the open position23a. Then, the hydraulic oil is supplied from the hydraulic pump motor17to the bottom chamber4bof the lift cylinder4, so as to extend the lift cylinder4. This operation results in raising the fork6. When the solenoid-controlled proportional valve23is at the open position23a, the solenoid-controlled proportional valve23opens with a valve travel according to the control signal. A check valve24for allowing the hydraulic oil to flow only in the direction from the solenoid-controlled proportional valve23to the lift cylinder4is provided between the solenoid-controlled proportional valve23and the lift cylinder4on the hydraulic pipe22.

A solenoid-controlled proportional valve26for tilt is connected through a hydraulic pipe25to a branch portion located between the hydraulic pump motor17and the solenoid-controlled proportional valve23on the hydraulic pipe22. The hydraulic pipe25is provided with a check valve27for allowing the hydraulic oil to flow only in the direction from the hydraulic pump motor17to the solenoid-controlled proportional valve26.

The solenoid-controlled proportional valve26is connected to rod chambers9aand bottom chambers9bof the tilt cylinders9through hydraulic pipes28,29, respectively. The solenoid-controlled proportional valve26is switched among an open position26a, an open position26b, and a close position26c. At the open position26a, the solenoid-controlled proportional valve26allows the hydraulic oil to flow from the hydraulic pump motor17to the rod chambers9aof the tilt cylinders9. At the open position26b, the solenoid-controlled proportional valve26allows the hydraulic oil to flow from the hydraulic pump motor17to the bottom chambers9bof the tilt cylinders9. At the close position26c, the solenoid-controlled proportional valve26interrupts the flow of the hydraulic oil from the hydraulic pump motor17to the tilt cylinders9.

The solenoid-controlled proportional valve26is normally at the close position26c(as shown). When a control signal (solenoid current command value for tilt depending upon a control input of a backward tilt control of the tilt control lever12) is fed to a solenoid control unit26don the open position26aside, the solenoid-controlled proportional valve26is switched to the open position26a. When a control signal (solenoid current command value for tilt depending upon a control input of a forward tilt control of the tilt control lever12) is fed to a solenoid control unit26eon the open position26bside, the solenoid-controlled proportional valve26is switched to the open position26b. When the solenoid-controlled proportional valve26is switched to the open position26a, the hydraulic oil is supplied from the hydraulic pump motor17to the rod chambers9aof the tilt cylinders9, so as to contract the tilt cylinders9. This operation results in inclining the mast3backward. When the solenoid-controlled proportional valve26is switched to the open position26b, the hydraulic oil is supplied from the hydraulic pump motor17to the bottom chambers9bof the tilt cylinders9, so as to extend the tilt cylinders9. This operation results in inclining the mast3forward. When the solenoid-controlled proportional valve26is at the open position26aor26b, the solenoid-controlled proportional valve26opens with a valve travel according to the control signal.

A solenoid-controlled proportional valve31for attachment is connected through a hydraulic pipe30to an upstream point of the check valve27on the hydraulic pipe25. The hydraulic pipe30is provided with a check valve32for allowing the hydraulic oil to flow only in the direction from the hydraulic pump motor17to the solenoid-controlled proportional valve31.

The solenoid-controlled proportional valve31is connected to a rod chamber15aand a bottom chamber15bof the attachment cylinder15through hydraulic pipes33,34, respectively. The solenoid-controlled proportional valve31is switched among an open position31a, an open position31b, and a close position31c. At the open position31a, the solenoid-controlled proportional valve31allows the hydraulic oil to flow from the hydraulic pump motor17to the rod chamber15aof the attachment cylinder15. At the open position31b, the solenoid-controlled proportional valve31allows the hydraulic oil to flow from the hydraulic pump motor17to the bottom chamber15bof the attachment cylinder15. At the close position31c, the solenoid-controlled proportional valve31interrupts the flow of the hydraulic oil from the hydraulic pump motor17to the attachment cylinder15.

The solenoid-controlled proportional valve31is normally at the close position31c(as shown). When a control signal (solenoid current command value for attachment depending upon a control input of a one-side control of the attachment control lever) is fed to a solenoid control unit31don the open position31aside, the solenoid-controlled proportional valve31is switched to the open position31a. When a control signal (solenoid current command value for attachment depending upon a control input of the other-side control of the attachment control lever) is fed to a solenoid control unit31eon the open position31bside, the solenoid-controlled proportional valve31is switched to the open position31b. The operation of the attachment cylinder15is omitted from the description herein. When the solenoid-controlled proportional valve31is at the open position31aor31b, the solenoid-controlled proportional valve31opens with a valve travel according to the control signal.

A solenoid-controlled proportional valve36for PS is connected through a hydraulic pipe35to an upstream point of the check valve32on the hydraulic pipe30. The hydraulic pipe35is provided with a check valve37for allowing the hydraulic oil to flow only in the direction from the hydraulic pump motor17to the solenoid-controlled proportional valve36.

The solenoid-controlled proportional valve36is connected to a rod chamber14aand a bottom chamber14bof the PS cylinder14through hydraulic pipes38,39, respectively. The solenoid-controlled proportional valve36is switched among an open position36a, an open position36b, and a close position36c. At the open position36a, the solenoid-controlled proportional valve36allows the hydraulic oil to flow from the hydraulic pump motor17to the rod chamber14aof the PS cylinder14. At the open position36b, the solenoid-controlled proportional valve36allows the hydraulic oil to flow from the hydraulic pump motor17to the bottom chamber14bof the PS cylinder14. At the close position36c, the solenoid-controlled proportional valve36interrupts the flow of the hydraulic oil from the hydraulic pump motor17to the PS cylinder14.

The solenoid-controlled proportional valve36is normally at the close position36c(as shown). When a control signal (solenoid current command value for PS depending upon a control speed to either of the right and the left of the steering wheel of the steering mechanism13) is fed to a solenoid control unit36don the open position36aside, the solenoid-controlled proportional valve36is switched to the open position36a. When a control signal (solenoid current command value for PS depending upon a control speed to the other of the right and the left of the steering wheel of the steering mechanism13) is fed to a solenoid control unit36eon the open position36bside, the solenoid-controlled proportional valve36is switched to the open position36b. The operation of the PS cylinder14is omitted from the description herein. When the solenoid-controlled proportional valve36is at the open position36aor36b, the solenoid-controlled proportional valve36opens with a valve travel according to the control signal.

A branch portion located between the hydraulic pump motor17and the solenoid-controlled proportional valve23on the hydraulic pipe22is connected through a hydraulic pipe40to the tank19. The hydraulic pipe40is provided with an unloading valve41and a filter42. The solenoid-controlled proportional valves26,31, and36are connected through respective hydraulic pipes43-45to the hydraulic pipe40. The solenoid-controlled proportional valves23,26,31, and36are connected through a hydraulic pipe46to the hydraulic pipe40.

The inlet port17aof the hydraulic pump motor17and the bottom chamber4bof the lift cylinder4are connected through a hydraulic pipe47. The hydraulic pipe47is provided with a solenoid-controlled proportional valve48for lift descent. The solenoid-controlled proportional valve48is switched between an open position48aand a close position48b. At the open position48a, the solenoid-controlled proportional valve48allows the hydraulic oil to flow from the bottom chamber4bof the lift cylinder4to the inlet port17aof the hydraulic pump motor17. At the close position48b, the solenoid-controlled proportional valve48interrupts the flow of the hydraulic oil from the bottom chamber4bof the lift cylinder4to the inlet port17aof the hydraulic pump motor17.

The solenoid-controlled proportional valve48is normally at the close position48b(as shown). When a control signal (solenoid current command value for lift descent depending upon a control input of a descent control of the lift control lever11) is fed to a solenoid control unit48c, the solenoid-controlled proportional valve48is switched to the open position48a. Then, the fork6descends because of its own weight and the lift cylinder4contracts with the descent of the fork6. This causes the hydraulic oil to flow out from the bottom chamber4bof the lift cylinder4. The solenoid-controlled proportional valve48, when being at the open position48a, opens with a valve travel according to the control signal.

A branch portion located between the hydraulic pump motor17and the solenoid-controlled proportional valve48on the hydraulic pipe47is connected through a hydraulic pipe49to the tank19. The hydraulic pipe49is provided with a pressure compensation valve50. The pressure compensation valve50is a flow control valve50with a pressure compensation function.

The pressure compensation valve50is switched among an open position50a, a close position50b, and a throttle position50c. At the open position50a, the pressure compensation valve50allows the hydraulic oil to flow. At the close position50b, the pressure compensation valve50interrupts the flow of the hydraulic oil. At the throttle position50c, the pressure compensation valve50regulates the flow rate of the hydraulic oil. A pilot control unit on the close position50bside of the pressure compensation valve50and the upstream side (front side) of the solenoid-controlled proportional valve48are connected through a pilot flow passage51. A pilot control unit on the open position50aside of the pressure compensation valve50and the downstream side (rear side) of the solenoid-controlled proportional valve48are connected through a pilot flow passage52. The pressure compensation valve50opens with a valve travel depending upon a pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve48. Specifically, the pressure compensation valve50is normally at the close position (as shown). As the pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve48increases, the valve travel of the pressure compensation valve50becomes smaller.

A solenoid-controlled directional control valve (valve means)53for bypass is provided between the pressure compensation valve50and the tank19on the hydraulic pipe49. The solenoid-controlled directional control valve53is an on-off valve and is switched between an open position53aand a close position53b. At the open position53a, the solenoid-controlled directional control valve53allows the hydraulic oil to flow. At the close position53b, the solenoid-controlled directional control valve53interrupts the flow of the hydraulic oil. The solenoid-controlled directional control valve53is normally at the close position53b(as shown). The solenoid-controlled directional control valve53is switched to the open position53awith input of an ON signal to a solenoid control unit53c. The hydraulic pipe49is provided with a filter54. Each of the hydraulic pipes20,22,25,28-30,33-35,38-40,43-47, and49constitutes an oil passage.

FIG. 3is a configuration diagram showing a control system of the hydraulic drive device16. In the same drawing, the hydraulic drive device16has a lift control lever control input sensor55for detecting a control input of the lift control lever11, a tilt control lever control input sensor56for detecting a control input of the tilt control lever12, an attachment control lever control input sensor57for detecting a control input of the attachment control lever (not shown), a steering control speed sensor58for detecting a control speed of the steering wheel of the steering mechanism13, a rotation rate sensor (rotation rate detecting means)59for detecting an actual rotation rate of the electric motor18(motor actual rotation rate), and a controller60.

The controller60receives detected values from the control lever control input sensors55-57, steering control speed sensor58, and rotation rate sensor59and performs predetermined processing to control the electric motor18, solenoid-controlled proportional valves23,26,31,36,48, and solenoid-controlled directional control valve53. Namely, the controller60is configured to control the electric motor18, each of the solenoid-controlled proportional valves23,26,31,36,48, and the solenoid-controlled directional control valve53. The controller60is an electronic control unit that generally controls the hydraulic drive device16. The controller60has a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and so on, and performs various controls by loading a program stored in the ROM, onto the RAM and executing the program in the CPU.

FIG. 4is a flowchart showing a control process procedure executed by the controller60. This control process is focused on only an operation involving the descent of the fork6(lift descent). The period of execution of this control process is appropriately determined by experiment or the like.

First, the controller60acquires the respective control inputs of the lift control lever11, tilt control lever12, and attachment control lever detected by the control lever control input sensors55-57, and the control speed of the steering wheel of the steering mechanism13detected by the steering control speed sensor58(S101).

Subsequently, the controller60determines a lift descent mode as a control condition, based on the respective control inputs of the lift control lever11, tilt control lever12, and attachment control lever and the control speed of the steering wheel acquired by the process in S101(S102). Lift descent modes include the following controls: lift descent single control, lift descent+tilt control, lift descent+attachment control, lift descent+power steering (PS) control, and, lift descent+tilt+power steering (PS) control.

Next, the controller60determines solenoid-controlled proportional valve solenoid current command values depending upon the respective control inputs of the lift control lever11, tilt control lever12, and attachment control lever and the control speed of the steering13acquired by the process in S101and the determined lift descent mode (S103). The solenoid-controlled proportional valve solenoid current command values include a lift descent solenoid current command value depending upon the control input of the descent control of the lift control lever11, a tilt solenoid current command value depending upon the control input of the tilt control lever12, an attachment solenoid current command value depending upon the control input of the attachment control lever, and a power steering (PS) solenoid current command value depending upon the control speed of the steering13.

After that, the controller60obtains a motor necessary rotation rate corresponding to the control condition determined by the process in S102(S104). Motor necessary rotation rates include a lift necessary motor rotation rate, a tilt necessary motor rotation rate, an attachment necessary motor rotation rate, and a power steering (PS) necessary motor rotation rate. The lift necessary motor rotation rate is a rotation rate of the electric motor18necessary for execution of the lift operation. The tilt necessary motor rotation rate is a rotation rate of the electric motor18necessary for execution of the tilt operation. The attachment necessary motor rotation rate is a rotation rate of the electric motor18necessary for execution of the attachment operation. The PS necessary motor rotation rate is a rotation rate of the electric motor18necessary for execution of the PS operation.

Next, the controller60sets a motor rotation rate command value (motor command rotation rate), based on the lift descent mode determined by the process in S102and the motor necessary rotation rate obtained by the process in S104(S105). At this time, the motor command rotation rate is set as shown inFIG. 5. Specifically, in the case of lift descent single control, the controller60sets the motor command rotation rate to lift necessary motor rotation rate N_lift. In the case of lift descent+tilt control, the controller60sets the motor command rotation rate to tilt necessary motor rotation rate N_tilt. In the case of lift descent+attachment control, the controller60sets the motor command rotation rate to attachment necessary motor rotation rate N_atmt. In the case of lift descent+power steering control, the controller60sets the motor command rotation rate to PS necessary motor rotation rate N_ps. In the case of lift descent+tilt+power steering control, the controller60sets the motor command rotation rate to a maximum of tilt necessary motor rotation rate N_tilt and PS necessary motor rotation rate N_ps.

Next, the controller60sets a power drive torque limit value of the electric motor18, based on the lift descent mode determined by the process in S102(S106). The power drive torque limit value is an allowable value of power drive torque. In the case of lift descent single control, as shown inFIG. 5, the controller60sets the power drive torque limit to ON. At this time, the power drive torque limit value is set, for example, to 0 Nm (0%) or a value close to 0 Nm. In the cases of lift descent+tilt control, lift descent+attachment control, lift descent+power steering control, and, lift descent+tilt+power steering control, the controller60sets the power drive torque limit to OFF. Namely, the power drive torque limit value is set to 100%.

Thereafter, the controller60controls opening and closing (ON and OFF) of the bypass solenoid-controlled directional control valve53, based on the lift descent mode determined by the process in S102, the motor command rotation rate set by the process in S105, and the motor actual rotation rate detected by the rotation rate sensor59(S107).FIG. 6shows the details of the control process procedure of the solenoid-controlled directional control valve53performed by the controller60.

InFIG. 6, first, the controller60determines whether the lift descent mode is the lift descent single control (S111). When determining that the lift descent mode is the lift descent single control, the controller60acquires the motor actual rotation rate detected by the rotation rate sensor59(S112).

Next, the controller60determines whether a difference between the motor command rotation rate set by the process in S105and the motor actual rotation rate is not less than a constant value N (S113). When determining that the difference between the motor command rotation rate and the motor actual rotation rate is not less than the constant value N, the controller60sends an ON signal to the solenoid control unit53cof the solenoid-controlled directional control valve53. By this, the controller60turns the solenoid-controlled directional control valve53to the open position53a(ON) (S114). When determining that the difference between the motor command rotation rate and the motor actual rotation rate is less than the constant value N, the controller60sends an OFF signal to the solenoid control unit53cof the solenoid-controlled directional control valve53. By this, the controller60turns the solenoid-controlled directional control valve53to the close position53b(OFF) (S115).

When determining by the process in S111that the lift descent mode is not the lift descent single control, or that the lift descent mode is any one mode of the lift descent+tilt control, lift descent+attachment control, lift descent+power steering control, and, lift descent+tilt+power steering control, the controller60sends an ON signal to the solenoid control unit53cof the solenoid-controlled directional control valve53, as also shown inFIG. 5. By this, the controller60turns the solenoid-controlled directional control valve53ON (S114).

In the present process, the controller60is configured to turn the solenoid-controlled directional control valve53ON when the difference between the motor command rotation rate and the motor actual rotation rate is not less than the constant value N and to turn the solenoid-controlled directional control valve53OFF when the difference between the motor command rotation rate and the motor actual rotation rate is less than the constant value N, but the process herein does not have to be limited to this example. For example, in order to prevent chattering, the controller60may be configured to turn the solenoid-controlled directional control valve53ON when the difference between the motor command rotation rate and the motor actual rotation rate is not less than a constant value N_on and to turn the solenoid-controlled directional control valve53OFF when the difference between the motor command rotation rate and the motor actual rotation rate is not more than a constant value N_off (<N_on).

Referring back toFIG. 4, after execution of the process in S107, the controller60sends the solenoid-controlled proportional valve solenoid current command value obtained by the process in S103, to the solenoid control unit of the corresponding solenoid-controlled proportional valve (S108). At this time, the controller60sends the lift descent solenoid current command value to the solenoid control unit48cof the solenoid-controlled proportional valve48. When obtaining the tilt solenoid current command value, the controller60sends the current command value to either of the solenoid control units26d,26eof the solenoid-controlled proportional valve26. When obtaining the attachment solenoid current command value, the controller60sends the current command value to either of the solenoid control units31d,31eof the solenoid-controlled proportional valve31. When obtaining the PS solenoid current command value, the controller60sends the current command value to either of the solenoid control units36d,36eof the solenoid-controlled proportional valve36.

Next, the controller60obtains the output torque of the electric motor18, based on the motor rotation rate command value (motor command rotation rate) set by the process in S105, the motor actual rotation rate detected by the rotation rate sensor59, and the power drive torque limit value of the electric motor18set by the process in S106. The controller60sends the obtained output torque as a control signal to the electric motor18(S109). The process in S109is executed by a motor torque output unit61included in the controller60, as shown inFIG. 7.

As shown inFIG. 7, the motor torque output unit61has a comparison unit62, a PID operation unit63, an output torque determination unit64, and a motor controlling unit65. The comparison unit62calculates a rotation rate deviation between the motor command rotation rate set by the process in S105and the motor actual rotation rate detected by the rotation rate sensor59. The PID operation unit63performs the PID operation of the rotation rate deviation between the motor command rotation rate and the motor actual rotation rate to obtain a power drive torque command value of the electric motor18to make the rotation rate deviation zero. The PID operation is a combinational operation of proportional operation, integral operation, and derivative operation.

The output torque determination unit64compares the power drive torque command value obtained by the PID operation unit63with the power drive torque limit value of the electric motor18set by the process in S106, to determine the output torque of the electric motor18. Specifically, when the power drive torque command value is not more than the power drive torque limit value, the output torque determination unit64determines that the power drive torque command value is the output torque of the electric motor18. When the power drive torque command value is higher than the power drive torque limit value, the output torque determination unit64determines that the power drive torque limit value is the output torque of the electric motor18. The motor controlling unit65converts the output torque of the electric motor18determined by the output torque determination unit64, into a current signal and sends the current signal to the electric motor18.

In the above configuration, the lift control lever control input sensor55, tilt control lever control input sensor56, attachment control lever control input sensor57, steering control speed sensor58, and controller60constitute setting means for setting the command rotation rate of the electric motor18and determining means for determining whether the descent control of the up-and-down control means11is performed singly or the controls of the plurality of manual control means including the descent control of the up-and-down control means11are performed simultaneously. The controller60constitutes electric motor controlling means for controlling the electric motor18, based on the command rotation rate set by the setting means and the actual rotation rate detected by the rotation rate detecting means59.

The controller60constitutes valve opening and closing controlling means for controlling the solenoid-controlled directional control valve53as follows: when the determining means determines that the descent control of the up-and-down control means11is performed singly, the valve opening and closing controlling means controls the solenoid-controlled directional control valve53so as to be switched to the open position53aif the difference between the command rotation rate of the electric motor18set by the setting means and the actual rotation rate of the electric motor18detected by the rotation rate detecting means is not less than the predetermined value; when the determining means determines that the controls of the plurality of manual control means including the descent control of the up-and-down control means11are performed simultaneously, the valve opening and closing controlling means controls the solenoid-controlled directional control valve53so as to be switched to the open position53a.

The controller60constitutes torque limit controlling means operating as follows: when the determining means determines that the descent control of the up-and-down control means11is performed singly, the torque limit controlling means imposes a limit on the power drive torque of the electric motor18; when the determining means determines that the controls of the plurality of manual control means including the descent control of the up-and-down control means11are performed simultaneously, the torque limit controlling means removes the limit on the power drive torque of the electric motor18.

The processes in S101and S102shown inFIG. 4function as a part of the determining means. The processes in S101and S103to S105function as a part of the setting means. The process in S106functions as the torque limit controlling means. The process in S107functions as the valve opening and closing controlling means. The process in S109functions as the electric motor controlling means.

Next, the operation of the hydraulic drive device16of the present embodiment will be described with reference toFIGS. 8 to 11.FIG. 8is a drawing showing a timing chart in the case where the lift descent single control is carried out in a large cargo load state (high load state).

InFIG. 8, the lift descent single control is started first at time t1 to turn the power drive torque limit of the electric motor18ON. Furthermore, the lift descent solenoid current command value (cf. dashed line B) depending upon the control input of the lift control lever11(cf. solid line A) is obtained and the current command value is output to the solenoid-controlled proportional valve48. Then, the solenoid-controlled proportional valve48is switched from the close position48bto the open position48a. At this time, since the cargo load is large, the pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve48becomes large and the pressure compensation valve50is switched from the open position50ato the close position50b. Then the lift necessary motor rotation rate according to the lift descent solenoid current command value is obtained as a motor command rotation rate (cf. dashed line C0) and the motor command rotation rate is output to the electric motor18.

At time t2, when the difference between the motor command rotation rate and the motor actual rotation rate (cf. solid line C) becomes not less than the constant value N, the solenoid-controlled directional control valve53turns ON (cf. solid line D). At this time, the hydraulic pump motor17is rotated by intake pressure of the hydraulic pump motor17, whereby the motor actual rotation rate becomes closer to the motor command rotation rate.

At time t3, the difference between the motor command rotation rate and the motor actual rotation rate becomes smaller than the constant value N and thus the solenoid-controlled directional control valve53turns OFF (cf. solid line D). Then, the flow rate Q3 of the hydraulic oil returning to the tank19(bypass flow rate) becomes zero and the flow rate Q1 of the hydraulic oil from the lift cylinder4comes to be the flow rate Q2 of the hydraulic oil supplied to the hydraulic pump motor17. Therefore, the hydraulic pump motor17becomes likely to operate as a hydraulic motor and the electric motor18functions as an electric generator. This allows the hydraulic drive device to efficiently perform the aforementioned cargo-handling regeneration.

FIG. 9is a drawing showing a timing chart in the case where the lift descent single control is carried out in a small cargo load state (low load state).

InFIG. 9, the lift descent single control is started first at time t1 and thus the power drive torque limit of the electric motor18turns ON. Furthermore, the lift descent solenoid current command value (cf. dashed line B) depending upon the control input of the lift control lever11(cf. solid line A) is obtained and the current command value is output to the solenoid-controlled proportional valve48. Then, the solenoid-controlled proportional valve48is switched from the close position48bto the open position48a. At this time, since the cargo load is small, the pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve48is small and thus the pressure compensation valve50is open. Then, the lift necessary motor rotation rate according to the lift descent solenoid current command value is obtained as a motor command rotation rate (cf. dashed line C0) and the motor command rotation rate is output to the electric motor18.

At time t2, when the difference between the motor command rotation rate and the motor actual rotation rate (cf. solid line C) becomes not less than the constant value N, the solenoid-controlled directional control valve53turns ON (cf. solid line D). Since at this time the intake pressure of the hydraulic pump motor17is low, the hydraulic pump motor17is not rotated and the motor actual rotation rate does not reach the motor command rotation rate. For this reason, the solenoid-controlled directional control valve53is maintained in the ON state.

In that state, the valve travel of the pressure compensation valve50increases until the bypass flow rate Q3 of the hydraulic oil returning to the tank19reaches a necessary flow. Then, the flow rate Q1 of the hydraulic oil from the lift cylinder4is ensured by a degree of necessity. This ensures a necessary lift descent speed. In addition, the electric motor18is inhibited from undergoing power drive, which can keep the power consumption low.

FIG. 10is a drawing showing a timing chart in the case where the controls of lift descent and tilt are performed simultaneously, in the large cargo load state (high load state). Since the operations at times t1 to t3 inFIG. 10are the same as those shown inFIG. 8, the description thereof is omitted herein.

At time t4, the tilt control is started, so as to turn the power drive torque limit of the electric motor18OFF and turn the solenoid-controlled directional control valve53from OFF to ON (cf. solid line D). Then, a tilt solenoid current command value depending upon a control input of the tilt control lever12(cf. solid line E) is obtained and the current command value is output to the solenoid-controlled proportional valve26. Then, the solenoid-controlled proportional valve26is switched from the close position26cto either of the open positions26a,26b. Then, the tilt necessary motor rotation rate according to the tilt solenoid current command value is obtained as a motor command rotation rate (cf. dashed line C0) and the motor command rotation rate is output to the electric motor18.

Since the power drive torque limit of the electric motor18is OFF, the motor actual rotation rate follows the motor command rotation rate. The cargo-handling regeneration can be implemented depending upon the cargo load.

Since the motor command rotation rate is lowered from the lift necessary motor rotation rate to the tilt necessary motor rotation rate, the motor actual rotation rate becomes decreased. Since the decrease of the motor actual rotation rate leads to decrease in the flow rate Q2 of the hydraulic oil supplied to the hydraulic pump motor17, the pressure compensation valve50comes to open to a valve travel enough to compensate for the decrease of flow rate Q2. Therefore, the bypass flow rate Q3 of the hydraulic oil returning to the tank19increases and thus the flow rate Q1 of the hydraulic oil from the lift cylinder4becomes nearly constant. By this, the lift descent speed can be kept constant even in execution of the simultaneous controls of lift descent and tilt. In addition, the lift descent operation and the tilt operation can be simultaneously carried out by making maximum use of regenerative energy of the cargo.

FIG. 11is a drawing showing a timing chart in the case where the controls of lift descent and tilt are performed simultaneously, in the small cargo load state (low load state). Since the operations at times t1 and t2 inFIG. 11are the same as those shown inFIG. 9, the description thereof is omitted herein.

At time t4, the tilt control is started, so as to turn the power drive torque limit of the electric motor18OFF and maintain the solenoid-controlled directional control valve53in the ON state (cf. solid line D). Furthermore, the tilt solenoid current command value depending upon the control input of the tilt control lever12is obtained and the current command value is output to the solenoid-controlled proportional valve26. Then, the solenoid-controlled proportional valve26is switched from the close position26cto either of the open positions26a,26b. In addition, the tilt necessary motor rotation rate according to the tilt solenoid current command value is obtained as a motor command rotation rate (cf. dashed line C0) and the motor command rotation rate is output to the electric motor18.

Since the power drive torque limit of the electric motor18is OFF, the motor actual rotation rate follows the motor command rotation rate. The cargo-handling regeneration can be implemented depending upon the cargo load.

Since the motor actual rotation rate does not reach the motor command rotation rate before the time t4, the motor actual rotation rate increases (cf. solid line C). Since the increase of the motor actual rotation rate leads to increase in the flow rate Q2 of the hydraulic oil supplied to the hydraulic pump motor17, the valve travel of the pressure compensation valve50becomes smaller to decrease the bypass flow rate Q3 of the hydraulic oil returning to the tank19by the increase of flow rate Q2. Therefore, the flow rate Q1 of the hydraulic oil from the lift cylinder4becomes substantially constant. By this, the lift descent speed can be kept constant even in execution of the simultaneous controls of lift descent and tilt.

The operations in the cases of execution of the simultaneous controls of lift descent and attachment and the simultaneous controls of lift descent and tilt are almost the same as the operation in execution of the simultaneous controls of lift descent and tilt. The simultaneous controls are not limited only to situations in which the lift control lever and, another control lever or the steering wheel are controlled at the same timing. For example, the simultaneous controls also include situations in which another control lever or the steering wheel is controlled in a state in which the lift control lever has been controlled. Namely, the simultaneous controls also include situations in which another control lever or the steering wheel is controlled in a state in which the lift cylinder4has operated based on a control of the lift control lever.

According to the present embodiment, as described above, the whole flow rate of the hydraulic oil from the lift cylinder4is fed to the hydraulic pump motor17during the lift descent single operation in the case of the cargo load being enough to implement the cargo-handling regeneration. This allows the cargo-handling regeneration to be implemented with high efficiency. During the lift descent single operation in the case of the cargo load being light, most of the flow rate of the hydraulic oil from the lift cylinder4returns to the tank19. This ensures the necessary lift descent speed with necessary minimum power.

During the lift descent single operation, even if the cargo is one with a load capable of implementing the cargo-handling regeneration at low lift descent speed, the pressure of the hydraulic oil will be lowered to pressure incapable of implementing the regeneration at high lift descent speed. In this case, however, most of the flow rate of the hydraulic oil from the lift cylinder4returns to the tank19, which prevents the electric motor18from undergoing power drive. Therefore, it can reduce power consumption.

Even in the case where another cargo-handling operation, such as tilt and attachment, or the steering operation is performed during the lift descent operation, a part of the flow rate of the hydraulic oil from the lift cylinder4returns to the tank19, which can suppress variation of lift descent speed. Since the other cargo-handling operation or the steering operation can be performed by making maximum use of regenerative energy of the cargo, depending upon the cargo load and the control input of the control lever, power consumption can be reduced.

When the lift descent operation overlaps with the other cargo-handling operation or the steering operation, the controller60sets the necessary motor rotation rate other than the lift necessary motor rotation rate, as the motor command rotation rate even if the lift necessary motor rotation rate is higher than the tilt necessary motor rotation rate, the attachment necessary motor rotation rate, and the power steering necessary motor rotation rate. This prevents the hydraulic oil from being supplied more than necessary to the tilt cylinders9, the attachment cylinder15, and the PS cylinder14. As a result, increase of loss can be prevented.

The pressure compensation valve50keeps change of lift descent speed small, even with variation in the pressure difference between pressures upstream and downstream of the solenoid-controlled proportional valve48. Therefore, it can suppress variation in lift descent speed due to variation of cargo load.

The controller60performs the determination on ON or OFF of the solenoid-controlled directional control valve53, based on whether the difference between the motor command rotation rate and the motor actual rotation rate is not less than the constant value N. This configuration requires neither of a pressure sensor and a stroke sensor, or the like. Furthermore, there is no need for provision of a plurality of hydraulic pump motors17and electric motors18. Therefore, the hydraulic drive device16can be constructed at low cost.

FIG. 12is a configuration diagram showing a control system of the hydraulic drive device according to the second embodiment. In the same drawing, the hydraulic drive device16of the present embodiment is further equipped with a direction sensor (running direction detecting means)90, in addition to the configuration shown inFIG. 3.

The direction sensor90detects a running direction (forward, backward, or neutral) of the forklift1selected through the direction switch (described above). When the direction switch is switched to forward or backward, the direction sensor90turns ON. When the direction switch is switched to neutral, the direction sensor90turns OFF.

The controller60receives the detected values from the control lever control input sensors55-57, the steering control speed sensor58, and the rotation rate sensor59and the detection signal from the direction sensor90, and performs predetermined processing to control the electric motor18, the solenoid-controlled proportional valves23,26,31,36,48, and the solenoid-controlled directional control valve53. The controller60executes the control processes of operations including the lift descent, in accordance with the processes in S101to S109shown inFIG. 4. The processes in S101to S104are the same as in the aforementioned first embodiment.

The motor command rotation rate (motor rotation rate command value) set by the process in S105is as shown inFIG. 13. Specifically, when the direction sensor90is OFF, the motor command rotation rates for all the lift descent modes are the same as in the above first embodiment. When the direction sensor90is ON, the controller60sets a maximum of lift necessary motor rotation rate N_lift and PS idle rotation rate N_psi as the motor command rotation rate in the case of lift descent single control. The PS idle rotation rate N_psi is preliminarily determined by experiment or the like. When the direction sensor is ON, the motor command rotation rates in the cases of the other lift descent modes are the same as in the above first embodiment.

The power drive torque limit value of the electric motor18set by the process in S106is set by two-valued control, as shown inFIG. 13.FIG. 14shows the details of the setting process procedure of the power drive torque limit value of the electric motor18.

InFIG. 14, the controller60first determines whether the lift descent mode is the lift descent single control (S121). When determining that the lift descent mode is the lift descent single control, the controller60determines whether the direction sensor90is ON (S122). When determining that the direction sensor90is ON, the controller60acquires the motor actual rotation rate detected by the rotation rate sensor59(S123).

Subsequently, the controller60determines whether the motor actual rotation rate is not less than the idle rotation rate (S124). When determining that the motor actual rotation rate is not less than the idle rotation rate, the controller60sets the power drive torque limit value of the electric motor18to a minimum setting value SL(cf.FIG. 15) (S125). At this time, the minimum setting value SL(first setting value) is, for example, a value (torque) that maintains the rotation rate of the hydraulic pump motor17at the idle rotation rate even if the oil temperature of the hydraulic oil is significantly high. The minimum setting value SLis a value obtained by experiment or the like.

When determining that the motor actual rotation rate is less than the idle rotation rate, the controller60sets the power drive torque limit value of the electric motor18to a maximum setting value SU(cf.FIG. 15) (S126). At this time the maximum setting value SU(second setting value) is a value larger than the minimum setting value SL. The maximum setting value SUis a value obtained by experiment or the like. The maximum setting value SUis, for example, a value (torque) that is necessary for rotating the hydraulic pump motor17at the idle rotation rate even if the oil temperature of the hydraulic oil is significantly low.

When determining by the process in S122that the direction sensor90is not ON but OFF, the controller60turns the power drive torque limit to ON as in the above first embodiment (S127). When determining by the process in S121that the lift descent mode is not the lift descent single control, the controller60turns the power drive torque limit to OFF as in the first embodiment (S128).

In the present process, when the motor actual rotation rate is not less than the idle rotation rate, the power drive torque limit value of the electric motor18is set to the minimum setting value SL. This reduces the power drive torque permitted by the electric motor18. When the motor actual rotation rate is less than the idle rotation rate, the power drive torque limit value of the electric motor18is set to the maximum setting value SU. This increases the power drive torque permitted by the electric motor18. Therefore, as shown inFIG. 15, the motor actual rotation rate P becomes closer to the idle rotation rate, in a situation in which the motor actual rotation rate P cannot follow the motor command rotation rate Q, e.g., in the low load state in which the cargo load is light.

The processes in S107to S109are the same as in the first embodiment.

The process in S127shown inFIG. 14functions as first power drive torque limit value setting means for setting the power drive torque limit value of the electric motor18to the predetermined value when the determining means determines that the descent control of the up-and-down control means is performed singly, in a state in which the running direction detecting means90detects the running direction of the cargo handling vehicle1being neutral. The processes in S123to S126function as second power drive torque limit value setting means for setting the power drive torque limit value of the electric motor18, based on the actual rotation rate detected by the rotation rate detecting means59and based on the idle rotation rate of the hydraulic pump17or the target rotation rate corresponding to the rotation rate higher than the idle rotation rate, when the determining means determines that the descent control of the up-and-down control means is performed singly, in a state in which the running direction detecting means90detects the running direction of the cargo handling vehicle1being forward or backward.

When the direction switch (described above) is in the forward or backward state, the hydraulic pump motor17needs to be rotated at a rotation rate not less than the idle rotation rate, in preparation for handling (steering) of the steering wheel of the steering mechanism13. Incidentally, as the oil temperature of the hydraulic oil becomes lower, the viscosity of the hydraulic oil increases and the pressure loss increases. This increases the power drive torque necessary for rotating the hydraulic pump motor17at a rotation rate not less than the idle rotation rate. Therefore, the power drive torque limit value needs to be set so as to ensure the rotation rate of the hydraulic pump motor17not less than the idle rotation rate at low oil temperatures of the hydraulic oil. In this case, however, the hydraulic pump motor17permits a large power drive torque even at ordinary oil temperatures of the hydraulic oil and, for this reason, the rotation rate of the hydraulic pump motor17rises more than necessary, resulting in increase of power consumption.

In contrast to it, the present embodiment is configured so that when the lift descent single operation is performed with the direction switch being in the forward or backward state, the controller60compares the motor actual rotation rate with the idle rotation rate and sets the power drive torque limit value of the electric motor18to the minimum setting value SLor the maximum setting value SUaccording to the result of the comparison. Therefore, the rotation rate of the hydraulic pump motor17becomes closer to the idle rotation rate, irrespective of the oil temperature of the hydraulic oil. This allows smooth steering during the lift descent single operation.

Since the hydraulic pump motor17rotates at the necessary minimum rotation rate close to the idle rotation rate in the light cargo load state, increase of power consumption is suppressed. Since the hydraulic pump motor17rotates at the motor command rotation rate higher than the idle rotation rate in the sufficiently heavy cargo load state, cargo-handling regeneration is implemented with high efficiency.

FIG. 16is a flowchart showing a modification example of the power drive torque limit value setting process procedure shown inFIG. 14. The present flowchart is different only in the process in S124from the flowchart shown inFIG. 14.

By the process in S124, the controller60determines whether the motor actual rotation rate is not less than a threshold rotation rate (the idle rotation rate+α herein). α is a constant rotation rate determined in advance. When determining that the motor actual rotation rate is not less than the foregoing threshold rotation rate (idle rotation rate+α), the controller60sets the power drive torque limit value of the electric motor18to the minimum setting value SL(cf.FIG. 17) (S125). When determining that the motor actual rotation rate is less than the foregoing threshold rotation rate (idle rotation rate+α), the controller60sets the power drive torque limit value of the electric motor18to the maximum setting value SU(cf.FIG. 17) (S126).

In the present modification example, the threshold rotation rate for switching of the power drive torque limit value is set to the idle rotation rate+α. As shown inFIG. 17, the motor actual rotation rate P becomes closer to the threshold rotation rate (idle rotation rate+α) being the target rotation rate in the situation in which the motor actual rotation rate P cannot follow the motor command rotation rate Q. This ensures that the rotation rate of the hydraulic pump motor17is certainly kept not less than the idle rotation rate, even with some pulsation of the motor actual rotation rate P near the idle rotation rate.

FIG. 18is a block diagram showing a configuration of a part of the controller60in the hydraulic drive device according to the third embodiment. The present embodiment is different only in the process in S106in the flowchart shown inFIG. 4from the second embodiment. The process in S106is executed by torque limit value setting unit80included in the controller60.FIG. 18is a drawing corresponding only to the process in the case where the direction sensor90is ON and where the lift descent mode is the lift descent single control. The processes except for the process in the case where the direction sensor90is ON and where the lift descent mode is the lift descent single control are the same as in the above second embodiment (cf.FIG. 19).

The torque limit value setting unit80has a comparison unit81and a PID operation unit82. The comparison unit81calculates a rotation rate deviation between the aforementioned idle rotation rate and the motor actual rotation rate detected by the rotation rate sensor59. The PID operation unit82performs the PID operation of the rotation rate deviation between the idle rotation rate and the motor actual rotation rate to obtain the power drive torque limit value of the electric motor18to make the rotation rate deviation zero. The power drive torque limit value is sent to the output torque determination unit64of the motor torque output unit61which executes the process in S109shown inFIG. 4.

The torque limit value setting unit80constitutes second power drive torque limit value setting means for setting the power drive torque limit value of the electric motor18, based on the actual rotation rate detected by the rotation rate detecting means59and based on the idle rotation rate of the hydraulic pump motor17or the target rotation rate corresponding to the rotation rate higher than the idle rotation rate, when the determining means determines that the descent control of the up-and-down control means is performed singly, in the state in which the running direction detecting means90detects the running direction of the cargo handling vehicle1being forward or backward.

In the present embodiment, when the lift descent single operation is carried out with the direction switch being in the forward or backward state, the controller60sets the power drive torque limit value of the electric motor18by the PID control. By this, as shown inFIG. 20, in the state in which the motor actual rotation rate P cannot follow the motor command rotation rate Q, the motor actual rotation rate P undergoes little pulsation around the idle rotation rate and the motor actual rotation rate P smoothly and stably follows the idle rotation rate. Therefore, the rotation rate of the hydraulic pump motor17can be maintained at the idle rotation rate, irrespective of the oil temperature of the hydraulic oil. As a consequence of this, smooth steering can be performed during the lift descent single operation. In the light cargo load state, the hydraulic pump motor17rotates at the necessary minimum rotation rate being the idle rotation rate and, for this reason, increase of power consumption is suppressed.

The present embodiment adopts the PID control for the power drive torque limit value of the electric motor18, but the present invention does not have to be limited to this example. It is sufficient that feedback control such as PI control be performed so as to make the rotation rate deviation between the idle rotation rate and the motor actual rotation rate zero.

The above described some embodiments of the hydraulic drive devices for cargo handling vehicle according to the present invention, but it is noted that the present invention is not limited solely to the above embodiments.

The above embodiments used the bypass solenoid-controlled directional control valve53, but there is no need to be limited to this example. Instead of the solenoid-controlled directional control valve53, a lift lock valve70and a pilot solenoid-controlled directional control valve71may be used as shown inFIG. 21. The lift lock valve70is disposed between the pressure compensation valve50and the tank19on the hydraulic pipe49. The pilot solenoid-controlled directional control valve71is disposed on a pilot flow passage72. The pilot flow passage72is connected to the lift lock valve70and to a downstream point (on the tank19side) of the lift lock valve70.

The pilot solenoid-controlled directional control valve71is switched between an open position71aand a close position71b. The pilot solenoid-controlled directional control valve71is normally at the close position71b(as shown). While the pilot solenoid-controlled directional control valve71is at the close position71b, the lift lock valve70is closed. When an ON signal is supplied to a solenoid control unit71cof the pilot solenoid-controlled directional control valve71, the pilot solenoid-controlled directional control valve71is switched to the open position71a. This opens the lift lock valve70.

When the lift lock valve70and the pilot solenoid-controlled directional control valve71are used as valve means, the opening and closing control of the oil passage between the pressure compensation valve50and the tank19can be implemented with a small flow rate. This achieves downsizing and lowers the drive power.

The above embodiments are equipped with the attachment and the power steering mechanism, but the hydraulic drive devices of the present invention can also be applied to forklifts without the attachment and the power steering. The hydraulic drive devices of the present invention are applicable to any battery-powered cargo handling vehicles other than the forklifts.