CN110966151A

CN110966151A - Potential energy recovery system and control method thereof

Info

Publication number: CN110966151A
Application number: CN201911373009.6A
Authority: CN
Inventors: 杨浪; 靳三川; 吴冠宇
Original assignee: Sany Marine Heavy Industry Co Ltd
Current assignee: Sany Marine Heavy Industry Co Ltd
Priority date: 2019-12-25
Filing date: 2019-12-25
Publication date: 2020-04-07

Abstract

The invention relates to the technical field of equipment with a potential energy recovery function, in particular to a potential energy recovery system and a control method thereof. The potential energy recovery system comprises an oil tank, a first power supply, a first motor, a lifting oil cylinder and a controller, wherein the first power supply is a storage battery, the first motor can be switched between a motor mode and a generator mode, and the lifting oil cylinder is used for driving a lifting part to lift; the storage battery is electrically connected with the first motor, the first motor is connected with the motor, a first interface of the motor is communicated with the lifting oil cylinder through the valve assembly, and a second interface of the motor is communicated with the oil tank; install first pressure sensor between valve member and the lift cylinder, install the second pressure sensor between motor and the valve member, first pressure sensor, second pressure sensor, valve member and first motor all are connected with the controller electricity. The control method is applied to the potential energy recovery system. The potential energy recovery system and the control method thereof provided by the invention have higher energy conversion efficiency.

Description

Potential energy recovery system and control method thereof

Technical Field

The invention relates to the technical field of equipment with a potential energy recovery function, in particular to a potential energy recovery system and a control method thereof.

Background

Many devices (such as a stacking machine) with potential energy recovery function use a hydraulic system to drive a lifting component (such as a lifting component of the stacking machine) to lift, and use the hydraulic system to recover the gravitational potential energy of the lifting component and convert the gravitational potential energy into other energy (such as electric energy) to be stored.

The existing equipment with the potential energy recovery function can be additionally provided with a balance valve in a hydraulic system in order to prevent a stalling phenomenon of a lifting component, however, the pressure loss of the balance valve is large, and the energy recovery efficiency is low.

In summary, how to overcome the above-mentioned defects of the hydraulic system in the existing equipment with potential energy recovery function is a technical problem to be solved urgently by those skilled in the art.

Disclosure of Invention

The invention aims to provide a potential energy recovery system and a control method thereof, which are used for solving the technical problem of low energy recovery efficiency of a hydraulic system in equipment with a potential energy recovery function in the prior art.

The potential energy recovery system comprises an oil tank, a first power supply, a first motor, a lifting oil cylinder and a controller, wherein the first power supply is a storage battery, the first motor can be switched between a motor mode and a generator mode, and the lifting oil cylinder is used for driving a lifting part to lift.

The battery with first motor electricity is connected, first motor with the motor meets, the motor has first interface and second interface, first interface with lift cylinder passes through the valve member intercommunication, the second interface with the oil tank intercommunication.

The valve assembly with install first pressure sensor between the lift cylinder, the motor with install second pressure sensor between the valve assembly, first pressure sensor second pressure sensor the valve assembly with first motor all with the controller is electric to be connected.

Preferably, as an embodiment, the potential energy recovery system further comprises a second power source, a second motor and a pump; the second power with the second motor electricity is connected, the second motor with the pump meets, the pump has inlet and liquid outlet, the inlet with the oil tank intercommunication, the liquid outlet with the first interface of motor passes through the check valve intercommunication.

Preferably, as an embodiment, the second motor is electrically connected to the controller; alternatively, the pump has a displacement less than the displacement of the motor.

Preferably, as an implementation mode, the valve assembly comprises a logic valve and a solenoid directional valve, a pipeline connecting port a of the logic valve is communicated with the first interface, a pipeline connecting port B of the logic valve is communicated with the lift cylinder, a control port C of the logic valve is communicated with the oil tank through the solenoid directional valve, and the solenoid directional valve is electrically connected with the controller.

Preferably, as an implementation mode, the valve assembly further comprises an overflow valve, and the overflow valve is located between the lifting cylinder and the oil tank; and/or the potential energy recovery system further comprises a speed limiting valve, and the speed limiting valve is positioned between the lifting oil cylinder and the valve assembly.

Preferably, as an embodiment, the first electric machine has an operating handle capable of adjusting a rotation speed of the first electric machine when the first electric machine is in a motor mode, and the operating handle is further capable of adjusting a torque of the first electric machine when the first electric machine is in a generator mode.

Correspondingly, the invention provides a control method of a potential energy recovery system, which comprises the following steps:

when the lifting part needs to ascend, the valve assembly is opened, meanwhile, the first motor is switched to be in a motor mode, the driving motor conveys oil in the oil tank to the lifting oil cylinder through the valve assembly, the lifting oil cylinder extends, and the extended lifting oil cylinder drives the lifting part to ascend; when the lifting component rises to a preset position, the valve assembly is closed, meanwhile, the first motor stops running, and the lifting component stops at the current position;

when the lifting component needs to descend, the first motor in the motor mode drives the motor to convey oil in the oil tank to the valve assembly, and the pressure at the second pressure sensor gradually rises; when the pressure value sensed by the second pressure sensor reaches the pressure value sensed by the first pressure sensor, the valve assembly is opened, and meanwhile, the first motor is switched to a generator mode; oil in the lifting oil cylinder flows back to the oil tank through the valve assembly and the motor in sequence, the oil drives the motor to run when passing through the motor, the running motor drives the first motor to generate electricity, and the first motor stores electric energy into the storage battery.

Preferably, as an embodiment, the step of the first electric motor driving the motor in the electric motor mode to deliver the oil in the oil tank towards the valve assembly is replaced by the steps of:

the second motor drives the pump to deliver oil from the tank to the valve assembly.

Preferably, as an embodiment, between the step of the first electric machine being switched to the motor mode and the step of the first electric machine driving the motor to supply the oil in the oil tank to the lift cylinder via the valve assembly, the control method further includes the steps of:

the second motor drives the pump to deliver oil from the tank to the lift cylinder via the valve assembly, wherein the displacement of the pump is less than the displacement of the motor.

Preferably, as an implementation mode, the step of the first electric motor driving motor delivering the oil in the oil tank to the lifting oil cylinder through the valve assembly specifically comprises the following steps:

when the second motor reaches the highest rotating speed, the first motor drives the motor to convey oil in the oil tank to the lifting oil cylinder through the valve assembly.

Compared with the prior art, the invention has the advantages that:

according to the potential energy recovery system and the control method thereof, when the lifting part needs to ascend, the valve assembly is opened, the first motor is converted into the motor mode, at the moment, the storage battery can supply power to the first motor in the motor mode, the first motor in the motor mode is used for driving the motor to run, the running motor can convey oil in the oil tank to the lifting oil cylinder through the valve assembly, so that the lifting oil cylinder extends, and the lifting part can be driven to ascend after the lifting oil cylinder extends; when the lifting component rises to a preset position, the valve component is closed, so that the lifting oil cylinder maintains the current state, further, the lifting component stops at the current position, and meanwhile, the first motor stops running, and the lifting action of the lifting component is completed.

When the lifting component needs to descend, the first motor in the motor mode is used for driving the motor to operate, the operating motor can convey oil in the oil tank towards the valve assembly, and therefore the valve assembly is in a closed state, the oil between the motor and the valve assembly has certain pressure, the second pressure sensor can sense the pressure value, and meanwhile, the first pressure sensor can sense the pressure value between the lifting oil cylinder and the valve assembly; when the pressure value that second pressure sensor sensing arrived reached the pressure value that first pressure sensor sensing arrived, open valve assembly to convert first motor into the generator mode, at this moment, fluid in the lift cylinder can flow back to the oil tank via valve assembly and motor in proper order under the effect of self oil pressure, fluid can the driving motor operation when the motor, the first motor electricity generation that the motor of operation can drive to be in the generator mode, the electric energy that first motor sent is enough stored in the battery, in order to realize energy recuperation's purpose.

The pressure value sensed by the first pressure sensor and the pressure value sensed by the second pressure sensor can be transmitted to the controller and can be used as a reference signal for controlling the valve assembly and the first motor by the controller. The opening and closing actions of the valve assembly, the mode conversion and the starting and stopping actions of the first motor can be completed by the controller, and especially when the pressure value sensed by the second pressure sensor reaches the pressure value sensed by the first pressure sensor, the opening action of the valve assembly and the action of converting the first motor into the generator mode are completed.

It should be noted that, the existence of the first pressure sensor and the second pressure sensor can enable the oil pressure at two sides of the valve component to reach the equal state or the state close to the equal state, and after the valve component is opened in the state, the oil in the lifting oil cylinder flows back to the oil tank through the motor, so that the stalling phenomenon of the lifting component caused by the large oil pressure difference at two sides of the valve component can be prevented, and a balance valve is not needed to be added, so that the loss of the oil pressure is small and the energy conversion efficiency is high in the descending process of the lifting component.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.

Fig. 1 is a schematic diagram of a potential energy recovery system according to an embodiment of the present invention.

Icon: 1-an oil tank; 2-a first power supply; 3-a first motor; 4-a motor; 5-lifting the oil cylinder; 6-a first pressure sensor; 7-a second pressure sensor; 8-a second power supply; 9-a second motor; 10-a pump; 11-a logic valve; 12-a solenoid directional valve; 13-relief valve; 14-a speed limiting valve.

Detailed Description

The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience in describing the present invention and simplifying the description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted" and "connected" are to be interpreted broadly, e.g., as being either fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.

The present invention will be described in further detail below with reference to specific embodiments and with reference to the attached drawings.

Referring to fig. 1, the present embodiment provides a potential energy recovery system, which mainly comprises an oil tank 1, a first power supply 2, a first motor 3, a motor 4, a lift cylinder 5 and a controller, wherein the first power supply 2 is a storage battery and can be charged and discharged; the first motor 3 can be switched between a motor mode and a generator mode, and the lifting oil cylinder 5 can drive the lifting component to lift when stretching.

The battery is connected with first motor 3 electricity, for first motor 3 charges or stores the electricity that first motor 3 sent, and first motor 3 meets with motor 4, and motor 4 has first interface and second interface, and first interface passes through the valve assembly intercommunication with lift cylinder 5, and the second interface communicates with oil tank 1 to, lift cylinder 5 just can be connected with oil tank 1 is indirect under the effect of valve assembly and motor 4, in order to obtain the oil in the oil tank 1, and can drain fluid back to oil tank 1.

A first pressure sensor 6 is arranged between the motor 4 and the valve assembly to sense the oil pressure between the motor 4 and the valve assembly by using the first pressure sensor 6, a second pressure sensor 7 is arranged between the valve assembly and the lifting oil cylinder 5 to sense the oil pressure between the valve assembly and the lifting oil cylinder 5 by using the second pressure sensor 7, and the first pressure sensor 6, the second pressure sensor 7, the valve assembly and the first motor 3 are all electrically connected with the controller, so that the controller can acquire the pressure value sensed by the first pressure sensor 6 and the pressure value sensed by the second pressure sensor 7 and can control the opening and closing of the valve assembly and the mode switching and starting and stopping of the first motor 3 according to the acquired pressure value signals.

The embodiment also provides a control method of the potential energy recovery system, when the lifting part needs to ascend, the valve assembly is opened, the first motor 3 is converted into the motor mode, at the moment, the storage battery can supply power to the first motor 3 in the motor mode, the first motor 3 in the motor mode is used for driving the motor 4 to operate, the operating motor 4 can convey oil in the oil tank 1 to the lifting oil cylinder 5 through the valve assembly, so that the lifting oil cylinder 5 extends, and the lifting oil cylinder 5 can drive the lifting part to ascend after extending; when the lifting component ascends to the preset position, the valve component is closed, so that the lifting oil cylinder 5 maintains the current state, further, the lifting component stops at the current position, and meanwhile, the first motor 3 stops running, and the lifting action of the lifting component is completed.

When the lifting component needs to descend, the first motor 3 in the motor mode is used for driving the motor 4 to operate, the operating motor 4 can convey oil in the oil tank 1 towards the valve assembly, therefore, the valve assembly is in a closed state, so that the oil between the motor 4 and the valve assembly has certain pressure, the second pressure sensor 7 can sense the pressure value, and meanwhile, the first pressure sensor 6 can sense the pressure value between the lifting oil cylinder 5 and the valve assembly; when the pressure value that second pressure sensor 7 sensing arrived reaches the pressure value that first pressure sensor 6 sensing arrived, open valve assembly, and convert first motor 3 into the generator mode, at this moment, fluid in the lift cylinder 5 can flow back to oil tank 1 through valve assembly and motor 4 under the effect of self oil pressure in proper order, fluid can drive motor 4 operation when motor 4, the first motor 3 electricity generation that is in the generator mode can be driven to motor 4 of operation, the electric energy that first motor 3 sent can be stored in the battery, in order to realize energy recuperation's purpose.

The pressure value sensed by the first pressure sensor 6 and the pressure value sensed by the second pressure sensor 7 can be transmitted to the controller, and can be used as a reference signal for controlling the valve assembly and the first motor 3 by the controller. The opening and closing actions of the valve assembly, and the mode switching and starting and stopping actions of the first motor 3 can be completed by the controller, and especially when the pressure value sensed by the second pressure sensor 7 reaches the pressure value sensed by the first pressure sensor 6, the opening action of the valve assembly and the action of switching the first motor 3 into the generator mode are completed.

It should be noted that the existence of the first pressure sensor 6 and the second pressure sensor 7 can make the oil pressure on both sides of the valve assembly reach the equal or nearly equal state, and after the valve assembly is opened in this state, the oil in the lifting oil cylinder 5 will flow back to the oil tank 1 through the motor 4, so that the stalling phenomenon of the lifting component caused by the large difference of the oil pressure on both sides of the valve assembly can be prevented, and no balancing valve needs to be added, therefore, in the process of descending the lifting component, the loss of the oil pressure is small, and the energy conversion efficiency is high.

The potential energy recovery system provided by this embodiment may further include a second power source 8, a second motor 9 and a pump 10, wherein the second power source 8 is electrically connected to the second motor 9, the second motor 9 is connected to the pump 10, the pump 10 has a liquid inlet and a liquid outlet, the liquid inlet is communicated with the oil tank 1, the liquid outlet is communicated with the first interface of the motor 4 through a one-way valve, and the first interface of the motor 4 is communicated with the lift cylinder 5 through a valve assembly, so that the second motor 9 can drive the pump 10 to pump the oil in the oil tank 1 into the lift cylinder 5.

On the basis of the above structure, the step "the first electric machine 3 in the motor mode drives the motor 4 to deliver the oil in the tank 1 towards the valve assembly" in the control method of the potential energy recovery system can be replaced by the following steps: the second motor 9 drives the pump 10 to deliver the oil in the tank 1 towards the valve assembly. Therefore, the problem of time required for converting the first motor 3 from the motor mode to the generator mode does not need to be considered, when the pressure at the second pressure sensor 7 rises to be equal to the pressure value sensed by the first pressure sensor 6, the first motor 3 can rapidly acquire gravitational potential energy generated by the descending of the lifting component, and further, the energy conversion efficiency can be further improved.

Specifically, the second motor 9 may be electrically connected to the controller, so as to control the second motor 9 to work by the controller, for example: when the lifting component is required to descend, the controller can send a starting signal to the second motor 9 to start the second motor 9; when the controller acquires the signal that the pressure sensed by the second pressure sensor 7 is equal to the pressure value sensed by the first pressure sensor 6, the controller can control the second motor 9 to stop in time so as to save the electric energy consumed by the second motor 9, therefore, the automation degree can be further improved, and the manpower is saved.

In addition, the displacement of the pump 10 can be set to be smaller than the displacement of the motor 4, and on the basis of the above step "the first electric machine 3 is switched to the electric motor mode" during the ascending process of the lifting part and the above step "the first electric machine 3 drives the motor 4 to deliver the oil in the oil tank 1 to the lifting cylinder 5 via the valve assembly", the control method of the potential energy recovery system further comprises the following steps: the second motor 9 drives the pump 10 to deliver the oil in the oil tank 1 to the lifting oil cylinder 5 through the valve assembly, that is, the pump 10 with small displacement is used for pumping the oil to realize the micro-motion lifting of the lifting part, and then the motor 4 with large displacement is used for delivering the oil to realize the rapid lifting of the lifting part, so that the micro-motion performance of the lifting part when the lifting part starts to lift can be optimized, and the lifting speed of the lifting part after the lifting part is lifted for a period of time can be ensured.

Specifically, in the ascending process of the lifting component, the first motor 3 can be started when the second motor 9 reaches the maximum rotating speed, the rotating speed of the second motor 9 can be monitored in real time by using the controller, when the controller detects that the rotating speed of the second motor 9 reaches the maximum rotating speed, the controller controls the first motor 3 to be started, the effect of the second motor 9 can be fully exerted, and the automation degree of the potential energy recovery system can be improved.

In particular, the motor 4 may be a fixed displacement plunger motor, and the pump 10 may be a gear pump.

The specific structure of the valve component comprises a logic valve 11 and an electromagnetic directional valve 12, wherein a port A of the logic valve 11 is communicated with a first interface, a port B of the logic valve 11 is communicated with the lifting oil cylinder 5, a control port C of the logic valve 11 is communicated with the oil tank 1 through the electromagnetic directional valve 12, and the electromagnetic directional valve 12 is electrically connected with a controller, so that the controller can control the electromagnetic directional valve 12 to act to open and close the logic valve 11, and therefore, the characteristic that the logic valve 11 is suitable for a high-pressure large-flow environment can be utilized to ensure the stability of a potential energy recovery system.

Furthermore, an overflow valve 13 can be additionally arranged in the valve component, and the overflow valve 13 is arranged between the lifting oil cylinder 5 and the oil tank 1, so that the overflow valve 13 can be used for carrying out constant-pressure overflow, pressure stabilization, system unloading and safety protection on the potential energy recovery system.

The potential energy recovery system can be additionally provided with a speed-limiting valve 14, and the speed-limiting valve 14 is arranged between the lifting oil cylinder 5 and the valve component, so that the speed of oil between the lifting oil cylinder 5 and the valve component can be limited, an emergency function is realized when a pipeline on one side of the limiting valve, which is far away from the lifting oil cylinder 5, leaks, and the lifting component is prevented from being out of control and descending.

Specifically, an operating handle may be provided on the first motor 3, and the operating handle may be capable of adjusting the rotation speed of the first motor 3 when the first motor 3 is in the motor mode, so as to adjust the rising speed of the lifting member; the operating handle can also adjust the torque of the first motor 3 when the first motor 3 is in a generator mode, so that the descending speed of the lifting component can be adjusted, and the micro-motion descending of the lifting component can be realized.

In summary, the present invention discloses a potential energy recovery system and a control method thereof, which overcome many technical defects of a hydraulic system in a conventional device with potential energy recovery function. The potential energy recovery system and the control method thereof can prevent the stalling phenomenon of the lifting component caused by the large oil pressure difference at two sides of the valve component, and the energy conversion efficiency is high.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. The potential energy recovery system is characterized by comprising an oil tank (1), a first power supply (2), a first motor (3), a motor (4), a lifting oil cylinder (5) and a controller, wherein the first power supply (2) is a storage battery, the first motor (3) can be switched between a motor mode and a generator mode, and the lifting oil cylinder (5) is used for driving a lifting part to lift;

the storage battery is electrically connected with the first motor (3), the first motor (3) is connected with the motor (4), the motor (4) is provided with a first interface and a second interface, the first interface is communicated with the lifting oil cylinder (5) through a valve assembly, and the second interface is communicated with the oil tank (1);

the valve assembly with install first pressure sensor (6) between lift cylinder (5), motor (4) with install second pressure sensor (7) between the valve assembly, first pressure sensor (6) second pressure sensor (7) the valve assembly with first motor (3) all with the controller electricity is connected.

2. The potential energy recovery system of claim 1, further comprising a second power source (8), a second motor (9) and a pump (10); the second power supply (8) is electrically connected with the second motor (9), the second motor (9) is connected with the pump (10), the pump (10) is provided with a liquid inlet and a liquid outlet, the liquid inlet is communicated with the oil tank (1), and the liquid outlet is communicated with the first interface of the motor (4) through a one-way valve.

3. The potential energy recovery system of claim 2, characterized in that the second electric machine (9) is electrically connected with the controller;

or the displacement of the pump (10) is smaller than the displacement of the motor (4).

4. The potential energy recovery system according to claim 1, wherein the valve assembly comprises a logic valve (11) and a solenoid directional valve (12), a pipeline connecting port A of the logic valve (11) is communicated with the first port, a pipeline connecting port B of the logic valve (11) is communicated with the lifting oil cylinder (5), a control port C of the logic valve (11) is communicated with the oil tank (1) through the solenoid directional valve (12), and the solenoid directional valve (12) is electrically connected with the controller.

5. The potential energy recovery system of claim 4, wherein the valve assembly further comprises an overflow valve (13), the overflow valve (13) being located between the lift cylinder (5) and the tank (1);

and/or the potential energy recovery system further comprises a speed limiting valve (14), and the speed limiting valve (14) is positioned between the lifting oil cylinder (5) and the valve assembly.

6. The potential energy recovery system of any one of claims 1-5, characterized in that the first electric machine (3) has an operating handle which is capable of adjusting the rotational speed of the first electric machine (3) when the first electric machine (3) is in motor mode, and which is further capable of adjusting the torque of the first electric machine (3) when the first electric machine (3) is in generator mode.

7. A method of controlling a potential energy recovery system according to any one of claims 1-6, characterized by the steps of:

when the lifting part needs to ascend, the valve assembly is opened, meanwhile, the first motor (3) is switched to be in a motor mode, the driving motor (4) conveys oil in the oil tank (1) to the lifting oil cylinder (5) through the valve assembly, so that the lifting oil cylinder (5) extends, and the extended lifting oil cylinder (5) drives the lifting part to ascend; when the lifting component rises to a preset position, the valve assembly is closed, meanwhile, the first motor (3) stops running, and the lifting component stops at the current position;

when the lifting component needs to descend, the first motor (3) in the motor mode drives the motor (4) to convey oil in the oil tank (1) to the valve assembly, and the pressure at the second pressure sensor (7) gradually rises; when the pressure value sensed by the second pressure sensor (7) reaches the pressure value sensed by the first pressure sensor (6), the valve assembly is opened, and meanwhile, the first motor (3) is switched to a generator mode; oil in the lifting oil cylinder (5) flows back to the oil tank (1) through the valve assembly and the motor (4) in sequence, the motor (4) is driven to operate when the oil passes through the motor (4), the first motor (3) is driven by the operating motor (4) to generate electricity, and the first motor (3) stores electric energy into the storage battery.

8. Control method according to claim 7, characterized in that the step of the first electric machine (3) in motor mode driving the motor (4) to convey oil in the tank (1) towards the valve assembly is replaced by the step of:

the second motor (9) drives the pump (10) to convey the oil in the oil tank (1) towards the valve assembly.

9. The control method according to claim 7, characterized in that between the step of switching the first electric machine (3) to electric motor mode and the step of the first electric machine (3) driving the motor (4) to feed oil in the tank (1) to the lift cylinders (5) via the valve assembly, the control method further comprises the steps of:

the second motor (9) drives the pump (10) to convey oil in the oil tank (1) to the lifting oil cylinder (5) through the valve assembly, wherein the displacement of the pump (10) is smaller than that of the motor (4).

10. A control method according to claim 9, characterized in that the step of the first electric machine (3) driving the motor (4) to feed oil from the tank (1) to the lift cylinder (5) via the valve assembly comprises in particular the steps of:

when the second motor (9) reaches the highest rotating speed, the first motor (3) drives the motor (4) to convey oil in the oil tank (1) to the lifting oil cylinder (5) through the valve assembly.