EP3178778B1

EP3178778B1 - Hydraulic system for energy regeneration and industrial truck with said hydraulic system

Info

Publication number: EP3178778B1
Application number: EP15199182.5A
Authority: EP
Inventors: Daniel Sundqvist
Original assignee: Unicarriers Europe AB
Current assignee: Mitsubishi Logisnext Europe AB
Priority date: 2015-12-10
Filing date: 2015-12-10
Publication date: 2019-05-22
Anticipated expiration: 2035-12-10
Also published as: EP3178778A1

Description

TECHNICAL FIELD

The present invention relates to a hydraulic system with energy regeneration, especially for forklift trucks, and to a method of energy regeneration in a hydraulic system.

BACKGROUND

Forklift trucks and other types of industrial trucks for lifting, lowering and transporting heavy loads are usually equipped with hydraulic systems. Such systems should operate energy efficiently, and one way of improving their energy efficiency is by energy regeneration during lowering. An example of a hydraulic lifting system with energy regeneration is disclosed in EP 1193211 B1 . This system has an electrical machine that operates as a motor driving a pump during lifting and as a generator recovering energy during lowering. The lowering movement is controlled by a valve arranged in a line branching off from a pressure medium line connecting the pump to a cylinder.
JP 2013/159418 is another example of a system for energy regeneration in a forklift truck. EP 2 657 412 is related to energy regeneration in a hybrid excavator boom actuating system.
Despite the efforts that have gone into developing energy regeneration techniques for hydraulic systems, further efforts aimed at finding innovative solutions to the various technical challenges associated with such efforts are warranted. In particular, there is a need for energy regeneration that operates more efficiently, and/or which provides less restrain on the operational speed of the hydraulic system. There is also a need for a hydraulic system allowing energy regeneration that is more cost-efficient and/or versatile, so that it can be used also in a wider range of systems than today, for example in smaller systems, such as in industrial trucks of smaller types.

SUMMARY

To meet the needs defined above, there is proposed a hydraulic system according to independent claim 1.
In view of the foregoing, and according to a first aspect of the invention, there is presented a hydraulic system for a forklift truck. The hydraulic system comprises a reservoir for a hydraulic fluid, the reservoir having an inlet and an outlet; a hydraulic apparatus operable as a pump and as a generator, the hydraulic apparatus having a first port and a second port, the first port being connected to the outlet; and at least one consumer operable in a lifting mode and in a lowering mode, the at least one consumer being connected to the second port. The inlet is connected to the first port via a first valve and to the at least one consumer via a second valve. Each of the first and second valves is switchable between a first state allowing fluid flow towards the reservoir and a second state blocking fluid flow towards the reservoir, and a fluid flow from the at least one consumer to the reservoir is controllable so as to pass through at least one of the first and second valves.
The first and second valves make it possible to control the amount of fluid flowing through the pump which drives the generator during lowering operations. This enables smooth and fast lowering operations of, for instance, the forks of a forklift truck, even when the forks carry no or only a small load and also during start of the lowering operation. When the energy regeneration starts immediately, as is common in the prior art, the force required to drive the pump and generator usually causes the lowering operation to start off by a jerking movement or by a very slow downward movement since enough pressure is not available. Such scenarios can be avoided by starting the energy regeneration only after a certain fluid pressure has been reached in the system, and this is made possible by the first and second valves and the way in which they are controlled to adjust level of energy recovered through the hydraulic apparatus during lowering.
The control may be provided in hardware, e.g. by means of valves automatically opening or closing at certain pressure levels. However, it may also be provided by means of a controller, such as a software controlled processor, receiving input of operational parameters of the system, and thereby automatically controlling the operation of the first and second valves.
Moreover, the number of revolutions per minute (RPM) of the generator can be controlled by switching the first and second valves so as to vary the amount of fluid flowing through the pump. The first and second valves thus make it possible to have the generator operate an RPM allowing the most efficient energy regeneration. Controlling and adjusting the RPM also makes it possible to reduce noise during operation of the hydraulic system and to increase the lifetime of the hydraulic apparatus.
Still according to the first aspect, the industrial truck is a forklift truck, and the lifting mode is for lifting the forks of said forklift truck and the lowering mode is for lowering the forks of said forklift truck. The system further comprises a control in the lowering mode arranged to determine a value of an operating parameter of the hydraulic system, to allow a fluid flowing from the consumer to the inlet of the reservoir via the second valve to bypass the pump of the hydraulic apparatus if the value is below a threshold value, and to allow the fluid to flow from the consumer to the inlet of the reservoir via the pump of the hydraulic apparatus and the first valve if the value is above the threshold value, wherein fluid flowing through the pump of the hydraulic apparatus drives the pump which, in turn, drives the motor operating as a generator.
According to one embodiment, at least one of the first and second valves is gradually switchable between the first and second states. This makes it possible to precisely control the fluid flow through the pump and, consequently, to control the lowering motion with high precision.
According to one embodiment, at least one of the first and second valves is a directional control valve. There are many different types of directional control valves that are inexpensive and readily commercially available.
According to one embodiment, the hydraulic system further comprises a pressure relief valve connected to the second port and to the inlet. The pressure relief valve is configured to allow fluid flow, from the at least one consumer to the inlet, to bypass the second valve when a pressure at the second port exceeds a threshold value. The pressure relief valve protects the pump against harmful pressure peaks.
According to a second aspect, there is provided an industrial truck having the hydraulic system according to the first aspect. The effects and features of the second aspect are similar to those of the first aspect. The industrial truck is a forklift truck, the hydraulic system according to the first aspect being particularly suitable for forklift trucks.
According to one embodiment, the operating parameter is continuously monitored. This makes it possible to precisely control the fluid flow through the pump and, consequently, regenerate energy efficiently.
According to one embodiment, the operating parameter is a rotation frequency of the power generator. A rotation frequency, such as the RPM, of the power generator is usually easy to determine precisely.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic diagram of an example of a hydraulic system.
Figure 2 shows a schematic diagram of another example of a hydraulic system.
Figure 3 shows a flowchart of some of the steps of a method of energy regeneration in a hydraulic system.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 shows a hydraulic system 1 for an industrial truck. The hydraulic system 1 includes a hydraulic apparatus 2 that has a battery 3, a motor 4 and a pump 5. The battery 3 is electrically connected, via a motor controller 4a, to the motor 4 which is an electric motor, for example an asynchronous motor, configured to be powered by the battery 3. The motor 4 is also operable as a generator configured to charge the battery 3. The motor 4 is connected to the pump 5 which is a reversible, i.e. bidirectional, pump. Hence, the pump 5 is operable as a pump, driven by the motor 4, and as a turbine driving the motor 4. The pump 5 has a first port 5a and a second port 5b, both of which are adapted to receive and discharge a hydraulic fluid, such as oil.
A first line 6 connects the first port 5a of the pump 5 to an outlet 7a of a reservoir 7, such as a tank for a hydraulic fluid. The first line 6 is some type of means for circulating a hydraulic fluid, typically an internal line in a valve manifold. Materials that the first line 6 can be made of include plastic materials and metals. A check valve 8 is arranged in the first line 6 between the reservoir 7 and the pump 5. The check valve 8 is configured to allow fluid to flow towards the pump 5. It should be noted that, in other examples, the check valve 8 may be replaced by some other component capable of allowing fluid to flow from the reservoir 7 towards the pump 5 and preventing fluid to flow in the opposite direction. For example, instead of the check valve 8 there may be an electrically operated directional valve capable of altering the return direction of the fluid.
A second line 9, similar to the first line 6, connects the first port 5a to an inlet 7b of the reservoir 7. A first valve 10 is arranged in the second line 9 between the reservoir 7 and the pump 5. The first port 5a and the inlet 7b are thus indirectly connected via the first valve 10. In this example, the first valve 10 is a two-position directional control valve that is actuated by a solenoid. More precisely, the first valve 10 is a unidirectional proportional poppet valve. It should be noted that, in other examples, the first valve 10 may be some other type of valve adapted to selectively allow and prevent fluid flow in the second line 9 between the pump 5 and the reservoir 7. For example, the first valve 10 may be a spool valve having one closed and one unidirectional control position. Moreover, in other examples, the first valve 10 may, instead of being actuated by a solenoid, be actuated by some other electrical, mechanical or electromechanical means. The first valve 10 may be directly actuated or indirectly actuated.
A third line 11, similar to the first line 6, connects the second port 5b of the pump 5 to a first consumer 12, such a hydraulic cylinder or some other type of actuator. The first consumer 12 is operable in a lifting mode and in a lowering mode. The first consumer 12 may for example be adapted to provide a force that lifts the forks of a forklift truck. As will be further discussed below, the first consumer 12 and the second port 5b are in this example indirectly connected to each other.
A fourth line 13, similar to the first line 6, connects the first consumer 12 to the inlet 7b. A second valve 14 is arranged in the fourth line 13 between the first consumer 12 and the reservoir 7. As will be further discussed below, there are more components between the first consumer 12 and the reservoir 7 than the second valve 14. In this example, the second valve 14 is a two-position directional control valve that is actuated by a solenoid. More precisely, the second valve 14 is a pressure compensated proportional valve allocated to control lowering motion of a load by gravity. It should be noted that, in other examples, the second valve 14 may be some other type of valve adapted to selectively allow and prevent fluid flow in the fourth line 13 between the pump 5 and the reservoir 7. For example, the second valve 14 may be a non-compensated directional control valve. Moreover, in other examples, the second valve 14 may, instead of being actuated by a solenoid, be actuated by some other electrical, mechanical or electromechanical means. The second valve 14 may be directly actuated or indirectly actuated.
A pressure relief valve 15 is connected to the second port 5b of the pump 5 and the inlet 7b of the reservoir 7, whereby a fluid flow from the first consumer 12 in the third line 11 can bypass the second valve 14 and go directly to the inlet 7b should the pressure in the third line 11 rise above a threshold value of the pressure relief valve 15.
In this example, a second consumer 16 is connected to the fourth line 13 and to the second valve 14. The second consumer 16 is typically adapted to provide a force for moving the forks of a forklift truck in some other way than lifting and lowering them, for example moving the forks horizontally and/or tilting the forks. The first and second consumers 12, 16 are indirectly connected to the second valve 14, but this may or may not be the case in other examples. More precisely, two control valves 17, 18 and a check valve 19 are arranged between the second valve 14 and the first and second consumers 12, 16. The first consumer 12 is connected to the second valve 14 via one of the control valves 17, 18 and the check valve 19, which is closer to the second valve 14 than the control valve, and the second consumer 16 is connected to the second valve 14 via the other one of the control valves 17, 18. The check valve 19 is adapted to allow fluid flow towards the inlet 7b and to prevent fluid flow in the opposite direction. The check valve 19 may or may not be included in other examples. The control valves 17, 18 are in this example two-position directional control valves. More precisely, the control valves 17, 18 are bidirectional poppet solenoid valves.
Figure 2 shows a hydraulic system 101 for an industrial truck. The hydraulic system 101 includes a hydraulic apparatus 102 that has a battery 103, a motor 104 and a pump 105. The battery 103 is electrically connected, via a motor controller 104a, to the motor 104 which is an electric motor, for example an asynchronous motor, configured to be powered by the battery 103. The motor 104 is also operable as a generator configured to charge the battery 103. The motor 104 is connected to the pump 105 which is a reversible, i.e. bidirectional, pump. Hence, the pump 105 is operable as a pump, driven by the motor 104, and as a turbine driving the motor 104. The pump 105 has a first port 105a and a second port 105b, both of which are adapted to receive and discharge a hydraulic fluid, such as oil.
A first line 106 connects the first port 105a of the pump 105 to an outlet 107a of a reservoir 107, such as a tank for a hydraulic fluid. The first line 106 is some type of means for circulating a hydraulic fluid, typically an internal line in a valve manifold. Materials that the first line 106 can be made of include plastic materials and metals. A check valve 108 is arranged in the first line 106 between the reservoir 107 and the pump 105. The check valve 108 is configured to allow fluid to flow towards the pump 105. It should be noted that, in other examples, the check valve 108 may be replaced by some other component capable of allowing fluid to flow from the reservoir 107 towards the pump 105 and preventing fluid to flow in the opposite direction. For example, instead of the check valve 108 there may be an electrically operated directional valve capable of altering the return direction of the fluid.
A second line 109, similar to the first line 106, connects the first port 105a to an inlet 107b of the reservoir 107. A first valve 110 is arranged in the second line 109 between the reservoir 107 and the pump 105. The first port 105a and the inlet 107b are thus indirectly connected via the first valve 110. In this example, the first valve 110 is a two-position directional control valve that is actuated by a solenoid. In the normal, or unactuated, position, the first valve 110 is configured to prevent fluid flow from the pump 105 towards the reservoir 107 and to allow fluid flow in the opposite direction. In the actuated position, the first valve 110 is configured to allow fluid flow from the pump 105 towards the reservoir 107 and to prevent fluid flow in the opposite direction. Hence, the motor 104 may drive the pump 105 when the first valve 110 is in its unactuated position, and the pump 105 may drive the motor 104 when the first valve 110 is in its actuated position. It should be noted that, in other examples, the first valve 110 may be some other type of valve adapted to selectively allow and prevent fluid flow in the second line 109 between the pump 105 and the reservoir 107. For example, the first valve 110 may be a spool valve that has one closed position and one unidirectional control position. Moreover, in other examples, the first valve 110 may, instead of being actuated by a solenoid, be actuated by some other electrical, mechanical or electromechanical means. The first valve 110 may be directly actuated or indirectly actuated.
A third line 111, similar to the first line 106, connects the second port 105b of the pump 105 to a consumer 112, such as a hydraulic cylinder or some other type of mechanical actuator. The consumer 112 is operable in a lifting mode and in a lowering mode. The consumer 112 may for example be adapted to provide a force that lifts the forks of a forklift truck. In this example there are no valves or similar components arranged in the third line 111, so the consumer 112 is directly connected to the second port 105b. However, the consumer 112 and the second port 105b may be indirectly connected to each other in another example. Also, it is readily appreciated that the consumer 112 may be replaced by several consumers in another example.
A fourth line 113, similar to the first line 106, connects the consumer 112 to the inlet 107a . A second valve 114 is arranged in the fourth line 113 between the consumer 112 and the reservoir 107. The consumer 112 and the inlet 107b are thus indirectly connected via the second valve 114. In this example, the second valve 114 is a two-position directional control valve that is actuated by a solenoid. More precisely, the second valve 114 is a pressure compensated proportional valve allocated to control lowering motion of a load by gravity. In the normal, or unactuated, position, the second valve 114 is configured to prevent fluid flow from the consumer 112 towards the reservoir 107 and to allow fluid flow in the opposite direction. In the actuated position, the second valve 112 is configured to allow fluid flow from the consumer 112 towards the reservoir 107 and to prevent fluid flow in the opposite direction. It should be noted that, in other examples, the second valve 114 may be some other type of valve adapted to selectively allow and prevent fluid flow in the fourth line 113 between the consumer 112 and the reservoir 107. For example, the second valve 114 may be a spool valve that has one closed position and one unidirectional control position. Moreover, in other examples, the second valve 114 may, instead of being actuated by a solenoid, be actuated by some other electrical, mechanical or electromechanical means. The second valve 114 may be directly actuated or indirectly actuated.
A pressure relief valve 115 is connected to the second port 105b of the pump 105 and the inlet 107b of the reservoir 107, whereby a fluid flow from the third line 111 can bypass the second valve 114 and go directly to the inlet 107b if the pressure in the third line 111 rise above a threshold value of the pressure relief valve 115. The functionality of the second valve 114 and the pressure relief valve 115 may be combined as one unit using for instance an electrically operated pressure control valve.
With reference to Figure 2 and 3, an example of the operation of the hydraulic system 101 will be discussed. For the sake of this discussion, it will be assumed that the lifting and lowering modes of the consumer 112 correspond to the lifting and lowering of the forks of a forklift truck.
In the lifting mode, the the battery 102 powers the motor 104 that drives the pump 105. The pump 105 draws a hydraulic fluid from the reservoir 107, the fluid flowing through the first line 106 from the outlet 107a to the first port 105a via the check valve 108. The fluid is discharged through the second port 105b and flows through the third line 111 into the consumer 112, whereby the consumer 112 provides a force that lifts the forks.
In the lowering mode, fluid inside the consumer 112 is discharged as the forks are lowered. The discharged fluid flows to the inlet port 107b of the reservoir 107, the flow path depending on whether an operating parameter of the hydraulic system is above or below a threshold value. That is to say, the flow path of the fluid is controllable, and the control is achieved by first determining the value of the operating parameter in a step S1 and then adjusting the flow path accordingly in a step S2. More specifically, if the value is determined to be below a threshold value, the second valve 114 is switched to a state allowing fluid flow so that the fluid can flow from the consumer 112 to the inlet 107b via the second valve 114. If, on the other hand, the value is determined to be above the threshold value, the first valve 110 switched to state allowing fluid flow so that the fluid can flow from the consumer 112 to the inlet 107b via the pump 105 and the first valve 110. Fluid flowing through the pump 105 drives the pump 105 which, in turn, drives the motor 104 operating as a generator that charges the battery 102. Thus, in the lowering mode, some of the energy of the flowing fluid is recovered and stored by the battery.
It should be noted that when one of the first and second valves 110, 114 is open the other valve may be closed or open. In the former case, all of the fluid flows via the open valve to the reservoir 107, whereas in the latter case some of the fluid flows via the first valve 110 and some of the fluid flows via the second valve 114. Hence, it is possible to control the amount of fluid that passes through the pump 105, something which makes it possible control the RPM of the generator 104 as well as the speed with which the forks are lowered. By increasing or decreasing the amount of fluid that passes through the pump 105, the RPM of the generator 104 can be increased or decreased, respectively, and thus controlled at a level at which the energy of the flowing fluid can be most efficiently recovered. The lowering speed of the forks may be increased or decreased by having more or less fluid pass through the second valve 114 so as to bypass the pump 105 and the first valve 110.
The person skilled in the art realizes that the present invention is not limited to the preferred embodiments described above. For example the regeneration system disclosed may be used in any type of industrial trucks, including low-lift trucks and tiller trucks. Further, additional valves for improving the control and operation even further may be included.
Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, a single unit may perform the functions of several means recited in the claims.

Claims

A hydraulic system (1; 101) for an industrial truck, comprising
a reservoir (7; 107) for a hydraulic fluid, the reservoir (7; 107) having an inlet (7b; 107b) and an outlet (7a; 107a);
a hydraulic apparatus (2; 102) operable as a pump and as a generator, the hydraulic apparatus (2; 102) having a first port (5a; 105a) and
a second port (5b; 105b), the first port (5a; 105a) being connected to the outlet (7a; 107a); and
at least one consumer (12; 112) operable in a lifting mode and in a lowering mode, the at least one consumer (12; 112) being connected to the second port (5b; 105b),
wherein the inlet (7b; 107b) is connected to the first port (5a; 105a) via a first valve (10; 110) and to the at least one consumer (12; 112) via a second valve (14; 114), wherein each of the first and second valves (10, 14; 110, 114) is switchable between a first state allowing fluid flow towards the reservoir (7; 107) and a second state blocking fluid flow towards the reservoir (7; 107), and wherein a fluid flow from the at least one consumer (12; 112) to the reservoir (7; 107) is controllable so as to pass through at least one of the first and second valves (10, 14; 110, 114),
characterized in that the industrial truck is a forklift truck, and the lifting mode is for lifting forks of said forklift truck and the lowering mode is for lowering the forks of said forklift truck, and in that it further comprises a control in the lowering mode arranged to:
determine a value of an operating parameter of the hydraulic system (1; 101);

allow a fluid flowing from the consumer (12; 112) to the inlet (7b; 107b) of the reservoir (7; 107) via the second valve (14; 114) to bypass the pump (5; 105) of the hydraulic apparatus (2; 102) if the value is below a threshold value; and

allow the fluid to flow from the consumer (12; 112) to the inlet (7b; 107b) of the reservoir (7; 107) via the pump (5; 105) of the hydraulic apparatus (2; 102) and the first valve (10; 110) if the value is above the threshold value, wherein fluid flowing through the pump (5; 105) of the hydraulic apparatus (2; 102) drives the pump (5; 105) which, in turn, drives the motor (4; 104) of the hydraulic apparatus (2; 102), said motor (4; 104) operating as a generator.
The hydraulic system (1; 101) according to claim 1, wherein at least one of the first and second valves (10, 14; 110, 114) is gradually switchable between the first and second states.
The hydraulic system (1; 101) according to claim 1 or 2, wherein at least one of the first and second valves (10, 14; 110, 114) is a directional control valve.
The hydraulic system (1; 101) according to any of the preceding claims, further comprising a pressure relief (15; 115) valve connected to the second port (5b; 105b) and to the inlet (7b; 107b), wherein the pressure relief valve is configured to allow fluid flow, from the at least one consumer (12; 112) to the inlet (7b; 107b), to bypass the second valve (14; 114) when a pressure at the second port (5b; 105b) exceeds a threshold value.
A forklift truck having the hydraulic system (1; 101) according to any of the preceding claims.
The forklift truck according to claim 5, wherein the control in the lowering mode is arranged to continuously monitor the operating parameter.
The forklift truck of claim 5 or 6, wherein the operating parameter is a rotation frequency of the power generator.