SHUTTLE VALVE FOR COMPENSATING DIFFERENTIAL FLOW RATE OF SINGLE-ROD ACTUATORS IN HYDROSTATIC SYSTEMS

Abstract

 The present invention relates to an improvement in the shuttle valve spool (4.2) of a hydraulic unit comprising a single-rod hydraulic actuator (1) having two ports directly connected to the pump (2) inlet-outlet ports, a flow rate controlled pump (2), which regulates the flow rate going to said single-rod hydraulic actuator (1), a hydraulic accumulator (3) used for compensating the differential flow rate formed in the hydrostatic circuit due to hydraulic actuator (1) movement, and a shuttle valve (4) providing bidirectional flow between the hydrostatic circuit and said hydraulic accumulator (3) and comprising shuttle valve spool (4.2) at the inner part thereof. The invention is characterized in that; with the operation of the hydraulic unit, in order to prevent the instability problem encountered during compensation of the differential flow rate occurring as a result of the asymmetric actuator (1) structure, at the center position of said shuttle valve spool (4.2), it comprises: a valve spool overlap (4.7) found between Port A (4.4) that is connected to the cap-side chamber (1.4) of the hydraulic actuator (1) and Port C (4.6) that is connected to the hydraulic accumulator (3); and preventing the flow between the hydraulic accumulator line (C) connected to said hydraulic accumulator (3) and the cap-side chamber line (A) connected to the cap-side chamber (1.4) of the hydraulic actuator (1), and a valve spool underlap (negative spool overlap) (4.8) found between Port B (4.5) that is connected to the rod-side chamber (1.5) of the hydraulic actuator (1) and Port C (4.6) that is connected to the hydraulic accumulator (3); and enabling flow between the hydraulic accumulator line (C) connected to the hydraulic accumulator (3) and the rod-side chamber line (B) connected to the rod-side chamber (1.5) of the hydraulic actuator (1).

Description

TECHNICAL FIELD

[0001] The present invention relates to closed circuit hydrostatic systems, in which the movement of a single-rod hydraulic actuator is controlled by means of regulating the flow rate of a single pump.

[0002] The invention particularly relates to an improvement provided in the shuttle valve spool structure in a closed circuit hydrostatic system, where the movement of a single-rod actuator is controlled by regulating the flow rate of a single pump, in order to solve the instability problem encountered while compensating the unequal flow rate, at two ports of the actuator, that occurs due to the asymmetric structure of the actuator.

THE PRIOR ART

[0003] In the known status of the art, several hydrostatic circuit solutions are found in the literature about the control of a single-rod hydraulic actuator movement by means of directly regulating the pump flow rate. The reasons of the problem to be solved by said hydraulic circuits is the occurrence of deficient/excessive flow rate in the closed circuit system. In case of a forward/backward movement, since the hydraulic actuator has an asymmetric structure, the entering and leaving flow rates at the two ports of the actuator are unequal and has to be compensated. The deficient/excessive flow rate occurring in a closed hydrostatic circuit is named as differential flow rate. The differential flow rate is directly determined by the cross sectional area of actuator rod and can be expressed as below:

[0004] In this equation; "V" is hydraulic actuator velocity, "A" is hydraulic actuator cylinder area, and "α" is the ratio of the actuator rod-side area to the cap-side area. In order to compensate the differential flow rate expressed as Δq in the equation, closed circuit hydrostatic solutions are found in the literature, wherein two pumps, a pump with 3 ports, or hydraulic transformers are used. Moreover, open circuit solutions, wherein multiple on/off valves are used, are also proposed.

[0005] The basic requirements to be met by hydrostatic circuit solutions wherein a single-rod actuator movement is controlled by a single pump are conceptually shown in Figure 4. In this system, the pump is directly connected to the hydraulic actuator. In this way, the actuator movement is controlled by means of regulating the pump flow rate, either by changing the pump displacement or pump speed. A hydraulic source (K) is used for compensating the differential flow rate (Δq). Said hydraulic source (K) is connected to the hydrostatic circuit via various hydraulic connection components (L). The deficient/excessive flow rates formed in closed hydraulic circuits are compensated by means of providing a bidirectional flow on these connection components (L). In the hydraulic circuit solutions found in the literature, generally, a hydraulic accumulator which is pressurized at a certain level is used as the hydraulic source (K). However, there might be differences in pressurizing the hydraulic accumulator of the prior art solutions. About connection of the hydraulic source (K) to the hydrostatic circuit, various solutions can be found. While, some of the solutions propose the use of pilot operated check valves, other solutions propose the use of shuttle valves. Among these solutions, the number of said valves and their way of connection to the circuit may vary.

[0006] The most common hydrostatic circuit solution wherein a single-rod hydraulic actuator motion is controlled by a single pump is provided by Rahmfeld and Ivantsynova [1]. This circuit solution is shown in Figure 5. The amount of flow rate going to the actuator (1) is determined by a variable displacement pump (2) that is able to operate in 4 quadrants. The secondary pump (M) which is shown to have a tandem connection is merely used for pressurizing the hydraulic accumulator (3), instead of controlling the flow rate going to the actuator (1), and therefore called as charge pump. The hydraulic accumulator (3) fed by a charge pump and maintained at a certain pressure level via a pressure relief valve is used for compensating the differential flow rate formed in the closed circuit system. The differential flow rate formed during actuator (1) movement is compensated through pilot operated check valves (N) found between the hydraulic accumulator (3) and the hydrostatic circuit. Besides these components, the system pressure is limited by a conventional method, wherein two pressure relief valves (O) are connected between the actuator chambers and the accumulator line.

[0007] In the patent research made about this subject, the invention no US5329767A, related to hydraulic circuit flow control, is encountered. The application, schematic view of which is given in Figure 6, comprises a closed circuit hydrostatic circuit solution wherein a shuttle valve (4) is used for a single-rod hydraulic actuator (1). This circuit solution also uses a variable displacement pump (2) and a low pressure line is formed by using a hydraulic accumulator in order to eliminate the differential flow rate. Different from the previous circuit solution, the flow that would eliminate the differential flow rate between the low pressure line and the hydrostatic system is provided by means of adjusting the position of a two-position three-way shuttle valve (4). Besides, a unidirectional flow path is provided from the low pressure line towards the hydrostatic circuit by means of using two check valves (6). While these check valve connections provide the deficient flow rate to be formed in the closed circuit system with the forward movement of the actuator, they also ensure maintaining the low actuator chamber pressure at a level that is close to the accumulator line pressure, and thus eliminate the risk of cavitation.

[0008] Use of shuttle valve for compensating/eliminating the differential flow rate occurring in closed circuit systems is also considered in various other studies. Among these, the application with publication no US20090120278A1 relates to an electro-hydrostatic actuator with 4-port, double-displacement pump, the application with publication no WO2009102740A3 relates to a flow control system for hydraulic machines, the application with publication no US8033107B2 relates to a hydrostatic drive embodiment with volumetric flow compensation, the application with publication no JPS58102806 (A) relates to a closed oil pressure circuit for actuator movement, the application with publication no US20110209471A1 relates to an embodiment about the velocity control of an unbalanced hydraulic actuator exposed to excessive central load conditions. In these studies, generally, internal pilot operated 3-position 3-way shuttle valves with closed center are used, instead of 2/3 shuttle valves. With the use of pilot operated shuttle valve, the position of the shuttle valve for compensation of deficient/excessive flow rates to be formed on the hydrostatic circuit is determined by system pressures and an external controller together with a valve actuator (solenoid) is not required.

[0009] Some of these studies are related to the flow rate control of single pump hydrostatic circuit designed for single-rod hydraulic actuator [WO2009102740A3] and the velocity control of the actuator [US20110209471A1], while some others are related to elimination of energy losses and cavitation [JPS58102806A]. In some other studies related to the use of shuttle valve in closed hydrostatic circuit, addition of a flow rate controlled secondary pump to the hydrostatic circuit [US8033107B2], and use of a 4-port, double displacement pump [US20090120278A1] are suggested. The common part of the hydrostatic circuits suggested or disclosed within the scope of these studies is the use of a closed center 3/3 shuttle valve for the purpose of eliminating/compensating differential flow rate.

[0010] Different from the previous studies where, a hydrostatic circuit is developed for single-rod hydraulic actuators, Wang and Book mentioned the internal stability issues, and disclosed that in order to eliminate the differential flow rate, a 3/3 displacement shuttle valve (4) has to be used, instead of a pilot operated check valve [2]. In his study, Wang first investigate the circuit solution disclosed previously by Ivantsynova [3], and has named the undesired and uncontrolled pressure and velocity oscillations formed in some situations due to switching of the system between the pumping and the motoring modes, as the internal instability of the system, and determined the reason of the problem as pilot operated check valves. Wang has defined the required operating region for problem-free operation of a hydrostatic circuit on a pressure plane defined by the cap-side (P_a) and the rod-side (P_b) chamber pressures of the actuator, as shown in Figure 8-a. He has proposed that this required pressure region can be obtained with the use of 3/3 shuttle valve (4). Otherwise, with the use of pilot operated check valves (6), the resulting operating region would be as shown in Figure-8-b. He further disclosed that, the instability situation occurring during the use of pilot operated check valve (6) is because the two check valves (6) are closed in the operating region determined by the accumulator line pressure (P₀) and pilot pressure (P₀₁) as shown in Figure 8-b.

[0011] In the same study, Wang L. has disclosed that, the shuttle valve system that he proposed could meet the required operating region defined in Figure 8-a on the pressure plane; however, couldn't eliminate undesired pressure oscillations. He has disclosed the reason for this as the presence of two different equilibrium point sets corresponding to a sngle input set acting on the system when the actuator chamber pressures are very close to each other [2]. These two different equilibrium points correspond to the pump and motor modes of the system and are stable; however, undesired pressure oscillations occur, since the system goes into a limit cycle between these two stable equilibrium points. In order to eliminate the limit cycle behavior, namely the undesired oscillations, two 2/2 flow control valves (8) are added to the hydrostatic circuit. In order to enable opening and closing of these two valves at certain chamber pressures, a controller was designed, which would use the data obtained from the sensors detecting the actuator chamber pressures, so that the problem of undesired pressure oscillations is solved. In a study made later on, Wang L. has disclosed that switching between two stable equilibrium points only occurs during the retraction of the actuator, and has mathematically shown this limit cycle [4]. Moreover, in this study, besides the physical leakage solution by using the two 2/2 flow control valves, he also proposed a control topology based on a virtual leakage flow method [4].

[0012] In these two studies, Wang L. has neglected the shuttle valve dynamics and formed the mathematical model of the system by accepting the shuttle valve as an ideal switching component, either opened to the left or right. Actually, the closed center 3/3 shuttle valve (4) he used in the studies do not provide the required operating region on the actuator chamber pressure plane that he defined as shown in Figure 8-a. This is because the valve spool stays still in the center position and do not do switching until the difference between the actuator chamber pressures correspond to a force that would overcome the pre-compression of the spring that holds the shuttle valve in the center. Since the valve he uses is a closed center one, when the pressure difference between the actuator chambers is less than the shuttle valve cracking pressure, both of the actuator chambers are closed to the accumulator line, and thus an operating region similar to the use of pilot operated check valve (Figure 8-b) is formed.

[0013] The analysis of the hydrostatic circuit developed for the single-rod hydraulic actuator taking the shuttle valve dynamics into account is made by Çali

kan H. [5]. In this study, the configurations of the hydraulic circuit changing according to the position of the shuttle valve are defined on load pressure vs actuator velocity (p_L - ν) plane by using the state variables of actuator velocity and the load pressure. In this study, it is mathematically shown that, when the shuttle valve spool is in fully open or central position, the system is stable and has a single equilibrium point; however, when the shuttle valve spool is partially open, the system is stable only during the extension of the actuator, and during the retraction of the actuator the corresponding equilibrium point is unstable. The region where the shuttle valve spool is not in fully open position is defined as critical load pressure region. It is disclosed that, when a closed center valve is used, the valve spool can not stay in the central position in this critical load pressure region, and would be partially opened in order to compensate the deficient flow rate formed during the retraction, and thus the system would bu unstable. For the solution of the instability problem, instead of using a closed center shuttle valve, use of a shuttle valve having a certain orifice opening at the central position is proposed. It is disclosed that the proposed partial orifice opening should be determined such that all of the differential flow rate to be formed at the maximum retracting velocity of the actuator would be met through the two orifices of the centrally positioned valve spool. In this way, instability problems could be solved by a physical technique without using extra valve, sensor, or control components. In this study, it is also disclosed that the orifice opening obtained by spool underlap at center position, shouldn't exceed a predetermined limit, and in the case of using a completely open center valve, a circuit structure wherein both of the two actuator chambers would be connected to each other directly, and thus dead pump flow rates would be formed at the critical load pressure region and the actuator movement can not be controlled.

[0014] As a result of studies, it is mathematically shown that, if the critical velocity (|ν| > |ν_cr|) is exceeded while the hydraulic actuator is moving backward (being retracted), the valve spool would be partially opened such that it would connect the actuator cap-side chamber to the accumulator line; and if the critical velocity is exceeded while it is moving forward, the valve spool is partially opened such that it would connect the actuator rod-side chamber to the accumulator line. Again, it is also disclosed and mathematically proven that the equilibrium point that corresponds to the partial valve opening formed while the hydraulic actuator is moving forward would be stable; however, the equilibrium point that corresponds to the valve opening formed while the hydraulic actuator is moving backward would be unstable.

[0015] In the hydrostatic system comprising a single-rod hydraulic actuator, a pump, a hydraulic accumulator, and a shuttle valve as main components, while the hydraulic actuator moves backward,

• choosing the shuttle valve as closed center valve causes instability problems, since the valve that does not have orifice opening in the central position opens partially in order to compensate the differential flow rate.
• the problem of instability can be solved up to a certain retraction speeds by means of providing orifice openings between the A-C and B-C ports of the shuttle valve at center position. However, since both A-C & B-C conduits are opened, flow rate passage can occur between A-B ports at certain parts of critical load pressure region. This unnecessary flow rate passage that does not affect the actuator movement but causes dead pump velocity and thus increases loss of energy and makes the control algorithm more difficult.

[0016] As a result; improvement is to be made in hydraulic systems, in order to solve the problems of dead pump speed and instability due to asymmetric hydraulic actuator structure, and therefore need has occurred for novel embodiments which would eliminate the above said drawbacks and bring in solutions to present problems.

PURPOSE OF THE INVENTION

[0017] The present invention relates to shuttle valve orifice openings, which meet the above said requirements, eliminate all of the drawbacks and bring about some additional advantages.

[0018] The primary purpose of the invention is to change the spool structure found in the shuttle valve of a hydrostatic circuit comprising interconnected single-rod hydraulic actuator, pump, hydraulic accumulator, and shuttle valve components, in order to solve the problems of instability and dead pump speed in hydraulic systems.

[0019] Another purpose of the invention is to provide a solution in hydraulic units comprising single-rod hydraulic actuator that wouldn't cause dead pump speed in the operating region defined as critical load region.

[0020] Another purpose of the invention is to eliminate the instability problem of the prior art by means of using a valve spool comprising a valve spool underlap (negative spool overlap) such that a certain flow rate passage would be provided between the accumulator line and the actuator rod-side chamber line, and a valve spool overlap such that flow rate passage would be avoided between the accumulator line and the actuator cap-side chamber line, in the center position of the shuttle valve.

[0021] Another purpose is to eliminate the problem of instability up to a certain velocity [ν_cr] while the hydraulic actuator is retacting in the region defined as critical load region, by means of the valve spool embodiment operating between the valve spool underlap (negative spool overlap) and the valve spool overlap.

[0022] Another purpose of the invention is to send the excess flow rate to be formed during the retraction of the hydraulic actuator to the hydraulic accumulator through the valve spool underlap found between port B and port C of the shuttle valve.

[0023] Another purpose of the invention is to create a valve spool overlap between port A and port C, and in this way, eliminate the unnecessary flow rate between port A and port B, so that the maximum flow rate [q_v|_xs=0] that can be provided to the system through the shuttle valve would be improved compared to the prior art solution, and thus higher critical velocities would be obtained.

[0024] Another purpose of the invention is to prevent unnecessary energy losses in the critical load region by means of using only one check valve in the system (would be used between the accumulator line and the rod-side chamber line, and wouldn't be used between the accumulator line and the cap-side chamber line).

[0025] In order to achieve the above said advantages which will be better understood from the below given detailed description, the present invention relates to an improvement in the shuttle valve spool of a hydraulic unit comprising a single-rod hydraulic actuator in order to prevent the instability problem encountered during compensation of the differential flow rate occurring as a result of the asymmetric actuator structure, such that at the center position of said shuttle valve spool, it comprises:

• a valve spool overlap found between Port A that is connected to the cap-side chamber of the hydraulic actuator and Port C that is connected to the hydraulic accumulator; and preventing the flow between the hydraulic accumulator line connected to said hydraulic accumulator and the cap-side chamber line connected to the cap-side chamber of the hydraulic actuator,
• and a valve spool underlap found between Port B that is connected to the rod-side chamber of the hydraulic actuator and Port C that is connected to the hydraulic accumulator; and enabling flow between the hydraulic accumulator line connected to the hydraulic accumulator and the rod-side chamber line connected to the rod-side chamber of the hydraulic actuator.

[0026] The structural and characteristic features of the invention and all advantages will be understood better in detailed descriptions with the figures given below and with reference to the figures, and therefore, the assessment should be made taking into account the said figures and detailed explanations.

BRIEF DESCRIPTION OF THE FIGURES

[0027] For better understanding of the embodiment of present invention and its advantages with its additional components, it should be evaluated together with below described figures.

Figure 1

; is a schematic view of the hydrostatic circuit with single-rod actuator comprising the shuttle valve spool embodiment of the invention.

Figure 2

; is a side profile sectional view of a preferred embodiment of the present invention shuttle valve comprising valve spool underlap and overlap.

Figure 3

; is the schematic view of the operating principle and possible circuit embodiments of the single-rod hydraulic actuator hydrostatic circuit.

Figure 4

; is the general structure schematic view of the prior art single-rod hydrostatic circuit solution controlling the movement of the single-rod hydraulic actuator.

Figure 5

; is the schematic view of the prior art closed circuit hydrostatic circuit solution, wherein pilot operated check valves are used for a single-rod hydraulic actuator.

Figure 6

; is the schematic view of the prior art closed circuit hydrostatic circuit solution, wherein shuttle valves are used for a single-rod hydraulic actuator.

Figure 7

; is the schematic view of the prior art closed circuit hydrostatic circuit solution, wherein 3/3 positioned shuttle valves are used for a single-rod hydraulic actuator.

Figure 8 a,b

; is the view of a: The operating region required for problem-free operation of a hydrostatic circuit defined by Longke W.; and b: The operating region provided by a pilot operated check valve circuit proposed by prior art studies [1], on a pressure plane defined by the actuator piston and the rod-side chamber pressures.

REFERENCE NUMBERS

[0028] 

1.

Hydraulic Actuator (Single-rod)

1.1.

Piston

1.2.

Piston shaft

1.3.

Piston cylinder

1.4.

Cap-side chamber

1.5.

Rod-side chamber

1.6.

Cap-side piston surface

1.7.

Rod-side piston surface

1.8.

Rod cross sectional area

2.

Pump (Flow rate controlled)

3.

Hydraulic Accumulator

4.

Shuttle valve

4.1.

Body

4.2.

Valve spool

4.3.

Centering spring

4.4.

Port A

4.5.

Port B

4.6.

Port C

4.7.

Valve spool overlap

4.8.

Valve spool underlap (negative spool overlap)

4.9.

Pilot pressure acting surface

5.

Pressure relief valve

6.

Check valve

7.

Accumulator Charge Circuit

8.

Flow Control Valves

A:

Cap-side chamber line

B:

Rod-side chamber line

C:

Accumulator line

D:

Stable operating region

E:

Unstable operating region

F:

Critical load region

K:

Hydraulic source

L:

Hydraulic connection components

M:

Second Pump

N:

Pilot operated check valve

O:

Pressure relief valve

DETAILED DESCRIPTION OF THE INVENTION

[0029] In this detailed description, the preferred embodiments of the invention will only be disclosed for better understanding of the subject, and will not form any limiting effect.

[0030] The present invention relates to closed circuit hydrostatic systems, wherein the movement of a single-rod hydraulic actuator (1) is controlled by means of regulating the flow rate of a single pump (2). The improvement of the invention is an embodiment, which solves the problem of instability encountered during compensation of the differential flow rate that occurs as a result of asymmetric hydraulic actuator (1) structure in closed circuit hydrostatic systems, wherein the single-rod actuator (1) movement is controlled via the flow rate of a single pump (2), by means of changing the shuttle valve spool (4.2) structure. For this purpose, valve spool underlap (4.8) and valve spool overlap (4.7) are formed on the shuttle valve spool (4.2) for the central position of the shuttle valve (4), that results with closed and partially open orifice forms between A-C and B-C conduits respectively.

[0031] Figure 1 shows the schematic view of the hydrostatic circuit comprising the shuttle valve (4) embodiment of the present invention. Single-rod actuator (1) is used in said hydrostatic circuit. Main components of the hydrostatic circuit consist of:

• a single-rod hydraulic actuator (1) formed of a piston (1.1), a piston rod (1.2), and a cylinder (1.3), and having two ports directly connected to the pump (2) ports,
• a flow rate controlled pump (2), which regulates the flow rate going to said single-rod hydraulic actuator (1), and which can operate at all four quadrants of the pressure-flow rate plane,
• a hydraulic accumulator (3) used for compensating the differential flow rate formed in the hydrostatic circuit due to hydraulic actuator (1) movement,
• an internal pilot operated shuttle valve (4) with 3 ways and 3 positions, which provides bidirectional flow between the hydrostatic circuit and the accumulator (3), and the position of which is determined by the cap-side chamber (1.4) and the rod-side chamber (1.5) pressures of the hydraulic actuator (1) so that it determines which actuator (1) chamber (1.4 or 1.5) would be connected to the accumulator (3) line.

[0032] The control of the position, speed, or force of the hydraulic actuator (1) is made by means of controlling the flow rate entering into/leaving from the single-rod hydraulic actuator (1) in the hydraulic unit. The two ports of the hydraulic actuator (1), found at the cap-side (1.4) and rod-side (1.5) are directly connected to the inlet and outlet ports of the hydraulic pump (2). In the flow rate controlled pump (2), the direction of the pump flow rates and the positions of the pump (2) pressure/suction ports can vary according to the velocity of the hydraulic actuator (1) and the load applied. Pump flow rate, and thus the hydraulic actuator (1) movement can be controlled by means of changing the pump (2) speed or displacement. Different from the conventional pumps, the pump (2) that performs flow rate control has the characteristic of operating in all 4 quadrants of the pressure-flow rate plane. As can be seen in Figure 1, the hydraulic unit comprises: a cap-side chamber line (A) providing direct connection between the pump (2) and the cap-side chamber (1.4), a rod-side chamber line (B) providing direct connection between the pump (2) and the rod-side chamber (1.5), and an accumulator line (C) providing connection between the shuttle valve (4) and the hydraulic accumulator (3). A line from the 3-way shuttle valve (4) is connected to the cap-side chamber line (A), while another line is connected to the rod-side chamber line (B).

[0033] Besides the main components of hydraulic actuator (1), flow rate controlled pump (2), hydraulic accumulator (3), and shuttle valve (4), the hydraulic unit also comprises auxiliary components such as pressure relief valve (5) restricting the pressures of the cap-side chamber (1.4) and the rod-side chamber (1.5) found in the hydraulic actuator (1), a check valve (6) providing unidirectional flow from the hydraulic accumulator (3) line to the rod-side chamber (1.5) of the actuator (1) in order to prevent cavitation formation, and an accumulator charge circuit (7) maintaining the hydraulic accumulator (3) at a certain pressure level.

[0034] Figure 2 shows a side profile view of a preferred embodiment of the shuttle valve (4) having an asymmetric partial opening at a central position. The shuttle valve (4) mainly consists of an outer body (4.1) and a cylindrically-shaped valve spool (4.2) embedded in said body (4.1). The valve spool (4.2) positioned centrally on the shuttle valve (4) determines the structure of the orifice opening. One end of the cylindrical valve spool (4.2) is connected to the valve centering spring (4.3). Said valve centering spring (4.3) is the factor determining the cracking pressure of the shuttle valve (4) and thus the size of the critical load region, by means of maintaining the valve spool (4.2) at a central position. The pilot pressure acting surfaces (4.9) found on the valve spool (4.2), together with the centering spring (4.3), play a role in determining the cracking pressure of the shuttle valve (4) and thus the size of the critical load region. The shuttle valve (4) is a 3-way, 3-position internal pilot operated valve. With this regard, three ports (4.4, 4.5, 4.6) are found on the shuttle valve (4) body (4.1). Among these ports:

• Port A (4.4) is connected to the cap-side chamber (1.4) through a port found on the piston cylinder (1.3) of the hydraulic actuator (1),
• Port B (4.5) is connected to the rod-side chamber (1.5) through a port found on the piston cylinder (1.3) of the hydraulic actuator (1), and
• Port C (4.6) is connected to the hydraulic accumulator (3) line.

[0035] The improvement of the invention is found at a central position on the shuttle valve (4) spool (4.2) as a valve spool underlap (4.8) between Port B (4.5) and Port C (4.6), and a valve spool overlap (4.7) between Port A (4.4) and Port C (4.6).

[0036] The valve spool underlap (4.8) enables flow between the hydraulic accumulator line (C) connected to the hydraulic accumulator (3) and the rod-side chamber line (B) connected to the rod-side chamber (1.5) of the hydraulic actuator (1), while the shuttle valve (4) is in central position. The valve spool overlap (4.7) prevents flow between the hydraulic accumulator line (C) connected to the hydraulic accumulator (3) and the cap-side chamber line (A) connected to the cap-side chamber (1.4) of the hydraulic actuator (1), while the shuttle valve (4) is in central position.

[0037] By means of the valve spool underlap (4.8) found between Port B (4.5) and Port C (4.6), at the region defined as critical load region, when the hydraulic actuator (1) is retracted, the differential flow rate formed in the system is sent to the hydraulic accumulator (3) through the hydraulic accumulator line (C). In this way, the shuttle valve (4) stays at the central position up to a certain velocity, and a stable operating region is obtained at the critical load region (F). By means of the valve spool overlap (4.7) found between Port A (4.4) and Port C (4.6), flow rate passage is prevented between Port A (4.4) and Port C (4.6) and unnecessary flow rate is not formed between Port A (4.4) and Port B (4.5). At the central position, when the valve spool overlap (4.7) is closed, critical velocity is increased with regard to the shuttle valves having partially open A-C and B-C orifice structures at the central position [5].

[0038] In the hydrostatic circuit of the present invention shown in Figure 1, movement of a single-rod hydraulic actuator (1) is controlled by means of regulating the flow rate of a two-port pump (2), in which the two ports are directly connected to the actuator. The pump (2) used in the system can generate flow rate in both directions and both of its ports can be pressurized. This component defined as pump (2) in hydraulic systems, can also act as a hydraulic motor. This component called as a pump (2) in the circuit and can operate in all four quadrants of the pressure-flow rate plane can preferably be a variable displacement pump or a variable speed constant displacement pump or a displacement and speed controlled hydraulic pump/motor. The requirement for the hydrostatic system of the invention is to be able to regulate/adjust the output flow rate of the component defined as pump (2), instead of its physical structure.

[0039] In a closed circuit system, the closed area found in front of the piston (1.1) of the hydraulic actuator (1) forms the cap-side chamber (1.4); while the closed area found at the rear part and including the piston rod (1.2) forms the rod-side chamber (1.5). Since the cap-side surface (1.6) of the piston (1.1) facing the cap-side chamber (1.4) and the rod-side surface (1.7) facing the rod-side chamber (1.5) have different areas, during the movement of the piston-rod (1.1, 1.2) assembly of the hydraulic actuator (1), unequal flow rates are formed at the inlet-outlet ports of the hydraulic actuator (1).

[0040] The difference between the flow rates entering and leaving the hydraulic actuator (1) at any moment is called as differential flow rate. The differential flow rate is directly determined by the rod cross sectional area (1.8) of the actuator rod (1.2). The differential flow rate formed by the hydraulic actuator (1) causes formation of deficient or excessive flow rate in the hydrostatic system having closed circuit structure. Therefore, a hydraulic accumulator (3) is used in order to eliminate differential flow rate and pump leakage etc. losses. Systems (7) having different hydraulic circuit structures can be used in order to charge or maintain the pressure level of said hydraulic accumulator (3). In order to eliminate the hydraulic actuator (1) differential flow rate, bidirectional flow rate passage between the closed circuit hydraulic system and the hydraulic accumulator (3) is provided through the 3-way, 3-position, and internal pilot operated shuttle valve (4). The position of the shuttle valve (4), and thus which one of the actuator chambers (1.4 or 1.5) that the hydraulic accumulator (3) would be connected is determined by the pressures of the actuator chambers (1.4 and 1.5), which are connected to the pilot lines of the shuttle valve (4). Pressures of the actuator chambers (1.4 and 1.5) are restricted in the hydraulic system by using pressure relief valve (5). Check valve (6) is used to prevent possible risk of cavitation.

The operating principle of the hydrostatic circuit comprising the shuttle valve (4) embodiment of the invention is as follows:

[0041] The operating principle of the present invention hydrostatic circuit is shown in Figure 3, on the (f_L - ν) plane formed by the variables: (f_L) for the external force applied on the hydraulic actuator (1) and (v) for actuator velocity. When factors such as leakage losses and compressibility of oil are neglected, all of the differential flow rate formulated as (1 - α)Aν formed in the hydrostatic system with the hydraulic actuator (1) movement, is met through the shuttle valve (4). This situation can be expressed with the below given flow continuity equation. In this equation, (q_v) is the total amount of flow rate passing through the shuttle valve, (α) is the ratio of the rod-side surface area (1.7) to the cap-side surface area (1.6), (A) is the cap-side surface area (1.6), and (ν) is the velocity of the actuator piston cylinder assembly (piston (1.1) and piston shaft (1.2)).

[0042] The velocity (v) of the hydraulic actuator (1) determines the direction of the flow rate passing through the shuttle valve (4). During forward movement (v>0), deficient flow rate necessity occurs, and this deficiency is met for the system by the hydraulic accumulator (3). In the backward movement (v>0), excess flow rate is formed and it is sent to the hydraulic accumulator (3).

[0043] In Figure 3, it can be seen that the hydrostatic system has 3 different circuit embodiments according to the position of the shuttle valve (4). The position of the shuttle valve (4) is determined by the pressures of the actuator chambers (1.4 and 1.5), and therefore by the external force (f_L) acting on the hydraulic actuator (1).

[0044] The accumulator line (C) is connected via the shuttle valve (4) to the actuator (1) cap-side chamber line (A) in the f_L ∈ (-∞, f_L1) operating region found at the left hand side in Figure 3, and to the rod-side chamber line (B) of the actuator (1) in the f_L ∈ (f_L2,∞) operating region found at the right hand side. Shuttle valve (4) is in fully open position in these two regions, and the valve spool (4.2) position is saturated at its end position. In the intermediate region defined by f_L ∈ (f_L1,f_L2), the shuttle valve (4) is either centered or partially opened. This region is defined as critical load region (F). The location of the critical load region (F) is determined by the hydraulic accumulator (3) pressure; while its size is determined by the cracking pressure of the shuttle valve (4) used.

[0045] Problems defined as system instability and pump mode oscillation are formed when the operating region of the hydrostatic system corresponds to the critical load region (F). With the novelty proposed within the scope of the present invention about the orifice structures at the central position of the valve spool (4.2), it is aimed to eliminate the problems encountered in the critical load region (F).

[0046] The value of the pressure difference between the actuator chambers (1.4 and 1.5) [|p_A - pB| = Δp] corresponding to the pre-compression force of the centering spring (4.3) that keeps the shuttle valve spool (4.2) at the center is defined as the cracking pressure [p_cr] of the shuttle valve (4). When the difference between the pressures of the actuator chambers (1.4 and 1.5) [Δp] is lower than the cracking pressure [p_cr] of the shuttle valve (4), the pre-compression force of the centering spring (4.3) can not be overcome, and thus the valve spool (4.2) would remain in the center position [x_s = 0]. In the critical load region (F) f_L ∈ (f_L1, f_L2), the valve spool (4.2) can remain in the center position up to a certain actuator velocity |v| < v_cr which is determined as critical velocity [ν_cr]. Physically, the differential flow rate [Δq] formed at the critical velocity [ν_cr] of the actuator (1) corresponds to the amount of maximum flow rate [q_v]|_xs=0] that can pass through the shuttle valve (4) when it is in center position [q_v|_xs=0 = (1-α)Av_cr]. Since the condition of shuttle valve (4) spool (4.2) staying [x_s = 0] in the center position is restricted with the valve cracking pressure [p_cr] , the critical velocity value of the hydraulic actuator (1) is obtained by means of equalization of the pressure difference [Δp] between the actuator chambers to the valve cracking pressure [Δp = p_cr] and thus solving the flow continuity and characteristic valve flow equations that define the system dynamics.

[0047] Since the differential flow rate to be formed when the actuator velocity exceeds the critical velocity [|ν| > ν_cr] would be greater than the amount of maximum flow rate that can pass through the shuttle valve (4) remaining in center position [q_v|_xs=0 < (1 - α)Aν_cr], the shuttle valve spool (4.2) would be partially opened in order to meet this flow rate demand.

[0048] Within the scope of the present invention, use of a shuttle valve (4) is proposed, which is found on the valve body (4.1), and which comprises a valve spool overlap (4.7) between Port A (4.4) and Port C (4.6), and a valve spool underlap (negative spool overlap) (4.8) between Port B (4.5) and Port C (4.6), when the valve spool (4.2) is in center position. The valve spool (4.2) operates between said positive valve spool underlap (4.7) and valve spool overlap (4.8). In this way, in the region that is defined as critical load region (F), the problem of instability would be eliminated up to a certain velocity [ν_cr] when the actuator (1) is retracted. The excess flow rate to be formed when the hydraulic actuator (1) is retracted would be sent to the hydraulic accumulator (3) through the valve spool underlap (4.8) between Port B (4.5) and Port C (4.6) of the shuttle valve (4). Since Port A (4.4) and Port C (4.6) of the shuttle valve (4) are closed in center position, unnecessary flow rate wouldn't be formed between the cap-side chamber (1.4) and the rod-side chamber (1.5) of the hydraulic actuator (1). By means of generating a spool overlap between Port A (4.4) and Port C (4.6) and thus eliminating the unnecessary flow rate between Port A (4.4) and Port C (4.6), the maximum flow rate [q_v|_xs=0] that can be provided to the system through the shuttle valve (4) would be increased compared to the prior art solutions, and thus higher critical velocities can be achieved.

[0049] In this detailed description, while said shuttle valve (4) shown in Figure 2 is preferably chosen as cartridge type, it is also possible to apply the improvements of the present invention to shuttle valves of different types and geometric structures. Besides, the centering spring (4.3) used for maintaining the valve spool (4.2) at the center position can be connected in another way or the valve spool underlap (4.8) and valve spool overlap (4.7) formed on the valve body via Port A (4.4), Port B (4.5), and Port C (4.6) within the scope of the invention can have a different geometric structure.

REFERENCES:

[0050] 

[1] Rahmfeld, R., and Ivantysynova, M., 2003, "Energy Saving Hydraulic Displacement Controlled Linear Actuators in Industry Applications and Mobile Machine Systems," The Fourth International Symposium on Linear Drives for Industry Applications, Birmingham, UK.

[2] Wang, L., Book, W. J., and Huggins, J. D., 2012, "A Hydraulic Circuit for Single Rod Cylinders," ASME J. Dyn. Syst., Meas., Control, 134(1), 011019, DOI: 10.1115/1.4004777.

[3] Williamson, C. and Ivantysynova, M., 2008, "Pump Mode Prediction for Four-quadrant Velocity Control of Valveless Hydraulic Actuators," Proceedings of the 7th JFPS International Symposium on Fluid Power, Toyama, Japan, Vol. 2, pp. 323-328, ISBN 4-931070-07-X.

[4] Wang, L. and Book, W. J., 2013, "Using Leakage to Stabilize a Hydraulic Circuit for Pump Controlled Actuators," ASME J. Dyn. Syst., Meas., Control, 135(6), 061007, DOI: 10.1115/1.4024900.

[5] Çali

kan, H., Balkan, T., and Platin, B. E., 2015, "A Complete Analysis and a Novel Solution for Instability in Pump Controlled Asymmetric Actuators," ASME J. Dyn. Syst., Meas., Vol. 137 (9), p.091008, DOI: 10.1115/1.4030544.

Claims

1. The invention relates to an improvement in the shuttle valve spool (4.2) of a hydraulic unit comprising a single-rod hydraulic actuator (1) having two ports directly connected to the pump (2) inlet-outlet ports, a flow rate controlled pump (2), which regulates the flow rate going to said single-rod hydraulic actuator (1), a hydraulic accumulator (3) used for compensating the differential flow rate formed in the hydrostatic circuit due to hydraulic actuator (1) movement, and a shuttle valve (4) providing bidirectional flow between the hydrostatic circuit and said hydraulic accumulator (3) and comprising shuttle valve spool (4.2) at the inner part thereof, and it is characterized in that; with the operation of the hydraulic unit, in order to prevent the instability problem encountered during compensation of the differential flow rate occurring as a result of the asymmetric actuator (1) structure, at the center position of said shuttle valve spool (4.2), it comprises:

• a valve spool overlap (4.7) found between Port A (4.4) that is connected to the cap-side chamber (1.4) of the hydraulic actuator (1) and Port C (4.6) that is connected to the hydraulic accumulator (3); and preventing the flow between the hydraulic accumulator line (C) connected to said hydraulic accumulator (3) and the cap-side chamber line (A) connected to the cap-side chamber (1.4) of the hydraulic actuator (1), and

• at a valve spool underlap (4.8) found between Port B (4.5) that is connected to the rod-side chamber (1.5) of the hydraulic actuator (1) and Port C (4.6) that is connected to the hydraulic accumulator (3); and enabling flow between the hydraulic accumulator line (C) connected to the hydraulic accumulator (3) and the rod-side chamber line (B) connected to the rod-side chamber (1.5) of the hydraulic actuator (1).

2. A shuttle valve (4) according to Claim 1, characterized in that; it comprises a check valve (6) positioned between the accumulator line (C) and the rod-side actuator chamber line (B) in the hydraulic unit where said shuttle valve (4) is used, and enabling unidirectional flow from said hydraulic accumulator (3) line to the rod-side chamber (1.5) of the actuator (1).

Drawing