DISPLACEMENT CONTROL UNIT

Abstract

A Control unit (2) of a hydraulic unit comprises a control piston (3) slidable arranged in a control cylinder (4) to provide a hydraulic fluid under servo pressure (P.sub.2) at a first outlet (8) of the control unit (2) suitable for the displacement of the hydraulic unit. The servo pressure (P.sub.2) is proportional to a sum of the inner forces of the hydraulic unit and the operational forces act on the control unit (2). The hydraulic fluid provided at the outlet (8) is fed by a charge pressure supply at charge pressure (P.sub.1) at an inlet (7) of the control unit (2). The control piston (3) has a blind bore (5) in the direction of a longitudinal axis (13) of the control unit (2), in which blind bore (5) a pressure relief valve (20) is arranged functioning as a pilot-control unit having a two-sided pressure relief piston (23) arranged in the blind bore (5) and slidable relative to the control piston (3) by means of an actuator (6), such that by displacement of the pressure relief piston (23) reduced charge pressure acts as a hydraulic set pressure (P.sub.4) both on the medial front face (26) and on the distal front face (27) of the pressure relief piston (23) and acts on the front faces (12) of the control piston (3). The control piston (3) is displaceable in the control cylinder (4) by the sum of the pressure forces generated by of the set pressure forces generated by of the set pressure (P.sub.4) and the force of the actuator (6).

Claims

1. A control unit of a hydraulic unit comprising a control piston slidable arranged in a control cylinder to provide a hydraulic fluid under servo pressure at a first outlet of the control unit suitable for the displacement of the hydraulic unit, wherein the servo pressure is proportional to a sum of the inner forces of the hydraulic unit and the operational forces acting on the control unit, and wherein the hydraulic fluid provided at the outlet is fed by a charge pressure supply at charge pressure at an inlet of the control unit, wherein the control piston has a blind bore in the direction of a longitudinal axis of the control unit, in which blind bore a pressure relief valve is arranged functioning as a pilot-control unit having a two-sided pressure relief piston arranged in the blind bore and slideable relative to the control piston by means of an actuator, such that by displacement of the pressure relief piston reduced charge pressure acts as a hydraulic set pressure both on the medial front face and on the distal front face of the pressure relief piston and acts on the front faces of the control piston, whereas the control piston is displaceable in the control cylinder by the sum of the pressure forces generated by of the set pressure and the force of the actuator.

2. The control unit as claimed in claim 1, wherein the actuator is a proportional solenoid that acts on the distal front face of the pressure relief piston.

3. The control unit as claimed in claim 2, wherein the actuator is intrinsically safe in the sense of being explosion proof.

4. The control unit as claimed in claim 1, wherein the force of the actuator is less than the force required to displace the control piston in the control cylinder.

5. The control unit as claimed in claim 1, wherein the pressure relief piston is biased and held in an initial position by a pressure relief spring acting on the second medial front face of said pressure relief piston.

6. The control unit as claimed in claim 1, wherein a lateral surface of the pressure relief piston is in fluid connection with an inlet for hydraulic fluid under charge pressure through a radial bore in the control piston and that hydraulic fluid is guidable to the distal front face of the pressure relief piston by a set pressure grove in the lateral surface.

7. The control unit as claimed in claim 1, wherein the pressure relief piston comprises a through bore in the direction of the longitudinal axis of the control unit, by which through bore hydraulic fluid is guidable from the distal front face to the medial front face of the pressure relief piston.

8. The control unit as claimed in claim 1, wherein the pressure relief piston is guided in a pressure relief bushing arranged in the blind bore of control piston, wherein hydraulic fluid under set pressure is guidable via set pressure relief groves in the pressure relief bushing to a second outlet of the control unit that is in fluid connection with a region of low pressure.

9. The control unit as claimed in claim 8, wherein pressure relief piston comprises a radial bore, by which radial bore hydraulic fluid is guidable from the through bore to the second outlet.

10. The control unit as claimed in claim 1, wherein pressure relief piston has two regions of different diameter, wherein the medial front face, the distal front face and a ring shaped area in a region connecting the regions of different diameter are exposable to said set pressure and a ring shaped area is permanently in fluid connection with the second outlet.

11. The control unit as claimed in claim 10, wherein the medial front face is the bigger front face of the pressure relief piston.

12. The control unit as claimed in claim 2, wherein the force of the actuator is less than the force required to displace the control piston in the control cylinder.

13. The control unit as claimed in claim 3, wherein the force of the actuator is less than the force required to displace the control piston in the control cylinder.

14. The control unit as claimed in claim 2, wherein the pressure relief piston is biased and held in an initial position by a pressure relief spring acting on the second medial front face of said pressure relief piston.

15. The control unit as claimed in claim 3, wherein the pressure relief piston is biased and held in an initial position by a pressure relief spring acting on the second medial front face of said pressure relief piston.

16. The control unit as claimed in claim 4, wherein the pressure relief piston is biased and held in an initial position by a pressure relief spring acting on the second medial front face of said pressure relief piston.

17. The control unit as claimed in claim 2, wherein a lateral surface of the pressure relief piston is in fluid connection with an inlet for hydraulic fluid under charge pressure through a radial bore in the control piston and that hydraulic fluid is guidable to the distal front face of the pressure relief piston by a set pressure grove in the lateral surface.

18. The control unit as claimed in claim 3, wherein a lateral surface of the pressure relief piston is in fluid connection with an inlet for hydraulic fluid under charge pressure through a radial bore in the control piston and that hydraulic fluid is guidable to the distal front face of the pressure relief piston by a set pressure grove in the lateral surface.

19. The control unit as claimed in claim 4, wherein a lateral surface of the pressure relief piston is in fluid connection with an inlet for hydraulic fluid under charge pressure through a radial bore in the control piston and that hydraulic fluid is guidable to the distal front face of the pressure relief piston by a set pressure grove in the lateral surface.

20. The control unit as claimed in claim 5, wherein a lateral surface of the pressure relief piston is in fluid connection with an inlet for hydraulic fluid under charge pressure through a radial bore in the control piston and that hydraulic fluid is guidable to the distal front face of the pressure relief piston by a set pressure grove in the lateral surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In the following, without limiting the inventive idea, the invention is described in more detail with the aid of a preferred embodiment and with reference to the appended drawings in which

[0021] FIG. 1 shows a partial cross section of an embodiment of a control unit according to the invention in a first operational state;

[0022] FIG. 2 shows a partial cross section of a control unit according to the invention in a second operational state; and

[0023] FIG. 3 shows a cross section of a symmetrical control unit according to the invention in the second operational state.

DETAILED DESCRIPTION

[0024] FIG. 1 shows a partial cross section of an embodiment of a control unit 2 according to the invention. Shown is the right side of generally symmetrical control unit 2 that comprises a common control cylinder 4 in which a movable and generally symmetrical control pistons 3 is arranged, as depicted in FIG. 3. The control piston 3 exemplarily shown in FIGS. 1 to 3 is used for the displacement control of hydraulic units, e.g. in hydrostatic pumps or motors with reversible hydraulic fluid flow.

[0025] The in FIG. 1 depicted right side of control unit 2 shows the right half of a control piston 3 that is slidable guided in a control cylinder 4. An inherently safe actuator 6 acts solely and indirectly on control piston 3 by means of a plunger 17. The displacement of the control piston 3 connects/disconnects certain conduits for hydraulic fluid to adjust the displacement of the hydraulic unit. These connections are opened or closed by control edges 15, 16 being defined by ring grove 10 machined in the outer surface of control piston 3. Hydraulic fluid is supplied for instance by charge pump (not shown) under charge pressure P.sub.1 to an inlet 7 arranged in control cylinder 4 and can be guided as a servo pressure P.sub.2 to an outlet 8. The pressure level of servo pressure P.sub.2 is proportional to the displacement of the control piston 3 relative to control cylinder 4. A further outlet 9 leads to a region of low pressure being at a pressure P.sub.3, generally denoted as sump, tank or casing. These features and the general mode of operation of a control unit are familiar to a person skilled in the art so that further details can be omitted here.

[0026] The control unit 2 according to FIG. 1 differs from a conventional control unit in that it includes a hydraulic pilot stage or a pre-control unit in form of a pressure relief valve 20 that is slidable arranged in a longitudinal blind bore 5 of control piston 3. The blind bore 5 is oriented along the longitudinal axis 13 of control unit 2. The pressure relief valve 20 comprises a pressure relief piston 23 slidable arranged in a bushing 21, which is mounted axially fixed in the blind bore 5 of control piston 3. One end face of bushing 21 is seated against the bottom 11 of blind bore 5. This end face of bushing 21 exemplarily is closed by an end cap 19, which also serves as a seat for a pressure relief valve spring 28. A radial bore 14 extend from the control piston 3 via the bushing 21 to the control piston 3 and connects inlet 7 for charge pressure P.sub.1 with a longitudinal bore 24 in bushing 21. Pressure relief piston 23 is slidable arranged in bore 24.

[0027] Exemplarily, pressure relief piston 23 is of a stepped design, what means, it has a first, distal front face 26 and a second, medial front face 27 of different diameter. Distal front face 26 is the smaller one of the two front faces 26, 27 and is oriented towards a plunger 17 of actuator 6. Plunger 17 solely contacts pressure relief piston 23 and only on distal front face 26. Plunger 17 can act to displace pressure relief piston 23 along the longitudinal axis 13 of control unit 2 by exerting medial oriented forces on distal end face 26. Pressure relief piston 23 further comprises a through bore 25 leading from the distal front face 26 to the medial front face 27. At the medial front face 27 through bore 25 shows an enlarged diameter in order to accommodate pressure relief spring 28 acting to bias pressure relieve piston 23 in the direction of plunger 17. Radial bores 34 in pressure relief piston 23 fluidly connect a recessed circumferential area 38 on pressure relief piston 23 with through bore 25. Area 38 can overlap a radial bore 35 in bushing 21 leading to outlet 9 connected with an area of low pressure. This overlap depends on the relative position of pressure relief piston 23 in bushing 21 and is present if pressure relief piston 23 is in the closed position. In case of an overlap of recessed area 38 and radial bore 35 a fluid connection is established between through bore 25 and second outlet 9 at case pressure P.sub.3. This will be described further on in more detail below.

[0028] The circumferential surface 29 of pressure relief piston 23 is stepped, by virtue of which different diameters of the medial front face 26 and the distal front face 27 are achieved. Simultaneously defining a ring shaped area 30 in the insection area of the two diameters. A corresponding ring shaped shoulder 39 is present in the longitudinal bore 24 of bushing 21 such that the ring area 30 can abut on the ring shaped shoulder 39, thus limiting the distal stroke of pressure relief piston 23 relative to bushing 21 and control piston 3 inside which bushing 21 is fixedly arranged.

[0029] At the distal end section of pressure relief piston 23 near distal front face 26 a notch 31 is formed on the outer surface of pressure relief piston 23 defining a set pressure control edge 33. Notch 31 can be brought into an overlapping relationship with radial bore 14 in bushing 21 thereby fluidly connecting inlet 7 with notch 31. When such an overlap occurs, hydraulic fluid from the inlet 7 is guided via radial bore 14 traversing control piston 3 and bushing 21 to a distal pressure chamber 36 in longitudinal blind bore 5 of control piston 3 closed by a flange 18. In distal pressure chamber 36 hydraulic fluid under set pressure P.sub.4 can act on the front side 12 of control piston 3, on the free front face 22 of bushing 21 and, guided further on along notch 31, on distal front face 26 of pressure relief piston 23. The pressure level of set pressure P.sub.4 is established by the magnitude of displacement of pressure relief piston 23 with respect to control piston 3. The bigger the displacement of pressure relief piston 23 relative to bushing 21 the higher the pressure level of set pressure P.sub.4 in distal pressure chamber 36 and in the regions in direct communication therewith, such as though bore 25 in pressure relief piston 23 and in medial pressure chamber 37. By means of through bore 25 set pressure P.sub.4 is guided from the distal chamber 36 to medial pressure chamber 37 located between the medial front face 27 of pressure relief piston 23 and the end cap 19 of bushing 21. Hence, set pressure P4 is also acting on medial front face 27 of pressure relief piston 23.

[0030] The state of operation of control unit 2 depicted in FIG. 1 shows the pilot stage or boost of control forces for the displacement of the hydraulic unit inactive. In this state ring area 30 of bushing 21 and ring shaped shoulder 39 of pressure relief piston 23 are in contact. Consequently pressure relief piston 23 is in its extreme distal position. In this inactive situation set pressure control edge 33 does not overlap with radial bore 14. Thus, no hydraulic fluid can flow from radial bore 14 via set pressure notch 31 into distal pressure chamber 36. In this closed position of pressure relief valve 20 the distal pressure chamber 36 is vented via through bore 25, via radial bores 34 and 35 and via set pressure relief notch 32 to outlet 9 connected to a region of low pressure, i.e. case pressure P.sub.3. Thus, low pressure at pressure level P.sub.3 is present in both pressure chambers 36 and 37. Consequently, set pressure P.sub.4 in this inactive state of the inventive control unit 2 is essentially equal to low pressure level P.sub.3. Further, servo pressure P.sub.2 is not influenced because control edge 15 does not overlap with inlet 7 for system pressure P.sub.1.

[0031] FIG. 2 shows a cross section of the left side of a control unit 2 according to the invention in a second active operational state. In this operational state the pilot stage or boost of control forces on control piston 3 is active on this side of control unit 2. Because control unit 2 is constructed symmetrically the same reference numerals used in FIG. 1 are used for corresponding features in FIG. 2 and, consequently, also in FIG. 3.

[0032] In FIG. 2 actuator 6 pushes plunger 17 actively against the first front end 26 of pressure relief piston 23. Pressure relief piston 23 is displaced in medial direction against the force of biasing spring 28. This causes an overlap between notch 31 and radial bore 14 conducting hydraulic fluid under system pressure P.sub.1. Thus the hydraulic pressure level in pressure chamber 36 raises and establishes a set pressure level P.sub.4 in pressure chamber 36 being fed by charge pressure P.sub.1 present at inlet 7. Simultaneously, with the overlap of notch 31 and radial bore 14 the overlap of recessed area 38 and radial bore 35 is set aside, so that the pressure level in pressure chamber 36 cannot be vented to the low pressure outlet 9. However, the pressure level P.sub.4 in pressure chambers 36 and 37 is communicated via through bore 25 in pressure relief piston 23. Hence, the pressure in pressure chamber 37 corresponds to set pressure P.sub.4 in pressure chamber 36 and acts on medial front face 27 of pressure relief piston 23.

[0033] The displacement of pressure relief piston 23 also closes a flow channel for hydraulic fluid leading from through bore 25 via radial bores 34 and 35 to pressure relief notch 32 defined as a region 38 of reduced outer diameter of bushing 21. Pressure relief notch 32 communicates in turn with second outlet 9 at case pressure P.sub.3. Because this flow channel is closed, pressure P.sub.4 is acting on the medial front face 27 of pressure relief piston 23. On the distal front face 26 of pressure relief piston 23 the same set pressure P.sub.4 acts on distal front face 26 as well as on front face 22 of bushing 21 and on the remaining ring front face 12 of control piston 3.

[0034] According to the operating condition/state of FIG. 2, the effective force acting in medical direction on control piston 3 is given by the sum of the hydraulic forces generated by set pressure P4 on front side 12 of control piston 3, on front face area 22 of bushing 21 and on front face 26 of pressure relief piston 23, i.e. the whole cross section area of pressure chamber 36, plus the mechanical force of actuator 6 on plunger 17. In distal direction the forces are given by the product of the area of the medial front face 27 of pressure relief piston 23 multiplied with the set pressure P.sub.4 plus the force of pressure relief spring 28. As the sum of pressure sensitive faces in the medial area is smaller than the sum of pressure sensitive faces in the distal area, the resulting force is oriented towards the center of control unit 2 and pushes control piston 3 to the right, as shown in FIG. 3.

[0035] The force acting on control piston 3 is triggered by the actuator force of actuator 6 acting on pressure relief piston 23 whose displacement is causing the hydraulic forces on the control piston 3. The role of actuator 6 is generally reduced to displace pressure relief piston 23 in bushing 21. The generated value of set pressure P.sub.4 is proportional to the amount of displacement of pressure relief piston 23 relative to control piston 3. Thus, the force provided by actuator 6, preferably a solenoid, is boosted considerably by the controlled supply of hydraulic pressure on the front faces of control piston 3. This permits the use of a relatively low actuator forces which require low electrical power, if using an inherently safe solenoid.

[0036] FIG. 3 shows a cross section of a complete control unit 2 according to the invention in the operational active state of FIG. 2. The symmetrical control piston 3 is displaced to the right, as shown in FIG. 2, which is indicated by the spacing of the dot-dashed lines representing the respective symmetry axes of control piston 3 and control cylinder 4. This displacement causes an overlap of inlet port 7 for hydraulic fluid under charge pressure P.sub.1 with a region 10 of reduced diameter on control piston 3 being bordered by control edge 15. Thus, hydraulic fluid is conducted from the right inlet port 7 to the first right outlet 8 under servo pressure P.sub.2, the pressure level of which is determinedas usualby the amount of displacement of control edge 15 of the control piston 3 relative to control cylinder 4.

[0037] However, the preferred embodiment shown in FIGS. 1 to 3 is explained in detail, a person skilled in the art detects that the scope of the inventive idea is not limited to this embodiment. In the shown embodiment, bushing 21, end cap 19, pressure relief piston 23 and pressure relief spring 28 can be fixed axially as a pre-mounted assembly group 20, i.e. as premounted pressure relief valve 20, into blind bore 5 of control piston 3. Nevertheless, a person skilled in the art easily would find a way to incorporate at least parts of pressure relief valve 20 into control piston 3, thereby reducing the amount of parts. Hence, all such embodiments of the present invention are also covered by the inventive idea, which is using a pressure relief valve 20 for generating an additional hydraulic force on control piston 3 triggered the actuator force for the controlled displacement of a hydraulic unit. Hereby, the use of charge pressure P.sub.1 is not limited to a pressure supply being generated by a charge pump. As a charge pressure P.sub.1, generally spoken, every pressure in a hydraulic unit being higher than ambient pressure serves to generate a boosting effect in the sense of the present invention.

[0038] It is understood that the invention can also be used in conjunction with other kinds of actuators instead of proportional solenoids using low currents. Also switching solenoids are covered by the inventive idea, opening the possibility of switchable hydraulic controls, however, eventually disabling a proportional control of the hydraulic unit. Also, an application for the operation of switchable valves using pneumatical, hydro-mechanical, mechanical, etc. actuator forces for initiating the inventive boost effect fall under the scope of the invention.

[0039] As already stated above, the inventive idea is not limited to hydrostatic machines as hydrostatic pumps or motors. The inventive idea is applicable as well to every kind of hydraulic units like hydraulic cylinders, e.g. for the movement of charges.

[0040] While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.