EP1176314B1 - Method and system for compensating the compressibility in hydraulic drives - Google Patents

Description

The invention relates to a device and a method for compensating the compressibility of the hydraulic fluid in hydraulic drives with at least one displacement chamber.

In hydraulic drives there is the problem that the hydraulic fluid used in each case is compressible within certain limits, which leads to a significantly reduced load stiffness compared to mechanical drives: sudden pressure changes in the at least one displacement chamber, the known regulations are too slow to order by appropriate supply or removing hydraulic fluid into the displacement chamber to maintain the desired desired position of a piston forming the displacement chamber together with a cylinder or a desired setpoint speed of the piston.

To compensate for the compressibility of the hydraulic fluid, various devices and methods are known, for example from DE 44 20 619 A1, which describes a drive system with a first and a second hydrostatic machine, the second machine has a speed control device with a signal generator and wherein for improvement the load stiffness is connected to each side of the signal generator in each case a hydraulic compensation volume.

From US 3,805,530 a hydraulic system is known in which two hydraulic motors are connected in series, and the second motor is used for supplying a small amount of hydraulic fluid to the first motor, whereby volume losses due to the compressibility of the hydraulic fluid to be compensated.

DE 42 28 308 A1 describes the compensation of the compressibility by piezo elements. Although the piezo elements can generate very high forces within a short time, the resulting paths are very short. In addition, a piezo element itself represents a spring. If it works against a pressure, the path is further reduced as it springs under pressure. This has the consequence that in practice only small load changes, which correspond to a maximum of 5 to 7 bar pressure change, be compensated. In general, however, depending on the supply pressure pressure changes in the range of 100 bar and more are required. The control described in DE 42 28 308 is therefore not suitable to meet the requirements occurring in practice.

Further methods and devices for increasing the load stiffness of hydraulic drives are known for example from DE 42 27 565, which proposes the use of a differential cylinder, and DE 35 32 931, which shows a special control control. Another example is disclosed in document US 4 630 523. However, the known solutions for compression compensation are sometimes structurally very expensive, maintenance-prone and expensive to manufacture, sometimes too slow to respond to load changes in the millisecond range.

Based on this, the object of the invention is to provide a device and a method which make it possible to increase the load stiffness of hydraulic drives reliably and to respond to load changes within a few milliseconds in order to compensate for compression-related position or speed deviations of a hydraulic drive.

The invention is achieved by a device having the features of claim 1 and by a method having the features of claim 15. Advantageous embodiments and embodiments are the subject of the dependent subclaims. The independent claim 21 relates to a hydraulic drive with a compensation device according to the invention.

While the compensation in the prior art is controlled, that is, without a return or a feedback, the compensation of the compression volume is carried out according to the invention in a closed volume flow control loop.

That is, the volumetric flow resulting from the pressure rise is calculated taking into account the compression coefficient and the volume under pressure, and represents the control variable in the control loop. The compensation volumetric flow generated by the compensating piston is constantly compared with the target volumetric (compression volumetric flow) and via the control circuit (three-point controller and fast control valves) this tracked. The result is that exactly the volume is supplied to the working cylinder, which is missing due to compression in the displacement. The drive cylinder remains largely in its position.

Compensation takes place over the entire pressure range up to the maximum load. It is effective on both controlled and position-controlled systems. In the case of position-controlled drives, the functions of the position control and of the compressibility compensation overlap.

Fig. 1

das Prinzipschaltbild eines hydraulischen Antriebs mit einem translatorisch bewegten Kolben zeigt, der zwei Verdrängerräume in einem Zylinder voneinander trennt und über einen geschlossenen Lageregelkreis mit stetigem Ventil verfügt, wobei eine erfindungsgemäße Kompensationsvorrichtung zur Erhöhung der Laststeife sowohl bei stillstehendem als auch bei bewegtem Kolben vorgesehen ist,

Fig. 1a

das Prinzipschaltbild einer zur Erhöhung der Laststeife nur bei stillstehendem Antrieb ausgebildeten Kompensationsvorrichtung wiedergibt,

Fig. 2

die Anwendung einer erfindungsgemäßen Vorrichtung bei einem Differentialzylinder verdeutlicht,

Fig. 3

einen stetig ausgeführten Volumenstromregelkreis für den zur Kompensation des Kompressibilitätsverlustes dienenden Ausgleichsstrom zeigt und

Fig. 4

die Anwendung der erfindungsgemäßen Vorrichtung bei einem Drehantrieb zeigt.

Further details and advantages of the invention will become apparent from the following purely exemplary and non-limiting description of various embodiments according to the invention equipped hydraulic drives in conjunction with the drawing corresponding block diagrams, wherein

Fig. 1

shows the block diagram of a hydraulic drive with a translationally moving piston, which separates two displacement chambers in a cylinder and has a closed position control loop with continuous valve, wherein a compensation device according to the invention is provided for increasing the load stiffness both stationary and moving piston,

Fig. 1a

shows the block diagram of a compensating device designed to increase the load stiffness only when the drive is stationary,

Fig. 2

illustrates the application of a device according to the invention in a differential cylinder,

Fig. 3

shows a continuously running volume flow control circuit for the compensating flow used to compensate for the loss of compressibility and

Fig. 4

shows the application of the device according to the invention in a rotary drive.

The hydraulic drive in Fig. 1 consists of a cylinder 1 and a guided therein piston 2, the two displacement chambers 3 and 4 separated from each other. In order to be able _to position the piston against an external load 5 according to a (mostly) electrically given signal X or to be able to follow a time-varying signal X _soll = f (t), the cylinder-piston drive is controlled with the steady valve 6 in the closed position control loop. by using a displacement measuring device 7, the piston stroke X _is measured and compared with the setpoint X _{is supposed to} . The difference X _soll - X _ist is the current error of the retracted position. This is modified by a controller 9 according to modern controller concepts, for example by a differentiating and / or integrating component is generated, which is supplied via an amplifier element 10 to the continuous valve 6, which controls the piston-cylinder unit via the lines 11 and 12 so that the error _{should be} X - X _{is made} as fast as possible to zero.

If the load F 5 changes at standstill of the drive, then this would lead to changes in the pressures p ₁ and p ₂ , since the equilibrium equation F = p ₁ A ₁ - p ₂ A ₂ . Here A ₁ and A _{2 are} the surfaces of the piston, act on the p ₁ and p ₂ . With slow load changes, the valve 6 can cause by a small shift that the drive stops near its target position. However, a small position error _is X _soll - X _is required to deflect the valve slightly. An integral part of the controller makes it possible to zero the error. But if the force F 5 changes very quickly, then the required pressure difference is generated by, for example, that the pressure medium in the chamber 3 is compressed and decompressed in the chamber 4. The piston 2 is displaced by the force change before the valve 6 can counteract the displacement, since some time passes before the valve 6 follows a control signal. This phenomenon of retreating with a rapidly increasing load is called dynamic load stiffness.

The invention is to cause according to Fig. 1, that despite rapid change in the load F 5, the piston 2 stops in its position or at a constant speed of the drive no speed drop occurs.

This is achieved by connecting a cylinder 14 to the piston chamber 3 via the line 13.

In the cylinder 14, a piston 15 is arranged, which is held by the springs 18 and 19 in the middle. The piston 15 separates two displacement chambers 16 and 17. The pressure in chamber 3 of the hydraulic drive is measured via the pressure sensor 21 and via the electrical line 22 forwarded. Pressure oscillations of the pressure sensor are damped by the damper element 23. In the differentiator 24, the temporal pressure gradient is calculated. From the pressure gradient dp ₁ / dt, the compression _flow Q _{Kompr can be} calculated, which disappears due to the time-related pressure change in the displacement chamber 3 or becomes free when the pressure is decreased.

The following applies: $${\mathrm{Q}}_{\mathrm{Compr}}=\frac{V_{\mathrm{O}}}{{\mathit{e}}_{\ddot{\mathit{O}}\mathit{l}}}\cdot \frac{d_{p1}}{d_t}$$

In this case, V _o is the volume of _oil in the displacer chambers 3, 16, 17 under the pressure p ₁ , _oil is the volume compression coefficient of the oil filling V _o , wherein the elasticity of the surrounding cylinders 1 and 14 is also taken into account. The oil filling V _o is of the piston travel X _is dependent, and the compression modulus E _oil is a function of the pressure p. ₁ Both influences are included in the block 25 in the result for Q _Kompr .

This calculated volume flow Q _Kompr , which would be absent in the displacement _chamber 3 when the pressure _rises , and therefore cause a displacement of the piston 3 in the event of sudden load changes, serves as a _reference variable in a control loop which _controls the compression flow Q _Kompr with a volume flow Q _Ausgl = a. y 'matches. In this case, y 'is determined by means of a speed sensor 20 integrated in the piston chamber 17. It can also be the way y of the piston 15 are measured and converted by the differentiating member 26 to the speed y '. The compensating _{volume flow} results in: Q _Ausgl = a. y '. The result of the difference formation Q _Kompr - Q _Ausgl is supplied to a three-position controller 27. Q _{Compr is} greater than Q _Comp . is connected via a very fast switching valve 28 with switching times of 1 to 2 milliseconds the displacer 17 with the pressure source P via the line 29. As a result, just as much volume flow is supplied to the displacement chamber 3 within milliseconds, as disappears in pressure increase by compression of the volume V _o .

$=\frac{1}{A_1}\left[F-p_2{A}_2\right]$

sowohl eine Funktion der Kraft F als auch des Druckes p₂ imSince the switching _{valve 28} switches only the low compression _flow Q _Kompr within a few milliseconds, its response is much faster than that of the steady valve 6, which must control the full volume flow for the drive. The result is that the piston is held in position and does not recede when the load increases. Because the pressure ${\mathrm{p}}_{}$${\mathrm{}}_1$

$=\frac{1}{A_1}\left[F-p_2{A}_2\right]$

Both a function of the force F and the pressure p ₂ in

Piston space is 4, the supply of a compensating volume flow is only necessary to a displacement.

In case of sudden load decrease causes the three-step controller 27, that moves over the switching valve 28, the piston 15 in the negative y-direction, and thus a corresponding compensating volume flow from space 17 and thus room 3 is discharged.

anliegt, ist für die Abdichtung dieses Kolbens kein Aufwand erforderlich. Dabei ist C die Gesamtfedersteifigkeit der Federn und a die Fläche des Kolbens 15. Das Schaltventil 30 kann gegebenenfalls durch ein erhöhtes Spiel zwischen Kolben 15 und Zylinder 14 ersetzt werden, wodurch eine Zentrierung des Kolbens 15 erfolgt.The springs 18 and 19 cause in phases in which the load F 5 does not change that by opening a small switching valve 30, the piston 15 moves in the middle position. This prevents it from moving to an end position over time. Because of the piston 15 only a very small pressure difference ${\mathrm{\Delta }}_p=\frac{C\cdot Y}{a}$

is applied, no effort is required for the sealing of this piston. Here, C is the total spring stiffness of the springs and a the surface of the piston 15. The switching valve 30 may optionally be replaced by an increased clearance between the piston 15 and cylinder 14, whereby a centering of the piston 15 takes place.

If a retraction of the position-controlled or controlled piston drive under load can be avoided only at standstill, then an Erxmittlung the compression flow is not required.

das Zurückweichen zu verhindern.In this case, the measured change in displacement to the piston Δ _Xist can according to Fig. 1a, are used on the quick switching valve 28 through the displacement of the piston 15 according to the algorithm $\mathrm{\Delta }\mathrm{y}=\mathrm{\Delta }\mathrm{Xist}\cdot \frac{A_1}{a}$

to prevent the backsliding.

angesteuert wird. Damit wird ein Abweichen von der Sollgeschwindigkeit verhindert. Die Geschwindigkeitsabweichung kann durch Differenzieren der Lageabweichung (X_soll - X_ist) nach der Zeit erhalten werden.Likewise, in constant velocity motions deviations Δx 'from this velocity can be compensated by load changes by the piston 15 in the closed velocity loop according to the algorithm: ${\mathrm{y}}^{\text{'}}=\frac{A_1}{a}\mathrm{\Delta }{\mathrm{x}}^{\text{'}}$

is controlled. This prevents a deviation from the setpoint speed. The speed deviation can be obtained by differentiating the positional deviation (X _soll - X _ist ) after the time.

The device for increasing the dynamic load stiffness is applicable to all types of cylinders, not only on the same-area cylinder, as in Fig. 1st

In Fig. 2, the application is shown in a differential cylinder. Even plunger cylinders with only one displacement chamber can be equipped with it. Fig. 2 shows two features. The compensating cylinder 14 with piston 15 can be integrated directly into the cylinder cover 33 of a piston side. In addition, in the cylinder cover instead of the 3/3-way valve 28 in Fig. 1, two 2/2-way valves 31 and 32 are integrated. These are characterized by a very small design and extremely short switching times in the range of 1 ms.

Fig. 3 shows that you can run the flow control circuit for the equalizing current also steadily. In this case, the continuous valve 36 is _actuated by the difference between the compression _flow Q _Compr and the compensating _flow Q _Ausgl via the blocks 34 and 35. By acting on the displacer 17, a compensating _flow Q _Ausgl corresponding to the compression _flow Q _{Compr is} supplied to the displacer 3 under pressure p ₁ .

Fig. 4 shows that the principle of the invention is also applicable to rotary actuators 38. In case of sudden changes in the load torque T causes the device 37, that the rotation angle φ _is not yield at standstill or is avoided a drop in speed at a constant speed. Since the volume V _o between the valve and motor (except for the internal pulse actions) is constant, only the pressure p has to be taken into account in the calculation of the compression flow in block 24a.

Formed in addition to the compression volume in the Kolbenverdrängem in Fig. 1 and 2 and in the rotary motor in Fig. 4 by the pressure increase Load increase a change in the leakage oil flow from room 3: ΔQ _leak = f (p _1- p ₂ ), the compensating _{volume flow} must be made higher by this value: Q _Ausgl = Q _Kompr + ΔQ _Leak .

Claims (22)

1. A device for compensating the compressibility of the hydraulic fluid in hydraulic drives with at least one displacement chamber (3), the device being coupleable via a passage (13) for hydraulic fluid with the displacement chamber (3) of the hydraulic drive, including means (21) for measuring the pressure in the displacement chamber (3), means (24, 25) for calculating the compression flow Q_Kompr of the hydraulic fluid appearing with pressure changes in the displacement chamber due to the compressibility of the hydraulic fluid, and means for supplying or removing a compensating volume flow Q_Ausgl of hydraulic fluid, regulated in the light of the calculated compression flow, to the displacement chamber.
2. The device according to claim 1, characterized in that a three position controller (27) is provided to regulate the compensating volume flow Q_Ausgl.
3. The device according to claim 2, characterized in that the means for supplying or removing the compensating volume flow Q_Ausgl include a cylinder (14), a piston (15) moving in the cylinder (14) and at least one valve (28; 31, 32; 36), the cylinder (14) being coupleable via the passage (13) with the displacement chamber (3) of the hydraulic drive and the piston (15) being adapted to be impinged with overpressure or with a partial vacuum via the at least one valve (28; 31, 32; 36).
4. The device according to claim 3, characterized in that the at least one valve is a valve (28; 31, 32; 36) with response times in the range of milliseconds.
5. The device according to claim 4, characterized in that the valve is a three-way switching valve (28).
6. The device according to claim 4, characterized in that two two-way switching valves (31, 32) are provided.
7. The device according to claim 4, characterized in that the valve is a proportional servo valve(36).
8. The device according to one of the claims 3 to 7, characterized in that a speed transducer (20) is provided for measuring the speed of the piston (15).
9. The device according to one of the claims 3 to 8, characterized in that the piston (15) is preloaded in a resting position by means of elastic elements, more specifically two springs (18, 19).
10. The device according to one of the claims 1 to 9, characterized in that the means for measuring the pressure in the displacement chamber (3) include a pressure sensor (21) and a damping element (23) for damping pressure oscillations of the pressure sensor (21).
11. The device according to one of the claims 1 to 10, characterized in that the at least one valve (28; 31, 32) and the means (21) for measuring the pressure are coupled via a wire (22) for transmitting electromagnetic signals.
12. The device according to one of the claims 1 to 11, characterized in that the means for calculating the compression flow Q_Kompr include a differentiator (24) for determining the time-dependent pressure gradient.
13. The device according to one of the claims 1 to 12, characterized in that the means for calculating the compression flow Q_Kompr include a block (25) configured to take into account the volume V₀ momentarily present in the displacement chamber (3) as well as the compression module E_Öl dependent on the pressure p₁ prevailing in the displacement chamber.
14. The device according to one of the claims 1 to 13, characterized in that a continuous valve (36) is adapted to be activated via two blocks (34, 35) to regulate the compensating volume flow Q_Ausgl.
15. A method for compensating the compressibility of the hydraulic fluid in hydraulic drives with at least one displacement chamber (3), characterized in that the pressure p₁ is measured in the displacement chamber, that the compression flow Q_Kompr appearing with pressure changes in the displacement chamber due to the compressibility of the hydraulic fluid is calculated and that the compression flow Q_Kompr is compensated by a volume flow Q_Ausgl supplied to the displacement chamber.
16. The method according to claim 15, characterized in that the volume flow Q_Ausgl is regulated by means of the calculated compression flow Q_Kompr.
17. The method according to claim 15 or 16 a piston-cylinder unit (14, 15) being provided for generating the volume flow Q_Ausgl, characterized in that the speed ý of the piston (15) is measured directly or indirectly and is used for measuring the volume flow Q_Ausgl according to the equation Q_Ausgl = ýa, a being the area of the cross-section of the cylinder (14).
18. The method according to claim 17, characterized in that Q_Ausgl is continuously compared to the calculated compression flow Q_Kompr and the result of the comparison is supplied to a three position controller (27) that impinges the piston (15) with an overpressure or a partial vacuum via a switching valve (28) depending on the result of the comparison.
19. The method according to one of the claims 15 to 18, characterized in that the dependence of the compression module on the momentary pressure in the displacement chamber is taken into account for calculating the compression flow Q_Kompr.
20. The method according to one of the claims 15 to 19, characterized in that an oil leakage flow ΔQ_Leck is taken into account for regulating the compensating volume flow Q_Ausgl.
21. A hydraulic drive with a device for compensating the compressibility of the hydraulic fluid used by the drive in case of a change of load according to one of the claims 1 to 14.
22. A hydraulic drive according to claim 21 with a device according to one of the claims 2 to 14, characterized in that the piston (15) of the device is integrated in a cylinder cover (33) of a piston side of the drive.

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