Method for operating an electro-hydraulic unit

Abstract

The invention relates to a method for operating a variable speed electro-hydraulic unit (10), in which a hydraulic pump (20) capable of displacing a displacement between a first relatively large displacement value and a second relatively small displacement value per operating cycle is driven by means of a variable speed electric drive (30), the method comprising: determining a setpoint speed of the variable speed electric drive (30) when the displacement has the first relatively large displacement value, determining an actual value of a pumping pressure via the hydraulic pump (20), determining an actual value of the speed of the hydraulic pump (20), determining whether an acceptable operating point results from the actual value of the pumping pressure and the actual value of the speed, and setting the second relatively small displacement value if an acceptable operating point does not result from the actual value of the pumping pressure and the actual value of the speed.

Description

Technical Field

[0001] The present invention relates to a method for operating an electrohydraulic unit.

Background Art

[0002] Background of the Invention The electrohydraulic unit (so-called motor pump unit) on which the present invention is based has a hydraulic pump (so-called hydraulic positive displacement machine, for example, axial piston machine) with a variable displacement volume for each operating cycle, and the hydraulic pump is driven at a variable rotational speed using an electric drive (motor). The displacement volume may be displaceable continuously or stepwise. During operation of such an electrohydraulic unit, usually, the volume flow rate and / or the pumping pressure (that is, the pressure difference between the inlet and the outlet) are closed-loop controlled by appropriately adjusting the rotational speed and the displacement volume of the hydraulic pump. That is, such an electrohydraulic unit basically has two degrees of freedom during closed-loop control.

[0003] German Patent Application Publication No. 102007007005 discloses an electrohydraulic control device including a displaceable fluid pump and an electric drive with variable rotational speed. The fluid pressure can be detected by a pressure detection device. The main control circuit of this electrohydraulic control device has a rotational speed operation unit of the electric drive as an operation unit. By this main control circuit, the fluid pressure, and thus, for example, the force exerted by a cylinder, or a lower operation amount such as, for example, position or speed, can be closed-loop controlled. The displacement volume operation unit of the fluid pump is driven depending on the detected fluid pressure by a sub-operation chain. A particularly simple embodiment assumes that the sub-operation chain includes control means, particularly a two-point adjustment unit, and by this two-point adjustment unit, the displacement volume of the displaceable fluid pump can be switched between a predetermined minimum value and a maximum value depending on a predetermined switching condition.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] German Patent Application Publication No. 102007007005 Specification [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Disclosure of the invention According to the present invention, a method for operating an electrohydraulic unit, an electrohydraulic unit, a computer program, and a data carrier are proposed, having the features of the independent claims. Advantageous embodiments are the subject of the dependent claims and the following specification. [Means for solving the problem]

[0006] The present invention enhances pump availability in electrohydraulic systems, prevents machine downtime, and avoids or reduces pump damage or additional wear. In particular, in the so-called pressure-volume flow mode (pQ) of a pump, where pressure is controlled in a closed loop using rotational speed as the manipulated variable, and volumetric flow rate is generated from this as the product of rotational speed and displaced volume, it is possible to ensure that the operating point (i.e., pressure / rotational speed combination) is only within an acceptable range. If the operating point is being approached by the definition or configuration of the target value, this is prevented within the framework of the present invention by switching, in particular, reducing, the displaced volume. When the displaced volume is small, there is usually no limitation on the operating point. The acceptable and unacceptable operating points are known in advance for each hydraulic pump, and are particularly listed in the datasheet. These operating points can be stored and monitored in the memory of the calculation unit that performs the operation.

[0007] To operate a variable-speed electrohydraulic unit, in which a hydraulic pump capable of displacing a displacement volume between a first relatively large displacement volume value and a second relatively small displacement volume value is driven using a variable-speed electric drive, a target value for the rotational speed of the variable-speed electric drive is determined and specified for the electric drive. This may be carried out based on a higher-level specification, for example, closed-loop control of pressure and / or volumetric flow rate, where the target value for the rotational speed of the variable-speed electric drive is determined as an operable variable of a higher-level pressure closed-loop control unit to which a target value of the pumping pressure via the hydraulic pump is supplied as an input, or generally to which pressure at a predetermined point in the system is supplied. In this case, the displacement volume is set to a first relatively large displacement volume value or a second relatively small displacement volume value, preferably also set by a higher-level specification, such as an operation strategy. For example, in the case of a small displacement volume value, the torque load of the electric drive is reduced. This is because the load torque is generated as the product of the pumping pressure and the displaced volume. For example, in the case of a small displaced volume, a high pressure can be maintained with a small torque. In this case, thermal overload of the electric motor and transducer of the electrical drive device does not occur.

[0008] Within the framework of the present invention, when the displacement volume has a first relatively large displacement volume value, monitoring is performed, in which the actual value of the pumping pressure via the hydraulic pump and the actual value of the rotational speed of the hydraulic pump (or the rotational speed of the electrical drive, since these rotational speeds are usually converted at a 1:1 ratio or have at least a known conversion ratio) are identified. It is determined from the actual value of the pumping pressure and the actual value of the rotational speed of the hydraulic pump whether an acceptable operating point occurs, and if an acceptable operating point does not occur, the displacement volume is set to a second relatively small displacement volume value.

[0009] Preferably, by monitoring the current torque, that is, by identifying the actual torque of the variable-speed electrically driven device, it can be determined whether or not setting a second, relatively small displacement volume value has been implemented or succeeded, because this will cause a change, particularly a decrease, in torque.

[0010] Alternatively or additionally, the currently set displacement volume value can be calculated using a calculation model or a model-based observer, to which current measurements such as rotational speed, torque or drive current, and pumping pressure can be supplied. This displacement volume value can also be used to determine whether or not setting a second, relatively smaller displacement volume value has been implemented or successful.

[0011] A further advantage is that the currently set displacement volume value can be considered as a characteristic of the controlled object in the pressure closed-loop control unit. In particular, a function for adjusting the controlled object is provided after the original adjustment unit that outputs the manipulated variable, and this function changes the manipulated variable (e.g., rotational speed target value) depending on the currently set displacement volume value. This eliminates the need to adjust parameters of the adjustment unit (coefficients such as proportional displacement and reset time), and allows the required dynamics of closed-loop control to be maintained.

[0012] According to a preferred embodiment of the present invention, when changing the target value of the rotational speed of a variable-speed electrically driven device, the rate of change of the target value is limited to the maximum rate of change of rotational speed. The change in the target value of rotational speed arises from a higher specification, especially during normal operation, but when the displaced volume is changed to a second, relatively smaller displaced volume value, i.e., when the displaced volume is reduced, it is necessary to increase the rotational speed to maintain the volumetric flow rate or pressure. The change in the target value of rotational speed is carried out with a predetermined limited angular acceleration. This makes it possible to achieve further component protection, in particular pump protection. The rate of change of rotational speed is, in particular, the rotational acceleration, [angle / time]. 2This can be expressed as [...]. Limiting the acceleration / deceleration of the pump rotation helps to avoid mechanical damage to internal components. If the acceleration is too high, a localized negative pressure may form in the working fluid within the system because the working fluid cannot dynamically follow the movement of the pump rotor. This can lead to cavitation. Limit values ​​for acceleration and deceleration can be included in the pump's datasheet.

[0013] According to a preferred embodiment of the present invention, in the pressure closed-loop control unit, the rate of change of the target pressure is limited to the maximum rate of change of pressure, and / or the target value itself is limited to the maximum pressure value (e.g., the nominal pressure of the pump). This represents two further advantageous component protection mechanisms for protecting the pump from overload or damage. If the rate of change of the target pressure is already limited (i.e., on the input side), this also automatically results in a time delay on the output side, i.e., in particular, the rate of change of rotational speed, and thus has the same advantages as described above. The rate of change of pressure can be expressed in particular as [pressure / time]. Since exceeding the maximum pressure value may be tolerated for a short time depending on the mode of construction and design, limiting the target pressure value to the maximum pressure value preferably involves first waiting for an acceptable overload time before limiting the target pressure value, i.e., before restricting it.

[0014] According to a preferred embodiment of the present invention, if the actual pumping pressure is high but corresponds to the minimum pressure value, the target value of the rotational speed of the variable-speed electrically driven device is limited to the minimum rotational speed. Typically, for energetic and dynamic reasons, it is desirable for the hydraulic pump to be operated at the maximum possible displacement volume, i.e., in this specification, it is desirable for the displacement volume to have a first relatively large displacement volume value. However, this means that the hydraulic torque is large, and therefore, in this case, if the pressure is high (e.g., above a threshold of 5 bar or 50 bar), particularly low rotational speeds or even negative rotational speeds (e.g., less than 200 rpm) can only be tolerated for a short time (e.g., a few seconds) (of course, each value depends on the specific pump and should be understood as merely an example). That is, to avoid cavitation of the pump, the minimum pressure value and minimum rotational speed can be monitored as a further component protection mechanism. If the rotational speed cannot be reduced further due to the limitations, the displacement volume is reduced (and correspondingly the rotational speed is increased) to maintain the volumetric flow rate. The restrictions may be implemented immediately or with a time delay; that is, a brief period below the minimum rotational speed may be permissible in some cases. However, in the latter case, if it is shown that the permissible time has been exceeded, the system will switch to a smaller displacement volume, taking transient time into consideration. Taking transient time into consideration means that the system will not exceed the permissible time until the displacement volume reaches a smaller displacement volume value (in which case there is a non-dangerous operating point, so it is no longer possible to exceed the smaller displacement volume value).

[0015] The computing unit according to the present invention, for example, a control device for an electrohydraulic unit, is configured to implement the method of the present invention, particularly in terms of programming techniques.

[0016] It is also advantageous to implement the method according to the invention in the form of a computer program or a computer program product comprising program code for carrying out all method steps. This is because, especially if the control device to be implemented is also used for further tasks and is thus in any case already existing, this results in particularly low costs. Suitable data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories such as, for example, hard disks, flash memories, EEPROMs, DVDs, etc. It is also possible to download the program via a computer network (Internet, intranet, etc.).

[0017] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0018] It is self-evident that the features described above, and the features to be further explained below, can be used not only in the presented combinations, but rather also in other combinations or alone without departing from the scope of the invention.

[0019] The invention is schematically illustrated in the drawings based on embodiments and will be explained in detail below while referring to the drawings.

Brief Description of the Drawings

[0020] [Figure 1] It is a schematic view of a preferred embodiment of a variable-speed electro-hydraulic unit according to the invention.

Embodiments for Carrying out the Invention

[0021] Detailed Description of the Drawings Figure 1 schematically shows a preferred embodiment of a variable-speed electro-hydraulic unit according to the present invention, which is generally designated by reference numeral 10. The electro-hydraulic unit 10 has a displaceable hydraulic pump 20 (which can pump in two directions in this embodiment), a variable-speed electric drive 30, and a computing unit 40 for operating the electro-hydraulic unit.

[0022] The hydraulic pump 20 is configured as an axial piston pump having a swivel angle α that is displaceable between two positions relative to an inclined plate or an inclined shaft. Thereby, the displacement volume can be displaced between a first relatively large displacement volume value and a second relatively small displacement volume value. For this purpose, an operating signal from the computing unit 40 to the axial piston pump 20 can be used. This operating signal may be, for example, an operating value for a so-called pilot valve 21 provided in the pump.

[0023] The electric drive is here configured as a so-called standard motor, which has a synchronous servo motor 31 and a frequency converter 32. In principle, it is also possible to use an asynchronous motor. The rotational speed n of the asynchronous motor 31 is variable. For this purpose, a target rotational speed is predefined from the computing unit 40 to the electric drive 30. The actual rotational speed can be determined by an angle generator. However, open-loop control without feedback is also possible.

[0024] The computing unit 40 is used to control the pump 10 in a closed loop. The computing unit 40 is provided with a target delivery pressure P S and / or a target volume flow rate Q SA power supply is provided. The calculation unit 40 may be configured as a so-called electrical drive adjustment unit, and within the framework of the present invention, the rotational speed n is determined from here as an operable variable. The calculation unit may be separate from the drive adjustment unit, or it may be located above the drive adjustment unit, or it may communicate with the drive adjustment unit via an interface (e.g., target rotational speed, actual rotational speed). The actual pumping pressure is preferably determined via a sensor. From here, closed-loop pressure control is possible.

[0025] Switching of the displacement volume during operation can be performed, for example, based on an operation strategy. For example, the calculation unit 40 can determine whether the electrical drive is operating at full load or partial load by monitoring the current to identify the electrical drive torque supplied from the electrical drive unit 30 and comparing it to the maximum possible electrical drive torque for the current rotational speed, or by monitoring the rotational speed. In particular, a comparison with a threshold can be assumed, in which case an electrical drive torque or rotational speed below the threshold (e.g., 75%) is identified as partial load. The rotational speed threshold is preferably defined according to the intended use.

[0026] During full-load operation, an operation signal is output for the swivel angle of the pump 20, which results in a first relatively large displacement volume. During partial-load operation, an operation signal is output for the swivel angle of the pump 20, which results in a second relatively small displacement volume. In particular, the dependence of the second relatively small displacement volume on the rotational speed can be extracted from a characteristic map in a calculation unit or the like.

[0027] The displacement volume can preferably be set by driving a pilot valve or a control valve 21. In this case, a switching valve (or proportional valve) can be used.

[0028] According to a preferred embodiment of the present invention, various protective mechanisms are used to protect the electrohydraulic unit or its components from damage. For this purpose, when the displacement volume has a first relatively large displacement volume value, the actual value of the pumping pressure via the hydraulic pump and the actual value of the rotational speed n of the asynchronous motor 31 (as the rotational speed of the hydraulic pump) are determined, from which it is determined whether an acceptable operating point exists. If no acceptable operating point exists (particularly because the rotational speed is too low and / or the pressure is too high), the displacement volume is set to a second relatively small displacement volume value. Pressure P S or volumetric flow rate Q S To maintain this, the rotational speed is increased accordingly to escape from an unacceptable operating point. As mentioned above, when the displacement volume is small, there is usually no limitation on the operating point.

[0029] Further advantageous protection mechanisms are, - Limiting the rate of change of the target value of the rotational speed of the variable-speed electrically driven device (30) to the maximum rate of change of rotational speed, • Limiting the rate of change of the target value of the pumping pressure via the hydraulic pump (20) to the maximum rate of change of pressure, • The target pressure of the hydraulic pump (20) is limited to the maximum pressure value, in some cases after exceeding an acceptable overload time. In some cases, if the actual pressure value is high but corresponds to the minimum pressure value after exceeding the allowable overload time, the target value of the rotational speed of the variable-speed electrically driven device (30) will be limited to the minimum rotational speed. Includes.

[0030] The present invention enhances the availability of pumps in electrohydraulic systems, prevents machine downtime, and avoids or reduces pump damage or excessive wear.

Claims

1. A method for operating a variable-speed electrohydraulic unit (10), wherein a hydraulic pump (20) capable of displacing a displacement volume between a first relatively large displacement volume value and a second relatively small displacement volume value in each operating cycle is driven using a variable-speed electrical drive device (30), The above method is used when the displaced volume has the first relatively large displaced volume value. To determine the target value of the rotational speed of the variable-speed electrically driven device (30), The actual value of the pumping pressure is determined via the aforementioned hydraulic pump (20), To determine the actual value of the rotational speed of the hydraulic pump (20), The determination of whether an acceptable operating point occurs based on the actual value of the pumping pressure and the actual value of the rotational speed of the hydraulic pump (20), If no acceptable operating point is obtained from the actual value of the pumping pressure and the actual value of the rotational speed of the hydraulic pump (20), the second relatively small displacement volume value is set as the displacement volume value of the hydraulic pump (20), Includes, The target value of the rotational speed of the variable-speed electrically driven device (30) is determined as the control input for the upper-level pressure closed-loop control unit to which the target value of the pumping pressure via the hydraulic pump (20) is supplied as an input amount. The aforementioned method, The method further includes limiting the target value of the pumping pressure via the hydraulic pump (20) to the maximum pressure value, A method for limiting the target value of the pumping pressure via the hydraulic pump (20) to a maximum pressure value, comprising waiting for an acceptable overload time before limiting the target value of the pumping pressure.

2. To determine the actual value of the torque of the variable-speed electrically driven device (30), Based on the actual value of the specified torque, it is determined whether the setting of the second relatively small displacement volume value has been carried out, The method according to claim 1, further comprising:

3. Limiting the rate of change of the target value of the rotational speed of the variable-speed electrically driven device (30) to the maximum rate of change of rotational speed. The method according to claim 1, further comprising:

4. Limiting the rate of change of the target value of the pumping pressure via the hydraulic pump (20) to the maximum rate of change of pressure The method according to claim 1, further comprising:

5. The control target characteristics of the higher-level pressure closed-loop control unit for changing the manipulated amount include considering the currently set displacement volume value. The method according to claim 1, further comprising:

6. The aforementioned displacement volume value, currently set, is calculated using a calculation model. The method according to claim 5, further comprising:

7. A method for operating a variable-speed electrohydraulic unit (10), wherein a hydraulic pump (20) capable of displacing a displacement volume between a first relatively large displacement volume value and a second relatively small displacement volume value in each operating cycle is driven using a variable-speed electrical drive device (30), The above method is used when the displaced volume has the first relatively large displaced volume value. To determine the target value of the rotational speed of the variable-speed electrically driven device (30), The actual value of the pumping pressure is determined via the aforementioned hydraulic pump (20), To determine the actual value of the rotational speed of the hydraulic pump (20), The determination of whether an acceptable operating point occurs based on the actual value of the pumping pressure and the actual value of the rotational speed of the hydraulic pump (20), If no acceptable operating point is obtained from the actual value of the pumping pressure and the actual value of the rotational speed of the hydraulic pump (20), the second relatively small displacement volume value is set as the displacement volume value of the hydraulic pump (20), Includes, The aforementioned method, If the actual value of the pumping pressure corresponds to the minimum pressure value, the target value of the rotational speed of the variable-speed electrically driven device (30) is further limited to the minimum rotational speed. A method for limiting the target value of the rotational speed of the variable-speed electric drive device (30) to a minimum rotational speed, comprising waiting for an acceptable overload time before limiting the target value of the rotational speed of the variable-speed electric drive device (30).

8. A computing unit (40) comprising a processor configured to carry out the method described in claim 1 or 7.

9. An electrohydraulic unit (10), A hydraulic pump (20) capable of displacing the displacement volume between a first relatively large displacement volume value and a second relatively small displacement volume value in each operating cycle, A variable speed electrically driven device (30), The calculation unit (40) according to claim 8, An electro-hydraulic unit (10) equipped with the following:

10. It is a computer program, When the computer program is executed by the computing unit (40), the computer program includes an instruction to cause the computing unit (40) to perform the method described in claim 1 or 7, Computer program.

11. A computer-readable data carrier storing the computer program described in claim 10.

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