Method for operating an electrohydraulic unit - Patents.com

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 technology]

[0002] 2. Background of the Invention The electrohydraulic unit (so-called motor-pump unit) on which the invention is based comprises a hydraulic pump (so-called hydraulic positive displacement machine, e.g. axial piston machine) with a variable displacement per operating cycle, which is driven at a variable speed by means of an electric drive (motor). The displacement can be variable continuously or in steps. During operation of such an electrohydraulic unit, the volumetric flow rate and / or the pumping pressure (i.e. the pressure difference between the inlet and the outlet) are usually controlled in a closed loop by correspondingly adjusting the speed and the displacement of the hydraulic pump, i.e. such an electrohydraulic unit basically has two degrees of freedom during closed loop control.

[0003] DE 10 2007 007 005 A1 shows an electrohydraulic control device with a displaceable fluid pump and an electric drive with a variable speed. The fluid pressure can be detected by a pressure detection device. A main control circuit of the electrohydraulic control device has a speed control of the electric drive as control element. This main control circuit allows closed-loop control of the fluid pressure and thus of subordinate control variables, such as, for example, the force exerted by the cylinder or, for example, the position or the speed. A displacement control element of the fluid pump is driven by a secondary control chain as a function of the detected fluid pressure. A particularly simple embodiment provides that the secondary control chain includes a control means, in particular a two-point adjustment, by means of which the displacement of the displaceable fluid pump can be switched between predefined minimum and maximum values ​​as a function of predefined switching conditions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] DE 102007007005 A1 Summary of the Invention [Problem to be solved by the invention]

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

[0006] The invention increases the availability of pumps in electrohydraulic systems, prevents machine breakdowns and avoids or reduces damage or additional wear of the pump. In particular in the so-called pressure-volume flow mode (pQ) of pumps, in which the pressure is controlled in a closed loop with the speed as the manipulated variable, from which the volume flow rate results as the product of the speed and the displacement, it can be ensured that only operating points (i.e. pressure / speed combinations) that lie within the permissible range are approached. If an unacceptable operating point is approached by the definition or configuration of the setpoint, this is prevented within the scope of the invention by switching over, in particular by reducing, the displacement. In the case of small displacements, no limitation of the operating point is usually performed. Permissible or unacceptable operating points are known in advance for each hydraulic pump and are specified in particular in the data sheet. These operating points can be stored and monitored accordingly in the memory of the computing unit that implements the drive.

[0007] To operate a variable speed electrohydraulic unit, in which a hydraulic pump 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, a setpoint speed of the variable speed electric drive is determined and prescribed to the electric drive. This can be carried out on the basis of higher-level specifications, for example a closed-loop control of pressure and / or volume flow, with the setpoint speed of the variable speed electric drive being supplied as an input variable to the hydraulic pump or the setpoint speed of the variable speed electric drive being determined as a manipulated variable of a higher-level pressure closed-loop control, which is generally supplied with pressure at a given point of the system. In this case, the displacement is set to the first relatively large displacement value or the second relatively small displacement value, preferably also by higher-level specifications, for example an operating strategy. For example, in the case of a small displacement value, the torque load of the electric drive is reduced. Since the load torque is the product of the pumping pressure and the displacement, for example, a high pressure can be maintained with a small torque in the case of a small displacement, without thermal overloading, particularly of the electric motor and converter of the electric drive.

[0008] If the displacement has a first relatively large displacement value, within the scope of the invention, a monitoring is carried out 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 electric drive, since these rotational speeds are usually converted 1:1 or at least have a known conversion ratio) are determined. It is determined whether an acceptable operating point results from the actual value of the pumping pressure and the actual value of the rotational speed of the hydraulic pump, and if not, the displacement is set to a second relatively small displacement value.

[0009] Preferably, by monitoring the current torque, i.e. by determining the actual value of the torque of the variable speed electric drive, it can be determined whether setting the second, relatively small displacement value has been implemented or has been successful, since this results in a change in the torque, in particular a decrease.

[0010] Alternatively or additionally, the currently set displacement value can be calculated using a computational model or a model-based observer, which can be supplied with current measurements such as the rotational speed, torque or drive current, pumping pressure, etc. This displacement value can also be used to determine whether setting the second, relatively small displacement value has been performed or has been successful.

[0011] It is furthermore advantageous to take into account the currently set displacement value as a controlled object characteristic in the pressure closed-loop control. In particular, a function for adjusting the controlled object is provided downstream of the actual control element which outputs the manipulated variable, which function varies the manipulated variable (e.g. the desired rotational speed value) as a function of the currently set displacement value. This makes it possible, for example, to avoid the need to adjust the parameters of the control element (coefficients such as proportional displacement, reset time, etc.) and to maintain the required dynamics of the closed-loop control.

[0012] According to a preferred embodiment of the invention, when changing the setpoint value of the speed of the variable speed electric drive, the rate of change of the setpoint value is limited to a maximum speed rate. The change of the setpoint value of the speed, which in particular in normal operation results from a higher-level regulation, has to be increased in order to maintain the volumetric flow rate or pressure as a result of the displacement being changed to a second, smaller displacement value, i.e. when the displacement is reduced. The change of the setpoint value of the speed is carried out with a predefined, limited angular acceleration. This makes it possible to achieve further component protection, in particular pump protection. The speed rate of change is in particular the rotational acceleration [angle / time 2If the acceleration / deceleration of the pump rotation is limited, mechanical damage to the internal components can be avoided. If the acceleration is too high, local negative pressures can form in the hydraulic fluid on the system side, because this fluid cannot dynamically follow the movement of the pump rotor. This can lead to cavitation. The pump data sheet can specify the limit values ​​for acceleration and deceleration.

[0013] According to a preferred embodiment of the invention, in the pressure closed loop control, the rate of change of the setpoint value of the pressure is limited to a maximum pressure rate of change and / or the setpoint value itself is limited to a 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 setpoint pressure rate of change is already limited (i.e. on the input side), this automatically results in a time delay on the output side as well, i.e. in particular the speed rate of change, and thus also has the above-mentioned advantages. The pressure rate of change can be expressed in particular as [pressure / time]. Since exceeding the maximum pressure value can be tolerated for a short time depending on the construction style and design, limiting the setpoint value of the pressure to the maximum pressure value preferably includes first waiting for an acceptable overload time before limiting, i.e. before limiting, the setpoint value of the pressure.

[0014] According to a preferred embodiment of the invention, if the actual value of the pumping pressure is high but corresponds to a minimum pressure value, the setpoint value of the speed of the variable speed electric drive is limited to a minimum speed. Normally, it is desirable for hydraulic pumps to be operated with the maximum possible displacement for energetic and dynamic reasons, i.e. in this case the displacement has a first relatively large displacement value. However, this means that the hydraulic torque is large, so that in this case, if the pressure is high (e.g. above a threshold value of 5 bar or 50 bar), particularly low or even negative speeds (e.g. below 200 rpm) can only be tolerated for a short time (e.g. for a few seconds) (of course, the respective values ​​depend on the concrete pump and should be understood as merely exemplary). That is, in order to avoid cavitation of the pump, the minimum pressure value and the minimum speed can be monitored as a further component protection mechanism. If the speed cannot be reduced further due to the limitation, the displacement is reduced (and the speed is correspondingly increased) in order to maintain the volumetric flow rate. The limitation may be immediate or may be implemented with a time delay, i.e. a short drop below the minimum speed may be tolerated in some cases. In the latter case, however, if it is indicated that the permissible time has been exceeded, a switchover to the small displacement is made, in particular taking into account the transient time. Taking into account the transient time means that the permissible time is not exceeded until the displacement reaches the small displacement value (in which case there is a non-critical operating point and therefore no further exceeding of the small displacement value is possible).

[0015] A computing unit according to the invention, for example a control device for an electrohydraulic unit, is in particular designed in terms of program technology to carry out the method according to the invention.

[0016] It is also advantageous to implement the method according to the invention in the form of a computer program or computer program product with program code for implementing all method steps, since this leads to particularly low costs, especially if the implementing control device is also used for further tasks and is therefore already existing in any case. 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. Downloading the program via a computer network (Internet, intranet, etc.) is also possible.

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

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

[0019] The invention is illustrated diagrammatically in the drawing on the basis of an embodiment and is explained in detail below with reference to the drawing. [Brief description of the drawings]

[0020] [Figure 1] 1 is a schematic diagram of a preferred embodiment of a variable speed electrohydraulic unit according to the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Detailed Description of the Drawings 1 a preferred embodiment of a variable speed electrohydraulic unit according to the invention is shown diagrammatically and is generally designated by the reference number 10. The electrohydraulic unit 10 comprises a displaceable hydraulic pump 20 (which in this example can pump in two directions), a variable speed electric drive 30 and a computing unit 40 for operating the electrohydraulic unit.

[0022] The hydraulic pump 20 is configured as an axial piston pump with a pivot angle α which can be displaced between two positions relative to a swash plate or an inclined axis, so that the displacement can be varied between a first, relatively large displacement value and a second, relatively small displacement value. For this purpose, an actuating signal can be used from the calculation unit 40 to the axial piston pump 20. This actuating signal can be, for example, an actuating 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 servomotor 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 setpoint rotational speed is predefined for the electric drive 30 by a calculation unit 40. The actual rotational speed can be determined by means of an angle generator. However, an open-loop control without feedback is also possible.

[0024] A calculation unit 40 is used for closed-loop control of the pump 10, which includes a target pumping pressure P S and / or target volumetric flow rate Q Sis supplied. The calculation unit 40 can be configured as a so-called electric drive regulation, from which the rotational speed n is determined as manipulated variable within the scope of the present invention. The calculation unit can be separate from the drive regulation, can be above the drive regulation or can communicate with the drive regulation via an interface (rotational speed setpoint, actual rotational speed, etc.). The determination of the actual delivery pressure is preferably carried out via a sensor. From this, a closed-loop pressure control is possible.

[0025] A displacement changeover during operation can be performed, for example, on the basis of an operating strategy. For example, the calculation unit 40 can determine whether the electric drive is operating at full load or at part load by monitoring the current to determine the electric drive torque provided by the electric drive 30 and comparing it with the maximum possible electric drive torque for the current speed or by monitoring the speed. In particular, a comparison with a threshold value can be envisaged, where an electric drive torque below a threshold value (for example 75%) or a speed below the threshold value is identified as part load. The speed threshold value is preferably defined depending on the intended use.

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

[0027] Setting of the displacement can preferably be carried out by actuating a pilot or control valve 21. In this case, a changeover valve (or a proportional valve) can be used.

[0028] According to preferred embodiments of the invention, different protection mechanisms are used to protect the electrohydraulic unit or components of the electrohydraulic unit against damage. For this purpose, 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 when (among other things) the displacement has a first relatively large displacement value, and from this it is determined whether an acceptable operating point exists. If no acceptable operating point exists (among other things because the rotational speed is too low and / or the pressure is too high), the displacement is set to a second relatively small displacement value. The pressure P S or volumetric flow rate Q S In order to maintain the above, the rotational speed is correspondingly increased to escape from the unacceptable operating point. As mentioned above, in the case of small displacements, no limitation of the operating point is usually performed.

[0029] Further advantageous protection mechanisms include: Limiting the rate of change of the target value of the speed of the variable speed electric drive (30) to a maximum rate of change of the speed; Limiting the rate of change of the target value of the pumping pressure through the hydraulic pump (20) to a maximum pressure rate of change; limiting the setpoint value of the pumping pressure via the hydraulic pump (20) to a maximum pressure value, possibly after an acceptable overload time has been exceeded; limiting the setpoint value of the speed of the variable speed electric drive (30) to a minimum speed if, possibly after an acceptable overload time has been exceeded, the actual value of the pumping pressure corresponds to a minimum pressure value at the highest, Includes.

[0030] The present invention increases pump availability, prevents machine stoppages, and avoids or reduces damage or excessive wear to the pump in electro-hydraulic systems.

Claims

1. A method for operating a variable speed electrohydraulic 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 further comprises, when the displacement has the first relatively large displacement value: determining a target value for the rotation speed of the variable speed electric drive (30); Determining an actual value of a pumping pressure through the hydraulic pump (20); Determining an actual value of the rotation 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 rotational speed of the hydraulic pump (20); setting the second relatively small displacement value when no acceptable operating point results from the actual value of the pumping pressure and the actual value of the rotational speed of the hydraulic pump (20); A method comprising:

2. determining an actual value of the torque of the variable speed electric drive (30); determining whether setting the second relatively small displacement value has been performed based on the determined actual value of the torque; and The method of claim 1 further comprising:

3. Limiting the rate of change of the target value of the rotation speed of the variable speed electric drive (30) to a maximum rate of change of the rotation speed. The method of claim 1 or 2, further comprising:

4. the setpoint value of the pumping pressure via the hydraulic pump (20) is supplied as an input to a higher-level pressure closed-loop control, to which is determined the setpoint value of the rotational speed of the variable-speed electric drive (30); 4. The method according to claim 1 .

5. Limiting the rate of change of the target value of the pumping pressure through the hydraulic pump (20) to a maximum pressure rate of change. The method of claim 4 further comprising:

6. Limiting the target value of the pumping pressure through the hydraulic pump (20) to a maximum pressure value. The method of claim 4 or 5, further comprising:

7. limiting the target value of the pumping pressure through the hydraulic pump (20) to a maximum pressure value includes waiting an allowable overload time before limiting the target value of the pumping pressure. The method according to claim 6.

8. Considering a currently set displacement volume value as a controlled characteristic of the pressure closed loop control unit for changing the manipulated variable. The method of claim 4 , further comprising:

9. Calculating the currently set displacement value using a computational model. The method of claim 1 , further comprising:

10. If the actual value of the pumping pressure is higher than the minimum pressure value, the setpoint value of the rotational speed of the variable speed electric drive (30) is limited to a minimum rotational speed. The method of claim 1 , further comprising:

11. limiting the target value of the rotation speed of the variable speed electric drive (30) to a minimum rotation speed comprises waiting an allowable overload time before limiting the target value of the rotation speed of the variable speed electric drive (30). The method of claim 10.

12. A computing unit (40) comprising a processor configured to carry out the method according to any one of claims 1 to 11.

13. An electrohydraulic unit (10), comprising: a hydraulic pump (20) capable of varying a displacement between a first relatively large displacement value and a second relatively small displacement value per operating cycle; a variable speed electric drive (30); A calculation unit (40) according to claim 12, An electrohydraulic unit (10) comprising:

14. A computer program comprising: The program comprises instructions for causing a computing unit of an electrohydraulic unit (10) according to claim 12 to carry out the method according to any one of claims 1 to 9 when the program is executed by said computing unit. Computer program.

15. 15. A computer readable data carrier on which a computer program according to claim 14 is stored.

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