Method for Operating a Hydraulic Drive

Abstract

The disclosure relates to a method for operating a hydraulic drive which comprises a hydraulic consumer with a positionable piston in a cylinder which is connected to a tank at one connection via a pump of variable rotational speed and at another connection via a proportional valve, wherein a position of the piston is controlled using a model-based control in which a rotational speed of the pump is used as a manipulated variable and in which a position of the proportional valve is preset.

Claims

1. A method for operating a hydraulic drive, the hydraulic drive including a hydraulic consumer having a positionable piston in a cylinder, the cylinder being connected to a tank at a first connection via a pump of variable rotational speed and connected to the tank at second connection via a proportional valve, the method comprising: controlling a position of the piston using a model-based control process in which a rotational speed of the pump is a manipulated variable and in which a position of the proportional valve is preset.

2. The method according to claim 1 further comprising: adjusting a setpoint value for a force on the positionable piston using the proportional valve and using an inverse system model of the hydraulic drive.

3. The method according to claim 2, the adjusting the setpoint value further comprising: inferring, to achieve the setpoint value for the force on the positionable piston, a setpoint profile of the position of the proportional valve from a planning of setpoint pressures in the hydraulic drive and from resultant positions of the proportional valve.

4. The method according to claim 1 further comprising: carrying out a feedforward control in a presetting of the position of the proportional valve, as part of a subordinate control.

5. The method according to claim 4, the carrying out the feedforward control in the presetting of the position of the proportional valve further comprising: carrying out the feedforward control taking into consideration at least one of (i) a model-based pressure setpoint value as input and (ii) dynamics of the proportional valve.

6. The method according to claim 1, the controlling the position of the positionable piston further comprising: carrying out a feedforward control of the position of the positionable piston.

7. The method according to claim 6, the carrying out the feedforward control of the position of the positionable piston further comprising: carrying out the feedforward control taking into consideration (i) a relationship between a through-flow rate of the cylinder and (ii) a state variable characterizing a movement of the positionable piston.

8. The method according to claim 1 further comprising: determining values of variables as part of a monitoring based on a model of the hydraulic drive; and comparing the determined values of the variables with corresponding measured values of the variables.

9. The method according to claim 1, wherein the pump of variable rotational speed is one of (i) a constant displacement pump and (ii) a variable displacement pump.

10. The method according to claim 1, wherein the hydraulic drive is used for an electrohydraulic axle.

11. A computer for operating a hydraulic drive, the hydraulic drive including a hydraulic consumer having a positionable piston in a cylinder, the cylinder being connected to a tank at a first connection via a pump of variable rotational speed and connected to the tank at second connection via a proportional valve, the computer being configured to: control a position of the piston using a model-based control process in which a rotational speed of the pump is a manipulated variable and in which a position of the proportional valve is preset

12. The method according to claim 1, wherein the method is performed by executing a computer program on a computer.

13. The method according to claim 12, wherein the computer program is stored on a non-transitory machine-readable storage medium.

14. The method according to claim 7, the state variable being a time derivative of the position of the positionable piston.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The disclosure is schematically illustrated in the drawing on the basis of an exemplary embodiment and will be comprehensively described below with reference to the drawing.

[0024] FIG. 1 schematically shows a hydraulic drive which is suitable for carrying out a method according to the disclosure.

[0025] FIG. 2 schematically shows a sequence of a method according to the disclosure in a preferred embodiment.

DETAILED DESCRIPTION

[0026] In FIG. 1 there is schematically illustrated a hydraulic drive 100 in which a method according to the disclosure can be carried out, as will also be explained below. The hydraulic drive 100 has in the present case a pump 110 of variable rotational speed, comprising an electric motor or drive 112 (of variable rotational speed) and a delivery unit 114 coupled thereto. The pump 110 is, for example, an axial piston pump, embodied as a constant displacement pump, with fixed delivery volume for each work cycle.

[0027] Furthermore, the pump 110 is connected to a hydraulic consumer 130 which, in the present case, is a cylinder 132 with a positionable piston 134. A position of the piston or of a reference point there is designated by x. A load 136, for example, can be moved by means of the piston. The cylinder 132 is connected to a tank A for hydraulic fluid at a connection A via the pump 110 and at a connection B via a proportional valve 140. A position of a slide of the proportional valve 140 is designated by y.

[0028] In the cylinder 132 or on the piston 134, the A-side piston surface is designated by A.sub.A, and the annular, B-side piston surface is designated by A.sub.B. A pressure on the A side is p.sub.A, and a pressure on the B side is p.sub.B. A delivery flow or a through-flow rate into the or out of the cylinder 132 is designated on the A side by Q.sub.A and on the B side by Q.sub.B.

[0029] It is possible via a computing unit 150, which takes the form of a control unit, for the electric motor 112 to be actuated, where appropriate via further components such as inverters or frequency converters, this likewise applying to the proportional valve 140. The hydraulic drive 100 can thus be used as an electrohydraulic axle. It will be understood that further pumps and/or valves can also be present at suitable points, but this is not absolutely necessary.

[0030] In FIG. 2 there is schematically illustrated a sequence of a method according to the disclosure in a preferred embodiment. To control the position x of the piston, use is made of a state controller 260 which, for example, can be implemented on the computing unit 150 and which employs an inverse model or system model 200 of the hydraulic drive 100. In the model 200 there is a distinction in particular according to the relevant components of the hydraulic drive, that is to say there is a model 230 for the hydraulic consumer 130, a model 210 for the pump 110 and a model 240 for the proportional valve 140. Additionally provided is a state observer 270, as is a unit 250 (a so-called interpolator) for presetting setpoint values for state variables. It is possible by means of an interpolator to generate trajectories and their derivatives, that is to say a further method, in addition to, for example, nth-order low-pass filters.

[0031] By means of the unit 250 there can be preset setpoint values for state variables (a setpoint trajectory) which here comprise in particular the position x of the piston, but also their time derivatives {dot over (x)} (speed) and {umlaut over (x)} (acceleration) (the setpoint trajectory should here be continuously differentiable n times). They are transferred to the state controller 260 which itself also obtains actual values of these state variables from the state observer 270. The latter in turn obtains as input variables the actual value x.sub.isl of the position x and also actual values of the pressures p.sub.A and p.sub.B and calculates therefrom actual values for {dot over (x)} (speed) and {umlaut over (x)} (acceleration). Using the model 200 there are then determined, from the setpoint value for the position x, values for a rotational speed n of the pump 110 and a position y of the proportional valve 140 which are set or adjusted.

[0032] In the hydraulic drive 100, the rotational speed n and the position y are then converted into the delivery rate Q.sub.A or the through-flow rate Q.sub.B, which results in a force F on the piston or the load 126. With the mass m of the moving mass (that is to say load 126 and piston 134) being taken into consideration, what results therefrom is also the speed and the position.

[0033] In the course of determining these values for the rotational speed n and the position y by means of the model 200, there are also determined model-based delivery flows or through-flow rates Q.sub.A* and Q.sub.B*. In these model-based calculations there can be suitable (inverse) nonlinear state space representations which describe the hydraulic drive and allow a linearization along the setpoint trajectory.

[0034] An acceleration of the setpoint trajectory (which thus indicates the force on the piston) can here be converted, for example, into setpoint values p.sub.A,soll and p.sub.B,soll for the pressures. It is possible here, for example, to preset one of the two pressures, for example p.sub.B,soll, which will then be established statically. It is thus possible to have influence on the pressure level and the stiffness of the hydraulic consumer. Although the stiffness cannot as a rule be directly influenced with a feedforward control, it is possible via pole presetting/impedance in the control in the state controller part to set the target stiffness by means of the controller.

[0035] Here, the through-flow rate Q.sub.B, which is a function of the position y, that is to say that Q.sub.B=Q.sub.B(y), can be achieved by the presetting of a position y of the proportional valve. The rotation speed in turn has an influence on the delivery flow Q.sub.A. Here, Q.sub.B is a function of the states of the trajectory planning filter which are then inserted into the feedforward control function. Assuming that p.sub.A in this case behaves constantly or is measurable, the target valve opening (position y) can be calculated over time.