METHOD FOR OPERATING A HYDRAULIC DRIVE

Description

[0001] The present invention relates to a method for operating a hydraulic drive comprising a hydraulic consumer with a positionable piston in a cylinder, as well as a computing unit and a computer program for carrying out the method. Background of the invention

[0002] An electro-hydraulic axle is a hydraulic drive with a pump (usually powered by an electric motor or drive) and a hydraulic cylinder that allows for electrical or electronic control of, for example, the position of the cylinder piston. Such electro-hydraulic axes are used, for example, in deep-drawing presses, injection molding machines, or other metal forming machines, as well as for moving heavy loads or machine parts.

[0003] Such electro-hydraulic axes usually feature alternating force-position control, meaning that, for example, force control or position control takes place depending on the operating point. Instead of force control, pressure control can also be provided, which are equivalent due to the relationship between force and pressure across the pressure application area, for example, in a hydraulic cylinder.

[0004] With such controls, for example, a valve can be used to change the volume flow of hydraulic fluid into or out of the cylinder. The speed of the pump (or the driving motor) can also be controlled. The pump can be designed, in particular, as a constant-displacement pump with a fixed delivery volume or as a variable-displacement pump with a variable delivery volume per working cycle. EP 1 930 604 B1, for example, discloses a method in which a variable-displacement pump with a constant speed is controlled to a delivery flow or pressure. A brake valve can be activated depending on this. DE 10 2016 223 099 A1 also deals with the operation of hydraulic drives. Disclosure of the invention

[0005] According to the invention, a method for operating a hydraulic drive, as well as a computing unit and a computer program for implementing the method, are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

[0006] The invention relates to a method for operating a hydraulic drive which comprises a hydraulic consumer with a positionable piston in a cylinder. As mentioned at the beginning, a speed of the pump can be used as a manipulated variable to control the position of the piston, or a position of a valve can be used to change the volume flow of a hydraulic fluid into or out of the cylinder. As has been shown, however, there are certain disadvantages in both variants. In the case of speed as the manipulated variable, these are, for example, insufficient stiffness of the hydraulic drive, inaccurate positioning dynamics and a varying resonance frequency. In the case of the valve, these are, for example, power losses in the valve due to the pressure drop in the valve, a non-linear characteristic and also a varying resonance frequency.

[0007] Against this background, it is proposed that the hydraulic consumer, e.g. an electro-hydraulic axle, is or is connected to a tank at one port (e.g. on the A side) via a variable-speed pump and at another port (e.g. on the B side) via a proportional valve. In the case of a hydraulic cylinder as the hydraulic consumer, this has two chambers separated by the piston. Each chamber is assigned to one of the ports (A side, B side). The position of the piston is then controlled using or within the framework of a model-based control system in which the speed of the pump is used as the manipulated variable and in which the position of the proportional valve is specified.Here, an inverse system model (or inverse system model) of the hydraulic drive is used to set, in particular regulate, a setpoint for a (braking) force on the piston using the proportional valve. In an inverse model, inputs and outputs are swapped; for example, volume flow and / or pressure as inputs of an inverse valve model can lead to a valve spool position as the output. The proportional valve is used in particular in a closed control loop, i.e. the adjustment of the spool in the valve can also take place in the closed (subordinate) control loop with feedback of an actual position value. This makes it possible to achieve greater rigidity of the hydraulic drive and, on the other hand, particularly good energy efficiency.

[0008] This type of position control allows for particularly good and rapid response to different phases or transitions between different phases, for example, during the operation of a casting cylinder. Furthermore, this type of control is particularly easy and quick to parameterize.

[0009] The variables for which the actual values ​​are required in this control system are in particular the position of the piston and the two pressures on the two connections. These can be recorded using suitable sensors, for example. Within the framework of model-based control, the speed and acceleration of the piston can then be determined from the position of the piston. From this, the desired flow rates (volume flows) on both sides - i.e. on the pump side (there also to be referred to as the delivery flow) and on the proportional valve side - can be determined and then specified accordingly. In this context, it should also be mentioned that with model-based control, a trajectory, i.e. the progression of variables (in this case the position), can be calculated in advance based on a model of the system (as mentioned, an inverse system model is used here in particular).In model-based control, controlled variables are also referred to as state variables, and state constraints can be taken into account. For a more detailed description of the model, please refer to the figure description.

[0010] According to the invention, in order to achieve (or realize) the desired value for the force on the piston - and thus the desired acceleration or braking effect - a desired curve of the position of the proportional valve is determined by planning desired pressures in the hydraulic drive and the resulting positions of the proportional valve.

[0011] Preferably, when the position of the proportional valve is specified (e.g. as part of a subordinate control system), feedforward control is carried out. Advantageously, a model-based pressure setpoint as input and / or a dynamics of the proportional valve are taken into account. A model-based value for the flow rate of the proportional valve can increase the valve dynamics. In this way, the valve can effectively be overridden, for example, in order to bring the actual flow rate as close as possible to the model-based value. While the underlying equations typically assume an infinitely fast actuator for the valve, feedforward control of this type can reduce these errors. A model used for the proportional valve can, in particular, comprise a dynamics component which, if necessary, simplifies the calculation of a spool position of the main stage orof the main valve and a limitation of the pilot valve (this is a control component for the main valve) at a specified system pressure. For example, a second-order system (e.g., PT2 behavior) with state restriction can be assumed. It can also include an opening equation that describes the flow rate depending on the pressures applied to the main stage and the geometry characteristic. The output of the pilot control is a setpoint for the valve position.

[0012] Preferably, a pilot control is used to control the position of the piston. This advantageously takes into account a relationship between a flow rate (volumetric flow) of the cylinder and a state variable characterizing the movement of the piston, in particular a time derivative of the position (e.g., speed, acceleration, and / or jerk). For the pilot controls of the piston and proportional valve, different or separate state and / or positioning restrictions can therefore be considered. One output of the pilot control is then preferably a setpoint for a volumetric flow of both ports (piston sides).

[0013] In the (inverse) plant or system model used, continuously differentiable trajectories for the flat output and its time derivatives are preferably required to implement the flatness-based feedforward control, in particular. To generate a manipulated variable trajectory in real time, an extended low-pass filter algorithm can be used, for example, and the nonlinear (flat) dynamics and limitations of the flat state can be taken into account. Such a method with real-time trajectory planning, taking into account state and manipulated variable constraints, is described, for example, in DE 10 2018 211 738 A1, where it is demonstrated using the example of an axial piston pump. For further explanations, please refer to the description therein.

[0014] In summary, there are the following particular advantages of the proposed control concept.

[0015] A pilot control can be used for controlling the cylinder (this affects the position of the piston) and the proportional valve, each of which also takes into account different state and control constraints. To realize the acceleration setpoint, a target valve opening curve can be deduced through extended planning of the target pressures and the resulting valve openings. By measuring a non-sensitive actual pressure on the A side (or on the pump side; non-sensitive if it is not directly controlled but depends on an accumulator and the B side only has one intervention point), the model can be compensated online for the simplification made. By measuring or specifying (target pressure in the case of pressure control of this chamber) one chamber pressure, the other chamber pressure can be clearly derived.If the pressure buildup equation, which is controlled to specify the flat output, is not directly measured and fed back, the flatness of the overall system can be approximated as flat within the limits of dead times and measured value processing time. This allows one to move from an ambiguous to an exact solution for the cylinder position. The proposed method is also online and real-time capable.

[0016] A further advantage is that the valve can be controlled much more dynamically than before, for example, with a response time of only about 7 ms in the small-signal range instead of about 14 ms. A non-flat system can still be controlled by a feedforward control using the proposed method and the adjustments made therein.

[0017] In addition, monitoring based on the hydraulic drive model can also compare certain values ​​with corresponding measured values, which can be performed as "condition monitoring." Furthermore, efficiency and precision can also be increased using a variable gear ratio.

[0018] A computing unit according to the invention, e.g. a control unit of a hydraulic axle, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0019] Implementing a method according to the invention in the form of a computer program or computer program product with program code for performing all method steps is also advantageous, as this entails particularly low costs, especially if an executing control unit is also used for additional tasks and is therefore already present. Suitable data storage devices for providing the computer program include, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0020] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0021] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0022] The invention is illustrated schematically in the drawing using an embodiment and is described in detail below with reference to the drawing.

[0023] Character description Figure 1 schematically shows a hydraulic drive which is suitable for carrying out a method according to the invention. Figure 2 schematically shows a sequence of a method according to the invention in a preferred embodiment. Detailed description of the drawing

[0024] In Figur 1 A hydraulic drive 100 is schematically shown, in which a method according to the invention can be carried out, as will also be explained below. The hydraulic drive 100 in this case has a variable-speed pump 110, comprising a (variable-speed) electric motor or drive 112 and a delivery unit 114 coupled thereto. The pump 110 is, for example, an axial piston pump designed as a fixed-displacement pump with a fixed delivery volume per working cycle.

[0025] Furthermore, the pump 110 is connected to a hydraulic consumer 130, which in this case is a cylinder 132 with a positionable piston 134. A position of the piston or a reference point there is denoted by x. A load 136, for example, can be moved by means of the piston. The cylinder 132 is connected to a tank 120 for hydraulic fluid at a port A via the pump 110 and at a port B via a proportional valve 140. A position of a spool of the proportional valve 140 is denoted by y.

[0026] In the cylinder 132 or on the piston 134, the A-side piston surface is designated AA and the annular, B-side piston surface AB. The pressure on the A-side is p A and the pressure on the B-side is p B . The flow rate into or out of the cylinder 132 is designated on the A-side by Q A and on the B-side with Q B designated.

[0027] The electric motor 112, optionally via additional components such as inverters or frequency converters, can be controlled via a processing unit 150 designed as a control unit, as can the proportional valve 140. The hydraulic drive 100 can thus be used as an electro-hydraulic axle. It is understood that additional pumps and / or valves can also be provided at suitable locations, but this is not absolutely necessary.

[0028] In Figur 2 a schematic representation of a sequence of a method according to the invention in a preferred embodiment is shown. To control the position x of the piston, a state controller 260 is used which can be executed, for example, on the computing unit 150 and which uses an inverse model or system model 200 of the hydraulic drive 100. In the model 200, a distinction is made in particular according to the relevant components of the hydraulic drive, ie 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. In addition, a state observer 270 is provided, as well as a unit 250 (a so-called interpolator) for specifying setpoints for state variables. By means of an interpolator, it is possible to generate trajectories and their derivatives, i.e. a further method besides or n-th order low-pass filters.

[0029] By means of the unit 250, setpoint values ​​for state variables (a setpoint trajectory) can be specified, which in particular include the position x of the piston, but also its time derivatives ẋ (speed) and ẍ (acceleration) (the target trajectory should be continuously differentiable n times). These are transferred to the state controller 260, which itself also receives actual values ​​of these state variables from the state observer 270. This in turn receives the actual value x ist the position x and actual pressure values p A and p B and calculates actual values ​​for ẋ (speed) and ẍ (Acceleration). Using the model 200, the setpoint for the position x is then converted into values ​​for a speed n of the pump 110 and a position y of the proportional valve 140 are determined, which are set or regulated.

[0030] In the hydraulic drive 100, the speed n and the position y then in the flow rate Q A or the flow rate Q B implemented, which results in a force F on the piston or the load 126. Taking into account the mass m The speed and position of the moving mass (i.e. load 126 and piston 134) also result from this.

[0031] In determining these values ​​for the speed n and the position y using the model 200, model-based flow rates or flow rates are also calculated Q A * and Q B * In these model-based calculations, suitable (inverse) nonlinear state-space representations can be used, which describe the hydraulic drive and enable linearization along the target trajectory.

[0032] An acceleration of the target trajectory (which indicates the force on the piston) can be converted into target values p A,soll and p b,soll for the pressures. For example, one of the two pressures, e.g. p b,soll , , which will then adjust itself statically. This allows the pressure level and stiffness of the hydraulic consumer to be influenced. While stiffness cannot usually be directly influenced with a pilot control, the target stiffness can be set by the controller via pole specification / impedance during control in the state controller section.

[0033] The flow rate Q B , which is a function of the position y, ie Q B = Q B ( y ) , can be achieved by specifying a position y of the proportional valve. The speed, in turn, influences the flow rate Q A . Q B a function of the states of the trajectory planning filter, which are then inserted into the feedforward control function. For the assumption that p A in this case is constant or measurable, the target valve opening (position y) can be calculated over time.

Claims

1. Method for operating a hydraulic drive (100) which has a hydraulic consumer (130) with a positionable piston (134) in a cylinder (132) which is connected to a tank (120) at one connection (A) via a pump (110) of variable rotational speed and at another connection (B) via a proportional valve (140), wherein a position (x) of the piston (134) is controlled using model-based control in which a rotational speed (n) of the pump (110) is used as a manipulated variable and in which a position (y) of the proportional valve (140) is preset, wherein an inverse system model (200) of the hydraulic drive (100) is used in order to set a setpoint value for a force on the piston using the proportional valve (140), and characterized in that, in order to achieve the setpoint value for the force on the piston (134), a setpoint profile of the position of the proportional valve (140) is inferred by means of planning setpoint pressures in the hydraulic drive (100) and resulting positions (y) of the proportional valve (140).

2. Method according to Claim 1, wherein the inverse system model (200) of the hydraulic drive (100) is used in order to adjust the setpoint value for the force on the piston using the proportional valve (140).

3. Method according to either of the preceding claims, wherein a pilot control operation is performed during presetting, in particular within the context of subordinate control, of the position (y) of the proportional valve (140).

4. Method according to Claim 3, wherein a model-based pressure setpoint value as input and / or the dynamics of the proportional valve are taken into consideration in the pilot control operation for the proportional valve (140).

5. Method according to any of the preceding claims, wherein a pilot control operation is performed during control of the position (x) of the piston (134).

6. Method according to Claim 5, wherein a relationship between a through-flow rate of the cylinder and a state variable characterizing the movement of the piston, in particular a time derivative of the position, is taken into consideration in the pilot control operation for the position (x) of the piston (134).

7. Method according to any of the preceding claims, wherein values of variables determined in the context of monitoring on the basis of a model of the hydraulic drive (100) are compared with corresponding measured values.

8. Method according to any of the preceding claims, wherein a constant displacement pump or a variable displacement pump is used as the pump (110) of variable rotational speed.

9. Method according to any of the preceding claims, wherein the hydraulic drive (100) is used for an electrohydraulic axle.

10. Computing unit (150) which is designed to carry out a method according to any of the preceding claims.

11. Computer program which causes a computing unit (150) according to Claim 10 to carry out a method according to any of Claims 1 to 9 when it is executed on the computing unit (150).

12. Machine-readable storage medium with a computer program according to Claim 11 stored on it.