JP7513843B2 - Method for operating a hydraulic drive - Patents.com - Google Patents

Description

The present invention relates to a method for operating a hydraulic drive including a hydraulic consumer with a piston positionable in a cylinder, as well as a computing unit and a computer program for implementing the method.

Electrohydraulic shafts are hydraulic drives with a pump (usually operated by means of an electric motor or drive) and a hydraulic cylinder, whereby for example the position of the cylinder piston can be controlled electrically or electronically in a closed loop. Such electrohydraulic shafts are used, for example, in so-called deep drawing presses, injection moulding machines, die casting machines or machines of other forming techniques, as well, for example for moving heavy loads or machine parts. It is also possible, instead of the pump, to use, for example, a pressure medium accumulator or to use both in combination. Furthermore, in this context, mention can also be made of hydraulic drives.

In such (electro)hydraulic shafts, a force release position control is usually envisaged, i.e. force or position control is performed, for example depending on the operating point. Instead of force control, a pressure control may also be envisaged, which is equivalent based on the relationship between force and pressure via the pressure acting surface, for example in a hydraulic cylinder. Instead of position control, a speed control can also be considered.

In such a closed-loop control, the pump (or the drive motor) can have a closed-loop control of its volumetric output and/or speed. However, it is also possible to use, for example, a control valve device for varying the volumetric flow rate of the working fluid at the inlet and/or outlet of the cylinder.

These components have usage limitations that are taken against themselves, such as, for example, maximum pressure, dead time, set speed limitations. For this purpose, limitations and components can also be described in the model and intelligently incorporated into the rule design. Essentially, the strong nonlinearity, dead time and limitations of such hydraulic systems present problems for closed-loop and open-loop control. To take into account these systematic limitations in open-loop and closed-loop control, continuous optimization problems can be solved in real time. Here, in particular, model predictive control (MPC) and reinforcement learning (RL) approaches are used, which require high computational power during execution time. In systems that should have a high bandwidth, this can no longer be meaningfully solved, or in any case becomes insignificant for high sampling rates.

According to the invention, a method for operating a hydraulic drive is proposed with the features of the independent patent claims, as well as a computing unit and a computer program for implementing said method. Preferred embodiments are the subject of the dependent claims and the following description.

The invention relates to a method for operating a hydraulic drive including a hydraulic consumer with a piston positionable in a cylinder. The hydraulic drive has two chambers, here for example denoted by A and B, separated by a piston. Each chamber is in this case assigned to one of a number of connections (A, B). The hydraulic drive can in particular be a hydraulic shaft. In this case, the cylinder is connected at one connection (e.g. A) to a pressure medium accumulator (here in particular a low-pressure accumulator) and/or a pump and at the other connection (e.g. B) to a pressure medium sink via a control valve device. In particular, the tank is considered here as the pressure medium sink. However, it is also conceivable (by corresponding connection) to use the first side (i.e. chamber A) as pressure medium sink (regeneration circuit).

The piston position or velocity is then closed-loop controlled using or within a model-based closed-loop control in which the setpoint of the controller is preset. The controller may include one or more continuously adjustable valves.

In this case, an inverted section model (or inverted system model) of the hydraulic drive is preferably used for adjusting, in particular for closed-loop control of, the setpoint value for the (braking) force on the piston using a control device. The control device, for example a proportional valve, is used here in particular in a closed-loop control circuit, i.e. in a (sub) closed-loop control circuit, the setting adjustment of the slider in the valve can also be carried out using feedback of the actual value of the position. In the inverted model, the actual output variable (for example the position) is used as the model input and the actual input variable (for example the volumetric flow rate or pressure) is used as the model output.

By using such a position/speed control, it is possible to react particularly well and quickly to the various phases or to the transitions between the various phases, for example during the operation of the casting cylinder. Furthermore, such a closed-loop control can be parameterized particularly simply and quickly.

To achieve the target value for the force on the piston, a target progression curve of the proportional valve's set position is preferably estimated via planning of the target pressure in the hydraulic drive and the resulting set position of the proportional valve. Planning of the target pressure here includes in particular planning a differential pressure between two connections or measuring the pressure on one side and planning the pressure on the other side. This thereby represents a kind of pilot control or hybrid pilot control and closed-loop control.

In this closed-loop control, the variables whose actual values are required are in particular the position of the piston and the two pressures on the two connections (A and B). These can be captured, for example, by means of suitable sensors. Thus, within the framework of model-based closed-loop control, the piston speed and acceleration can also be determined from the piston position. The desired flow rates (volume flow rates) can then be determined from this on both sides, which are therefore predefined accordingly. In this connection, it should also be mentioned that in the case of model-based closed-loop control, the trajectory, i.e. the progression curve of the variable (in this case the position), can be calculated in advance and also online based on a model of the system (here, as mentioned above, in particular an inversion section model is used). The controlled variables are also called state variables in the case of model-based closed-loop control, and state limitations can be taken into account. For a more detailed description of the model, please also refer to the description of the figures.

By connecting one connection (side A) to a pressure medium accumulator, for example via a switching valve, a constant pressure can be derived there, at least in a first approximation, or precalculated depending on the stroke and speed, so that only the pressure on the other side (side B) is relevant. In trajectory planning, therefore, the underlying mechanical differential equations, i.e., in particular pressure, position and speed, are taken into account. In this case, planning of the pressure target value can be limited to planning the B-side pressure, since the other side is directly connected to the pressure medium accumulator and thus corresponds (at least approximately) to the accumulator pressure (dynamic influences are taken into account here via the measurement of the A-side pressure in the calculation at each time stamp).

When presetting the set position of the proportional valve, as described above, particularly within the framework of the subordinate closed-loop control, a pilot control is preferably performed as well, which can be different depending on the selection of the proportional valve (direct open-loop controlled proportional valve or pilot controlled proportional valve). In this case, the set value for the position of the main stage of the proportional valve is pilot controlled expediently. Here, a PT2 control system with dead times and state limits can be written in the underlying system. The pilot control then uses the main stage set value of the valve as input. The dynamics of the proportional valve can be taken into account particularly well in this way.

In this regard, it should be noted that pilot control is effective as long as the internal state of the proportional valve is not saturated. However, an attempt is always made to open-loop control the proportional valve to saturation for as long as possible. Saturation here means, in particular, to fully open the pilot stage for as long as possible. With (conventional) closed-loop control, this would only be achieved for a very short period of time, since the position signal present beyond the maximum opening value would be lost. With pilot control, on the other hand, the system acquires dynamic behavior without becoming unstable. This allows the flow rate on side B (or on the proportional valve side) to be set or pre-set with sufficient accuracy.

A special advantage here is that inversion is possible (using the inversion section model) even for systems that are not themselves flat. Systems with hydraulic cylinders are not considered flat in principle, because the generation of the actuator forces (which are introduced into the mechanical differential equations) is polysemic, i.e. a given pressure difference on both sides is formed by a wide variety of combinations of pressures. However, in particular in the case of die-casting circuits (or generally in the case of the interconnections mentioned at the beginning), inversion nevertheless works, because in the cylinder the pressure is biased and the chamber on the pressure medium accumulator side (side A) is in direct communication with the accumulator pressure, for example via a switching valve. Therefore, in the state space representation, it can be assumed that the pressure in this chamber of the cylinder is constant. The force rise therefore depends only on the pressure in the other chamber (side B). However, since the pressure medium accumulator has (to some extent) inherent dynamics, whereby the pressure is not actually constant, further corrections can still be made in order to minimize errors in the pilot control. For example, the pressure can be measured or calculated in the chamber on the pressure medium accumulator side (side A) and this pressure can additionally be used for the pressure planning of the other chamber or side (side B). The valve opening or setting position of the proportional valve (on side B) can thus be used as the only setting variable (or actuator).

In this way, by using the system model and the load model, highly dynamic processes such as, for example, aluminum die casting can be handled even if the control loop dynamics of the involved system components are not optimal. No longer is everything closed-loop controlled, but partly pilot controlled (via the model), in particular taking into account known set variable limits and dynamics limits. In a manner of speaking, a switchable planning filter algorithm is provided, which allows real-time state trajectory planning, taking into account, among other things, cylinder pressure bias, accumulator pressure and dead time/setting dynamics of the actuators (i.e. proportional valves).

Besides, the above mentioned assumptions and linearization of the underlying system model allow the implementation of pilot control rules based on flatness, which makes the residual error dynamics sufficiently small. This allows a retrofittable parameterization with minimal use of look-up tables or characteristic curves. Additionally, this allows a significant improvement in the development of small and large cylinder units, since only the model parameters are variable or need to be changed, while the underlying procedures (e.g. mapped to software) remain essentially unchanged.

A further advantage is that the valve can be open-loop controlled much more dynamically than before. As long as the valve is in a small signal range, the actuation control time can be halved. Non-flat systems can nevertheless be controlled by pilot control by the proposed method or via the adjustments made therein.

Additionally, within the framework of model-based monitoring of the hydraulic drive, predefined values of variables can also be compared with corresponding measured values. This can be done as "condition monitoring". Furthermore, variable gear ratios can be used to increase efficiency and precision.

The calculation unit according to the invention, for example a hydraulic axis control device, is configured to carry out the method according to the invention, in particular with regard to the programming techniques.

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 carrying out all method steps, since this entails particularly low costs, especially if the executing control device is already existing anyway for reasons also used for the further tasks. Suitable data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as, for example, hard drives, flash memories, EEPROMs, DVDs, etc. Downloading the program via a computer network (Internet, intranet, etc.) is also possible.

Further advantages and embodiments of the present invention will become apparent from the following description and accompanying drawings.

It goes without saying that the features mentioned above and those further described below can be used not only in the combinations presented in each case, but also in other combinations or alone, without departing from the scope of the present invention.

The invention is illustrated in the drawings, which are schematic representations of embodiments, and will be described in detail below with reference to the drawings.

1 shows a schematic representation of a hydraulic drive suitable for implementing the method according to the invention; FIG. 2 is a diagram showing a schematic sequence of the method according to the present invention in a preferred embodiment. 2 shows a schematic representation of a second embodiment of a hydraulic drive suitable for pressure induction in the method according to the invention; FIG. 3 shows a schematic representation of a third embodiment of a hydraulic drive suitable for pressure induction in the method according to the invention;

1 shows a schematic representation of a hydraulic drive 100 in which the method according to the invention can be carried out, as will also be explained below, which in this embodiment comprises two pressure medium accumulators 110 and 112, where the pressure medium accumulator 110 is in particular a low-pressure accumulator (e.g. with a pressure below 300 bar, for example between 170 and 200 bar) and the pressure medium accumulator 112 is in particular a high-pressure accumulator (e.g. with a pressure above 400 bar).

Furthermore, the pressure medium accumulators 110, 112 can be connected to a hydraulic consumer 130, which in this case is a cylinder 132 with a positionable piston 134. The position of the piston or the position of the reference point therein is indicated by x. By means of the piston, for example, a load 136 can be displaced. The cylinder 132 can be connected to the pressure medium accumulator 110 (low pressure accumulator) via a valve 144 configured as a switching valve at the connection port A and to the pressure medium accumulator 112 (high pressure accumulator) via a proportional valve 146. Needless to say, during operation, preferably only one of the two pressure medium accumulators is connected, in which case in this embodiment, in particular the connection to the pressure medium accumulator 110 (low pressure accumulator) is observed when the switching valve 144 is open in this case.

Furthermore, the cylinder 132 is connected to the tank 120 for the working fluid via a proportional valve 140 at connection port B. The set position of the slider of the proportional valve 140 is indicated by y. Similarly, the cylinder 132 can be connected to connection port A at connection port B via a proportional valve 142. However, in the case of this embodiment, it should be observed that the illustrated situation is particularly in which the proportional valve 142 is closed.

However, in principle, various interconnections for various operating phases of the hydraulic drive 100 can be realized, for example, by suitable switching or conversion of valves, as is the case for example in the context of aluminum die casting.

At the cylinder 132 or piston 134, the A-side piston surface is designated by A _A and the annular B-side piston surface is designated by A _B. The A-side pressure (in the piston chamber) is p _A and the B-side pressure (in the annular chamber) is p _B. The discharge or flow rate to or from the cylinder 132 is designated at the A side by Q _A and at the B side by Q _B.

Valves 140, 142, 144 and 146, for example, can be driven and controlled via a computing unit 150 configured as a control unit. Thus, the hydraulic drive 100 can be used as a hydraulic shaft.

（速度）並びに圧力ｐ_Ａ及び／又はｐ_Ｂも含まれる。これらは、状態制御部２１０及びパイロット制御部２２０に伝送される。次いで、ここでは、位置ｘについての目標値から比例弁１４０の設定位置ｙについての値が求められる。 2 shows the sequence of the method according to the invention in a preferred embodiment. For closed-loop control of the piston position x, a state control 210 with a flatness-based pilot control 220 is used, which can be implemented for example on the computing unit 150 and uses an inverse or section model 200 of the hydraulic drive 100. Here, setpoints (setpoint trajectories) for the state variables are determined. This includes, in particular, not only the piston position x but also its time derivatives

(speed) and pressures _pA and/or _pB . These are transmitted to state control 210 and pilot control 220. A value for the set position y of proportional valve 140 is then determined here from the set value for position x.

が属する。圧力ｐ（ここでは、既に述べたように、Ａ側における圧力は一定であるとみなすことができるため、Ｂ側における圧力のみが考慮される（ただし、両方の圧力でも可能である））は、速度及び加速度から決定することができる。フィルタパラメータは、好適には、（液圧式の駆動装置の）目標システムの動特性が（圧力媒体アキュムレータからの）既存の設定エネルギによって達成し得るように選択される。 Within the framework of the above-mentioned trajectory planning, the setpoint value for the position x of the piston is, for example, subjected to n-th order low-pass filtering, whereby the n-th order derivative of the setpoint signal is then obtained.

The pressure p (here, as already mentioned, only the pressure on the B side is taken into account, since the pressure on the A side can be considered constant (but both pressures are also possible)) can be determined from the speed and acceleration. The filter parameters are preferably selected such that the dynamics of the target system (of the hydraulic drive) can be achieved with the existing set energy (from the pressure medium accumulator).

As soon as the correct setpoint variable (proportional valve setpoint) y can be determined depending on the inverse model 200 of the system, a setpoint for e.g. the volumetric flow rate _QB on side B can be determined based on the setpoint pressure on side B (using the volumetric flow rate equation) and from there a setpoint for the setpoint y can be determined. The system can be open-loop controlled so that it tracks the position with very little deviation. Phase shifts due to the deadtime of the system are maintained.

についての実際値が生じる。この下位の閉ループ制御の枠内においては、同様にパイロット制御も行われ、詳細には、メインステージ１４０．２の設定位置又は位置において行われる。図３及び図４は、本発明に係る方法のシーケンスに関しては図１と異なるものではない。図３及び図４は、高圧アキュムレータ１１２に対する代替的な変形形態のみを示しているので、参照記号は、実質的に同一である。 This set position y is implemented within the framework of a subordinate closed-loop control in which the pilot stage 140.1 and the main stage 140.2 are taken into account, whereby the piston force F and the speed x_act also represent the actual value of the position _{x_act.}

1. Within the framework of this subordinate closed-loop control, a pilot control is also carried out, in particular at the set position or position of the main stage 140.2. Figures 3 and 4 do not differ from Figure 1 in terms of the sequence of the method according to the invention. As Figures 3 and 4 only show alternative variants for the high-pressure accumulator 112, the reference symbols are substantially identical.

The hydraulic drive shown diagrammatically in Figures 3 and 4 differs from the hydraulic drive shown in Figure 1 in that instead of the high-pressure accumulator 112 a pressure intensifier 152 (also called boost cylinder) is used, which is configured, for example, as a differential cylinder. The structure of this type of pressure intensifier 152 is known and therefore does not require further description. Here, a further low-pressure accumulator 111 is required, which can be connected to the pressure intensifier pressure chamber via accumulator shut-off valves 146, 148, in order to tension the pressure intensifier pressure chamber in the support direction of the cylinder 132 when connected to the low-pressure accumulator 110. A control valve arrangement is assigned to the pressure intensifier 152 and to the cylinder 132, which differs from the control valve arrangement of Figure 1, which is disassembled into two proportional valves 140, 142 that can be operated independently of each other, in that further proportional valves 146, 148 are arranged at the outlet of the pressure intensifier 152. For example, the check valve 143, which may be configured as a logic valve, provides for rapid closure of the piston chamber 132 to the low pressure accumulator 110 upon pressure build-up in the piston chamber.

The closed-loop control of the pressure build-up is realized via a proportional valve 146, which in FIG. 3 is arranged at the outlet of the pressure booster, and in FIG. 4, as a further variant, at the inlet of the pressure booster 152.

Claims (12)

1. A method for operating a hydraulic drive (100) including a hydraulic consumer (130) with a piston (134) positionable within a cylinder (132), comprising:
said cylinder (132) is connected at one connection (A) to a pressure medium accumulator (110) and at the other connection (B) to a pressure medium sink (120) via a proportional valve (140);
The position (x) or velocity of the piston (134)

is closed-loop controlled using a model-based closed-loop control in which the set position (y) of the proportional valve (140) is preset ;
a target progression curve of a setting position (y) of the proportional valve (140) is estimated via a planning of a target pressure in the hydraulic drive (100) and the resulting setting position (y) of the proportional valve (140) in order to achieve a target value for the force on the piston (134) . The method according to claim 1, wherein a reversing section model (200) of the hydraulic drive (100) is used to set and adjust, in particular to control in a closed loop, a target value for the force on the piston (134) using the proportional valve (140). 3. The method according to claim 1 or 2, wherein the planning of the target pressure comprises planning a differential pressure between the two ports (A, B) or measuring the pressure on one side and planning the pressure on the other side. 4. The method according to claim 1 , wherein as the proportional valve (140) a proportional valve having a main stage (140.2) and one or more pilot stages (140.1) is used. 5. The method according to claim 4 , wherein a pilot control is performed when presetting the set position (y) of the proportional valve (140), in particular within the framework of a subordinate closed-loop control. 6. The method according to claim 5 , wherein in the pilot control for the proportional valve (140), a setpoint for the position of the main stage (140.2) of the proportional valve is pilot controlled. 7. The method according to claim 1 , wherein the cylinder (132) is connected at the connection (A) to the pressure medium accumulator (110) via a valve (144), in particular a switching valve. 8. The method according to claim 1, wherein within the framework of a model-based monitoring of the hydraulic drive (100), predefined values of variables are compared with corresponding measured values. The method according to any one of the preceding claims, wherein the hydraulic drive (100) is used for a hydraulic shaft. A computing unit (150) configured to perform the method according to any one of claims 1 to 9 . A computer program which, when executed on a computing unit (150), causes said computing unit (150) to carry out a method according to any one of claims 1 to 9 . A machine-readable storage medium having stored thereon the computer program of claim 11 .

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