DE102023203371A1 - Method for reducing power losses in an electrohydraulic system - Google Patents

Abstract

The present invention relates to a method for specifying a target engine speed for reducing power losses in an electro-hydraulic system of a mobile work machine, which has an electric drive (3) with at least one electric drive component and at least one hydraulic machine (8, 10) which is coupled to the electric drive and which sets a speed thereby, the method comprising the following steps: a. determining a target speed for the electric drive component on the basis of a volume flow value; b. setting the electric drive on the basis of the optimal engine speed determined in step a., characterized in that a pre-stored function which determines the optimal speed on the basis of the volume flow value is used to determine the target speed described in step a.

Description

TECHNICAL AREA

The present invention relates to a method for reducing power losses in an electrohydraulic system as well as a computing unit and a computer program for carrying out the method and an electrohydraulic system.

STATE OF THE ART

Hydraulic (mobile) work machines are becoming increasingly electrified, mainly due to increasingly strict emission standards.

Hydraulic working cylinders, which exert a very high force, cannot be directly replaced by electric linear motors and will remain in place. They are supplied with hydraulic pumps as with conventional work machines.

In conventional hydraulic drive systems, these pumps are driven by a diesel engine, typically at a constant speed. Changes in volume flow are implemented by variable displacement pumps (e.g. axial piston pumps). By adjusting the swivel angle, the stroke volume of the pistons and thus the volume flow are changed at a given speed.

In particular, the volume flow delivered by the hydraulic pump can be calculated using the following formula: $$Q=n_{Pmp}\ast VG\ast \alpha$$

Where Q is the required volume flow, n _Pmp is the pump speed, Vg is the maximum displacement of the hydraulic pump per revolution and α is the swivel angle of the hydraulic pump.

This formula also shows that speed and swivel angle equally determine the volume flow, i.e. the volume flow changes by changing the swivel angle or the speed or a combination of both.

As part of electrification, diesel engines are now being replaced by electric motors. The energy source (e.g. grid connection, battery or fuel cell) is not important here, as long as the required electrical power can be provided.

Since electric motors with their torque characteristics are much more dynamic than diesel engines, volume flow changes can now be implemented by changing the speed and/or swivel angle. A hydraulic load requirement (oil volume flow Q at a given pressure Δp) can therefore be generated with different combinations of pump speed (n _pmp ) and swivel angle (α). Each of these combinations has a different effect on the component operating points and thus also on their efficiency.

In an electrified pump system, this results in a degree of freedom in the implementation of the hydraulic operating point, which can be specifically exploited to increase energy efficiency.

This can and should be used specifically to optimize the overall system energy efficiency in order to find the best efficiency compromise for all components involved for each load requirement.

1. 1. mit einem großen Schwenkwinkel und einer niedrigen Drehzahl (hohes Drehmoment) oder
2. 2. mit einem kleineren Schwenkwinkel und einer im Vergleich zu 1. höheren Drehzahl (niedrigeres Drehmoment)

umgesetzt werden.The same flow rate requirement at a given pressure could, for example,

1. 1. with a large swivel angle and a low speed (high torque) or
2. 2. with a smaller swivel angle and a higher speed (lower torque) compared to 1.

be implemented.

In principle, there can be an “efficiency conflict” between the electric drive and the hydraulic pump: case 1 is positive for the efficiency of the pump and negative for the efficiency of the electric drive, whereas case 2 is negative for the efficiency of the pump and positive for the efficiency of the electric drive.

The aim here is to find the best compromise, i.e. the speed-swivel angle pairing that leads to the best overall efficiency.

For this reason, the patent application DE 102022213991.7 the applicant has developed a solution to determine an optimal speed for the electric motor. In particular, this patent application develops a control system and a method for controlling a pump that is operated with an electric motor.

The pressure and a target volume flow are given. An operating strategy uses this to determine both the target speed of the electric motor and, in a second step, the swivel angle for the pump. A speed control regulates the pump speed using the motor, and a pump control determines the speed from the target and the current speed. the swivel angle and regulates it. The leakage of the pump is taken into account and compensated.

An operating strategy continuously determines, for example, with the help of all component loss maps, an optimal speed for the entire system as a setpoint, which lies within a speed interval from a minimum speed to the maximum speed of the pump, or the maximum speed of the component that has the lowest maximum speed.

The optimum is calculated by considering the total losses in order to ensure maximum efficiency at system level for each load specification.

Optimization routines for selecting the most efficient speed for dynamically changing load requirements require a model-based approach in which the most favorable speed - swivel angle pairing - at the overall system level is determined online (in each calculation time step) and set as the target value.

Such approaches require a large amount of component data (typically multidimensional maps) and a high computational effort.

In contrast, the invention is based on the object of developing a method in which the selection of the most efficient speed can be determined with very little computational effort but at the same time with good accuracy.

SUMMARY

• a. Ermitteln einer Soll-Drehzahl für die elektrische Antriebskomponente auf Basis eines Volumenstromwertes;
• b. Einstellen des elektrischen Antriebes auf Basis der im Schritt a. ermittelten optimalen Motordrehzahl

dadurch gekennzeichnet, dass
für die Ermittlung von der im Schritt a. beschriebenen Soll-Drehzahl eine vorgespeicherte Funktion benutzt wird, die auf Basis des Volumenstromwertes die optimale Drehzahl ermittelt.According to one embodiment of the present invention, a method is claimed for specifying a target engine speed for reducing power losses in an electrohydraulic system of a mobile work machine, which has an electric drive (3) with at least one electric drive component and at least one hydraulic machine (8, 10) which is coupled to the electric drive and is set to a speed by it (or can be driven by it), the method comprising the following steps:

• a. Determining a target speed for the electric drive component based on a volume flow value;
• b. Adjusting the electric drive based on the optimal engine speed determined in step a.

characterized in that
To determine the target speed described in step a., a pre-stored function is used which determines the optimum speed based on the volume flow value.

The wording "adjust" can mean both "regulate" and "control". The wording "pre-stored function" means a function that can be stored in a control unit (i.e. was stored in the past), for example. How this function was determined is not specified here in this embodiment, since this function can be determined by various methods (as will become clearer as the description progresses).

SHORT DESCRIPTION OF THE CHARACTERS

• 1 zeigt beispielhaft eine Arbeitsmaschine, nämlich einen mobilen Bagger gemäß dem Stand der Technik.
• 2 zeigt ein beispielhaftes elektrohydraulisches System mit zwei Kreisen, wie es etwa im Bagger der 1 verwendet werden kann, gemäß dem Stand der Technik.
• 3 zeigt eine erste Funktion für die Ermittlung einer Soll-Drehzahl für die elektrische Maschine gemäß einer Ausführungsform der vorliegenden Erfindung;
• 4 zeigt eine zweite Funktion für die Ermittlung einer Soll-Drehzahl für die elektrische Maschine gemäß einer weiteren Ausführungsform der vorliegenden Erfindung.

The present invention is described with reference to the accompanying figures, wherein like reference numerals refer to like parts and/or to similar parts and/or to corresponding parts of the system. Regarding the figures:

• 1 shows an example of a work machine, namely a mobile excavator according to the state of the art.
• 2 shows an exemplary electro-hydraulic system with two circuits, as used in the excavator of 1 can be used according to the state of the art.
• 3 shows a first function for determining a target speed for the electric machine according to an embodiment of the present invention;
• 4 shows a second function for determining a target speed for the electric machine according to another embodiment of the present invention.

DETAILED DESCRIPTION

In the following, the present invention will be described with reference to specific embodiments as shown in the accompanying figures. Nevertheless, the present invention is not limited to the particular embodiments described in the following detailed description and shown in the figures, but the described embodiments merely illustrate some aspects of the present invention, the scope of which is defined by the claims.

Further changes and variations of the present invention will be apparent to those skilled in the art. The present description thus includes all changes and/or variations of the present invention. application, the scope of protection of which is defined by the claims.

The invention can be applied to various non-mobile and mobile work machines, such as excavators, wheel loaders, telehandlers or reach stackers.

1 shows an example of a work machine, namely a mobile excavator 30. The excavator 30 comprises a chassis 32 and a superstructure 34 rotatably mounted thereon. Wheels 36 are mounted on the chassis. A rotation of the superstructure relative to the chassis is made possible by a slewing gear 38. This rotation can be driven, for example, by an electric drive or electric slewing gear drive. A boom or excavator arm 40 is attached to the superstructure 34, at the end of which there is a bucket 42. The boom, arm and bucket are moved or driven here, for example, by means of an electro-hydraulic system 44, i.e. hydraulics driven by an electric drive, by means of hydraulic cylinders 46. A battery 48 (but could also be any other electrical energy source such as a fuel cell system or a cable) supplies the electric drive of the slewing gear and the electro-hydraulic system with electrical energy.

2 shows an example hydraulic system or electro-hydraulic system with two circuits, as used in the excavator of the 1 The two circuits can either be operated separately or connected together via a summing valve (ie a summing device).

An electric battery 2 is shown, through which an electric drive 3 of the electrohydraulic system for driving hydraulic pumps and an electric slewing gear drive for driving a slewing gear (not shown) are supplied with electrical energy. The battery 2 can be considered part of the electrohydraulic system. Alternatively, an external battery could also be used. Instead of a battery, another power supply could also be provided. The electric drive 3 of the hydraulic system has an inverter 4 (e.g. DC/AC converter) and an electric machine 6 as electrical drive components. The electric slewing gear drive has an inverter 5 (e.g. DC/AC converter) and an electric slewing gear machine 7 as electrical components.

The electric machine 6 is coupled via a shaft to a first hydraulic pump 8 (or hydraulic machine) and a second hydraulic pump 10. A first output channel 14 of the first hydraulic pump 8 is connected to a first hydraulic circuit in order to supply it with pressure medium (hydraulic fluid, typically a hydraulic oil). A second output channel 16 of the second hydraulic pump 10 is connected to a second hydraulic circuit in order to supply it with pressure medium. For example, further hydraulic pumps 12 or auxiliary pumps are driven via the shaft, which serve, for example, to provide a control pressure for actuating hydraulically or electrohydraulically actuated valves.

By way of example, a summing valve 18 is provided with which the two circuits can be hydraulically connected (for example, in order to meet a high power requirement in one of the circuits).

Two hydraulic cylinders 20 (which e.g. control the movement of the boom of the excavator of the 1 The flow to and from the hydraulic cylinders 20 is controlled by means of a first valve arrangement 24 which is hydraulically connected to the first output channel 14, to the summing valve 18 and to a tank for pressure medium.

The second circuit supplies two hydraulic cylinders 22, 23 (which, for example, control the movement of a middle arm element and the bucket of the excavator of the 1 The flow to and from the hydraulic cylinders 22, 23 is controlled by means of second valve arrangements 26, 27 which are hydraulically connected to the second output channel 14, to the summing valve 18 and to the tank for pressure medium.

The valve arrangements comprise adjustable or adjustable passages, shown as adjustable metering orifices, so that the flow of pressure medium to and from the hydraulic consumers can be controlled according to their adjustment. The metering orifices (passages) can be formed, for example, in valves and in particular directional control valves, whereby different switching positions, such as different positions of a slide or piston of a directional control valve, correspond to different sized cross sections of the metering orifices, i.e. the metering orifices or their cross sections can be adjusted by adjusting the (directional control) valves. The exact structure of the valve arrangements, which depends on the specific application, e.g. the machine in which the electro-hydraulic system is used, does not need to be explained here and is known to the person skilled in the art. Ultimately, settings of the valve arrangements correspond to certain hydraulic performance requirements, e.g. a required pressure and/or volume flow, which are to be met by the hydraulic pumps. pens 8, 10 or the electric drive (inverter 4, electric machine 6).

With reference to the 3 and 4 only one example of a method according to an embodiment of the present invention is explained.

The inventor came up with the idea of developing a function that makes it possible to determine a target speed for the electric machine, so that with little computational effort a target speed is determined that does not differ significantly from an optimal speed (which is calculated, for example, using a model that takes energy losses into account).

In 3 is an example of such a function. As can be seen in the figure, a target speed for the electric machine is determined depending on a volume flow Q, which can be an estimated, measured or target volume flow. In the figure, the dashed line shows a case in which the hydraulic pumps are fully swiveled and therefore the target speed is minimal.

The reason for this is that from the volume flow balance, neglecting leakage losses, the pump speed is calculated as follows: $$n_{Pmp}=\frac{Q}{VG\ast \alpha }$$

This means that the larger the swivel angle, for the same flow rate Q, the lower the speed will be. This simply means, as already mentioned, that for larger swivel angles the pump will work under perfect working conditions and, on the other hand, the electric machine will work under poor working conditions (low speeds, high torque).

The inventor observed a certain linearity in the optimal speeds (which were determined, for example, using a model) and therefore came up with the idea that, in order to save computational effort, the model could be replaced with a function that approximates the optimal speed values.

This function can be determined, for example, from the curve with the minimum speed, taking into account a multiplication factor, which is preferably multiplied directly by the swivel angle α. The multiplication factor can be, for example, equal to 0.9, and depends on various factors such as the type of machine, application, size of the hydraulic pumps and/or the electrical machine. An example is in 3 represented by the continuous line. This function can be used to determine a target speed based on a specific volume flow, which can be used for controlling or regulating (ie setting) the electrical machine.

It is clear to the expert that the representation in 3 is only an example and that this function can be replaced by a number of other functions.

Another function that is even better at determining an optimal target speed can also take pressure into account. The inventor has observed that by taking pressure into account, such as a delta pressure on the hydraulic pump, the function can determine results that are closer to the optimal speeds.

For this reason, the 3 The function shown can be expanded. In the expanded function, the volume flow is multiplied by a first factor and the pressure by a second factor. This results in the following function: $$n_{SOll}=k1\ast Q+\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="DE102023203371A1\_0005.png"\; state="\{\{state\}\}">k</figure-callout>}2\ast P$$

Where k1 and k2 are a first and a second multiplication factor respectively, Q is the volume flow and P is a pressure value (such as the delta pressure at the hydraulic pump).

The first and second factors can either be estimated or determined depending on component properties (e.g. factor for volume flow value of displacement volume). The first and/or the second factor can be dependent on the volume flow value and/or the pressure value.

Just as an example, two values are given for the first and second factors. The first factor k1 can be equal to 28, so that at 100 l/min a target speed of 2800 revolutions per minute is obtained. The second factor k2 can be equal to 3, so that at a pressure value of 300 bar 900 revolutions per minute are added to the target speed. This means that in this example, at a volume flow of 100 l/min and a pressure value of 300 bar, a target speed of 3700 revolutions per minute is determined.

Even if the description always refers to a linear function, it is clear to the expert that this function can be replaced by any continuous function (e.g. a freehand line). Alternatively, the linear function can be replaced by a characteristic curve (which of course also falls under the definition of function). The function can of course also be a single- or multidimensional lookup table depending on volume flow or pressure or both.

Another function that determines an optimal target speed even better can also take an offset into account. The inventor has observed that by taking into account an offset, which can be constant or variable, the function can determine results that are closer to the optimal speeds. If the offset is variable, this offset can depend on pressure or volume flow, for example. In 3 An example of such a function is shown.

It is clear to the person skilled in the art that all factors and offsets (or all equation variables or polynomials) mentioned in this description can be determined using a "gain scheduling" method. Gain scheduling makes it possible to design simple controls for non-linear control systems on the basis of linear system theory.

While the present invention has been described with reference to the embodiments described above, it will be apparent to those skilled in the art that it is possible to make various modifications, variations and improvements to the present invention in light of the above teachings and within the scope of the appended claims without departing from the scope of the invention.

Furthermore, the areas in which those skilled in the art would be knowledgeable have not been described here in order not to unnecessarily obscure the invention described.

Accordingly, the invention should not be limited to the specific illustrative embodiments, but only by the scope of the appended claims.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 1020222139917 [0015]

Claims (12)

Method for specifying a target engine speed for reducing power losses in an electro-hydraulic system of a mobile work machine, which has an electric drive (3) with at least one electric drive component and at least one hydraulic machine (8, 10) which is coupled to the electric drive and which has a speed set by the latter, the method comprising the following steps: a. determining a target speed for the electric drive component on the basis of a volume flow value; b. setting the electric drive on the basis of the optimum engine speed determined in step a., characterized in that a pre-stored function which determines the optimum speed on the basis of the volume flow value is used to determine the target speed described in step a. procedure according to claim 1 , whereby the above function continues to determine the target speed based on a pressure value. procedure according to claim 2 , whereby the pressure value and the volume flow value are each multiplied by a first and a second factor to determine the target speed. procedure according to claim 3 , wherein the first factor and/or the second factor is/are dependent on the volume flow value and/or on a pressure value. Method according to one of the Claims 3 or 4 , where the first factor and/or the second factor is estimated. Method according to one of the Claims 3 or 4 , where the first factor and/or the second factor is/are determined depending on component properties. Method according to one of the Claims 1 until 6 , where the function is a linear function. Method according to one of the Claims 1 until 7 , whereby an offset value is added to the determined target speed. Control unit which is configured to carry out a method according to one of the Claims 1 until 8 to execute. Mobile working machine comprising an electrohydraulic system having an electric drive (3) with at least one electric drive component and at least one hydraulic machine (8, 10) which is coupled to the electric drive and is set to a speed by the latter, wherein the working machine has a control unit according to claim 9 includes. Computer program designed to carry out the method according to one of the Claims 1 until 8 to execute and/or control. Machine-readable storage medium with a computer program stored thereon in accordance with claim 11 .

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