Method and system for transferring regenerative electrical power

Abstract

A method and system for transferring electrical power regenerated by an electrically driven first subsystem of a working machine (2) having a power source (8) is disclosed, where a second subsystem of the working machine (2) is controlled to receive at least a portion (120) of the regenerated electrical power when the power source (8) is not fully receiving the regenerated electrical power.

Description

Technical Field

[0001] The present invention relates to a method and a system for transferring electrical power or energy regenerated in a first subsystem. Background Art

[0002] Mobile working machines, such as excavators, stackers or pile drivers, may include components that are electrically driven by an electric motor. In such working machines, the electric motor regeneration energy can be used so that the regenerated electric power is fed back into the power supply. Examples include: regeneration during stopping or deceleration of the slewing drive of an excavator or pile driver; regeneration when lowering the boom or arm of an excavator with a hydraulic system driven by an electric motor; and regeneration when braking when driving downhill in a mobile working machine with an electrically driven travel drive. Summary of the invention

[0003] According to the invention, a method and a system for transferring electrical power (or energy) regenerated in a first subsystem are proposed, having the features of the independent claims. Advantageous embodiments form the subject matter of the dependent claims and the subsequent description.

[0004] In a method for transferring electric power regenerated by an electrically driven first subsystem (e.g. a tool, a (rotating) tool, a slewing drive or a transmission system) of a working machine (e.g. a mobile working machine) having a power supply, when the power supply of the working machine cannot fully receive the regenerated electric power (or electric energy), a second subsystem of the working machine is controlled to receive at least a portion of the regenerated electric power (or electric energy). Thus, overloading of the power supply or charging beyond the limits of the power supply (e.g. a battery) can be avoided, and no additional measures (e.g. a load resistor) are required for dissipating power / energy from the first subsystem.

[0005] In other words, a method for operating a working machine including an electrically driven first subsystem and a second subsystem having a power supply includes: controlling the first subsystem to regenerate electric energy, storing a first portion of the regenerated electric energy in the power supply, and transferring a second portion of the electric energy (at least a portion of the above) to the second subsystem.

[0006] In particular, at least a portion of the regenerative electrical power received by the second subsystem exceeds an actual (current or normal) power requirement of the second subsystem (during at least a portion of the receiving of the regenerative electrical power). The actual (current, normal) power requirement represents the power requirement of the second subsystem in order to drive the components of the second subsystem in accordance with control signals of the components (e.g., based on user input). That is, the actual (current, normal) power requirement is the power required by the components of the second subsystem in order to perform their normal functional operations. Therefore, the received regenerative power is higher than the power required by the components of the second subsystem in order to perform the normal (current) functions of the second subsystem at the time of receiving.

[0007] Optionally, the second subsystem is a hydraulic subsystem comprising an electric motor (or motor) driving a hydraulic pump, wherein the pressure and / or volume flow of the hydraulic fluid output by the hydraulic pump changes. In particular, the pressure and / or volume flow may increase relative to a normal pressure and / or volume flow provided by the hydraulic pump in (normal) operation of the second subsystem, i.e., when the second subsystem does not receive regenerative electrical power in excess of the actual power requirement of the second subsystem. That is, the received electrical power is converted into hydraulic power. The change in pressure and / or volume flow over time may comprise an increase in pressure and / or volume flow starting from its normal value (to an increased / higher value) followed by a decrease in pressure and / or volume flow back to its normal value.

[0008] Optionally, the second subsystem comprises a deceleration valve and / or an unloading valve, in particular connected to the hydraulic supply line and the hydraulic fluid tank; wherein the deceleration valve and / or the unloading valve is controlled to open so that throttling losses occur, in particular so that the throttling losses correspond to the power received during at least a portion of the reception of the regenerated electric power that exceeds the actual power requirement of the second subsystem. In other words, the electric power is converted into hydraulic power (by the electric motor and the hydraulic pump of the second subsystem), and the hydraulic power that exceeds the actual power requirement of the second subsystem (for example, currently required to drive the tool) is then converted into heat using the deceleration valve (for example, an adjustable throttle valve). In the case of the unloading valve, the excess volume flow can be directed to the tank.

[0009] Optionally, in response to detecting at least one predefined first operation of the working machine (i.e. an operating state or a specific working process), the deceleration valve and / or the unloading valve is opened before at least a portion of the regenerative electrical power is received by the second subsystem. This is helpful in the case where the regenerative power of the first subsystem is higher than the regenerative power that the (hydraulic) second subsystem can accept, in particular to avoid undesired actuation of hydraulic components of the second subsystem.

[0010] Optionally, the second subsystem comprises a hydraulic component provided with a hydraulic fluid; wherein, when controlling the hydraulic component, changes in the pressure and / or volume flow of the hydraulic fluid are taken into account, in particular so that the forces and / or torques and / or movements caused by the hydraulic component remain unchanged (relative to the forces and / or torques and / or movements caused if the pressure and / or volume flow were unchanged). Thus, undesired actuation of the hydraulic component can be avoided.

[0011] A system for transferring electric power regenerated by an electric drive first subsystem of a working machine having a power supply includes: a second subsystem of the working machine, which is configured to receive at least a portion of the regenerated electric power; and a controller, which is configured to perform a method for transferring electric power regenerated by a first electric drive subsystem of a working machine according to an embodiment of the present invention.

[0012] Further advantages and embodiments of the invention will become apparent from the description and drawings.

[0013] The invention is schematically illustrated in the drawings based on exemplary embodiments and will be described below with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A system for transferring regenerative electrical power according to an exemplary embodiment is shown.

[0015] Figure 2 Another system for transferring regenerative electrical power according to an exemplary embodiment is shown.

[0016] Figure 3 Yet another system for transferring regenerative electrical power according to an exemplary embodiment is shown.

[0017] Figure 4 A flow chart illustrating the steps of a method for diverting regenerative electrical power according to an exemplary embodiment is shown. DETAILED DESCRIPTION

[0018] Figure 1 A system for transferring regenerative electrical power according to an exemplary embodiment is shown. Figure 1 as well as Figure 2 and Figure 3 In FIG, possible flow paths of the regenerative power are indicated by arrows. Figures 1 to 3 Although not shown in the figure, the system may include at least one controller configured to execute or implement the method according to the present invention by controlling the system or at least part of the system (particularly the second subsystem).

[0019] The system includes an exemplary first subsystem that is powered (driven) by electricity. The first subsystem includes a motor 2, an inverter 4 (first inverter) and a slewing transmission 6. The inverter 4 is electrically connected to a power supply system including a power supply 8 to provide power to the motor 4. In the example shown, the power supply 8 is a battery. Alternatively or additionally, the power supply may also be a power supply network (e.g., a power grid) or include a power supply network (e.g., a power grid).

[0020] In the illustrated embodiment, the first subsystem is an electric swing drive of a working machine (e.g., an excavator) in which the cab of the excavator rotates relative to the chassis of the excavator. The rotation of the cab relative to the chassis can be slowed down by converting kinetic energy into electrical energy using the electric motor 2, which is supplied to the power supply system 8 via the inverter 4. That is, during certain operations of the working machine (in the illustrated example, the deceleration of the swing drive), power / energy can be regenerated (recovered) by the first subsystem and transmitted to the power supply system.

[0021] It will be appreciated that in addition to driving the swing transmission, the motor 2 can also drive other types of components. For example, the motor can drive a hydraulic machine that feeds a hydraulic system, such as a travel drive of a working machine (with a hydraulic motor) or a hydraulically actuated (by a hydraulic cylinder) tool of a working machine. In these examples, energy can be regenerated if the working machine is braking or traveling downhill, or if the tool or its components (such as the boom of an excavator) are lowered.

[0022] also, Figure 1 The system includes an exemplary second subsystem, which includes an electric motor 12 (or motor), an inverter 14 (second inverter), a hydraulic pump 16 and an adjustable deceleration valve 18 (adjustable throttle valve). The inverter 14 is electrically connected to the power system and the electric motor 12, thereby providing power to the electric motor 12. The electric motor 12 is arranged to drive the hydraulic pump 16 (e.g., coupled to a drive shaft of the pump. For example, the hydraulic pump 16 is a variable displacement pump. The hydraulic pump 16 is arranged to supply pressurized hydraulic fluid to a hydraulic component 22 of the second subsystem (not explicitly shown; for example, a hydraulic motor and / or a hydraulic cylinder) via a hydraulic supply line 20 connected to the output of the pump. The input of the pump can be connected to a tank.

[0023] An adjustable deceleration valve 18 is connected to the supply line 20 and the tank, and the second subsystem is also configured to control an adjustable flow of hydraulic fluid from the supply line 20 to the tank, wherein in a closed state of the deceleration valve 18 no flow to the tank occurs, and the flow can be adjusted by gradually opening the deceleration valve 18. For example, by adjusting the cross-sectional area of ​​the orifice of the deceleration valve 18.

[0024] Furthermore, the system may optionally include other (auxiliary) components 24, which are electrically connected to the power supply system in order to be provided with power. For example, the other components 24 may include one or more of a fan, a coolant pump, etc. Typically, the (average or peak) power requirements of these other components 24 are much lower (e.g., lower than 10% or 5%) than the (peak) power requirements of the first and second subsystems and / or the peak power regenerated by the first subsystem.

[0025] It may happen that a relatively high amount of power (in watts and / or energy) is regenerated by the first subsystem. For example, the slewing drive of an excavator may regenerate 50kW in 2 to 3s (seconds) under certain circumstances, or the slewing drive of a pile driver may regenerate up to 150kW in 0.1 to 0.2 seconds under certain circumstances. In these examples, the regenerated power may exceed the power consumed by the second subsystem in normal operation and optionally by other components at the same time. Depending on the type and / or state of the power supply, the power supply may not be able to fully receive the regenerated electric power, or it may not be able to receive the regenerated electric power that exceeds the power required by the second subsystem and / or other (electrical) components while regenerating the power. For simplicity and ease of description, the first way of elaboration ("cannot fully receive the regenerated electric power") is intended to also include the latter situation ("cannot receive the regenerated electric power that exceeds the power required by the second subsystem and / or other (electrical) components while regenerating the power").

[0026] In the case when the power supply cannot fully receive the regenerative electrical power, the second subsystem can be controlled to change (e.g. increase) the pressure and / or volume flow of the hydraulic fluid provided by the hydraulic pump. For example, the rotational speed of the electric motor 12 can be changed and / or the swash plate angle of the variable displacement pump 14 can be adjusted in order to change the pressure and / or volume flow. This can be seen as changing (in particular increasing) the hydraulic power in the (hydraulic) second subsystem, wherein the received power exceeds the actual power requirement of the hydraulic drive components of the second subsystem. The deceleration valve 18 can be adjusted to open (gradually) so that throttling of the hydraulic fluid through the deceleration valve 18 occurs. As a result, throttling losses occur, i.e. excess hydraulic power may be converted into heat (throttling losses).

[0027] Depending on the configuration of the (hydraulic) second subsystem, measures may be taken in order to avoid undesired actuation or disturbance of the hydraulic component 22 supplied by the pump 16. For example, the undesired actuation or disturbance may be a change relative to an expected value of the force or torque provided by the hydraulic component (hydraulic cylinder or motor) and / or an expected value of the linear or rotational speed of the hydraulic component. The expected value may be provided by a control unit of the second subsystem, for example based on a user input.

[0028] In some hydraulic systems, pressure and volume flow are provided to the hydraulic components independently of the pressure of the hydraulic pump by using a pressure compensator. In such a system, an increase in pressure in the supply line 22 generally does not result in an undesired actuation or disturbance of the hydraulic component 22. For example, in a load sensing system, the deceleration valve 18 can be opened and a higher pump pressure can be specified on the load sensing line. In turn, the displacement of the variable displacement pump 16 will increase, so that the pump's motor 14 has to provide more mechanical power and consume more electrical power. That is, the deceleration valve 18 acts as an additional hydraulic load in the load sensing system (i.e., in addition to the hydraulic component 22).

[0029] Alternatively or additionally, the increase in volume flow can be transferred, for example, to a tank using a deceleration valve 18 as shown and / or an unloading valve present in the hydraulic second subsystem. Furthermore, the control of the hydraulic component 22 can optionally be adjusted to take into account the increased pressure of the hydraulic fluid. For example, a proportional valve controlling the flow of hydraulic fluid to the hydraulic cylinder can be controlled to open less than with a constant pressure.

[0030] Another (alternative or additional) optional measure is to provide a bypass valve between the supply line 20 and the load sensing line of the load sensing system (not shown), in particular when the increase in pump displacement is not sufficient to absorb the regenerative power received in the (hydraulic) second subsystem. The pressure drop across the bypass valve can be used to increase the pump pressure and avoid opening the pressure relief valve of the load sensing line.

[0031] Similar measures may be implemented in an electronic load sensing control to increase the pressure of the hydraulic fluid provided by the hydraulic pump (pump pressure).

[0032] In other simpler hydraulic systems (e.g. without load sensing), an increase in the pump pressure may lead to undesired actuation or disturbance of the hydraulic component 22. In this case, the pressure of the hydraulic fluid should be increased only relatively slowly or avoided as much as possible, and the hydraulic power in the second subsystem should be increased mainly by an increase in the volume flow.

[0033] As described above, the increase in volume flow can be transferred to the tank using the reduction valve 18 as shown and / or an unloading valve present in the hydraulic second subsystem. In addition, the control of the hydraulic component 22 can be optionally adjusted to take into account the increased pressure of the hydraulic fluid. For example, a proportional valve that controls the flow of hydraulic fluid to the hydraulic cylinder can be controlled to open less than the constant pressure.

[0034] In such systems (i.e. without load sensing), a so-called "E-valve" can be used to bring the pump pressure into line with the target value. The additional power is then compensated by adjusting the pump displacement.

[0035] Another additional or alternative measure (particularly for a hydraulic second subsystem without load sensing) is to control the deceleration valve to start opening in advance before receiving regenerative electrical power through the second subsystem. The corresponding time point for starting such control can be determined based on the detection of a certain operation (operating state) of the working machine and / or based on the detection of a certain control signal (e.g. user input). For example, in the case of a slewing drive, when it is determined that the element driven by the slewing drive is rotating, the deceleration valve can be controlled to open so that once the rotation stops or slows down, it is possible to regenerate power. In this case, the opening of the deceleration valve can be started in response to the detection of a control signal to stop or slow down the rotation (due to the delay, this may result in the deceleration valve being opened before the time when regenerative electrical power is received at the second subsystem). The opening of the deceleration valve for pre-opening can also be determined based on the detected operation of the working machine (or its properties). For example, based on the rotation speed of the slewing drive, or based on the travel speed in the case of a hydraulic travel drive. In any case, the state of the power supply, such as the load state of the battery, can be taken into account.

[0036] Due to delays in the hydraulic second subsystem, for example when adjusting the displacement of the hydraulic pump, it may not be possible to receive short (e.g. 0.1 to 0.2 seconds) and high peak (e.g. 150 kW) regenerative power by converting the received electrical power into hydraulic power. To deal with this situation, alternative or additional measures can be used, which will be referred to below. Figure 2 and 3 Give a description.

[0037] exist Figure 2 and Figure 3 In the embodiment, the structure and function of the first subsystem (motor 2, inverter 4, rotary transmission device 6), the power supply system (power supply 8) and other components 24 (if any) are the same as those of the first subsystem (motor 2, inverter 4, rotary transmission device 6), the power supply system (power supply 8) and other components 24 (if any) Figure 1 Therefore, the description thereof will not be repeated. The only difference is that the slewing transmission device 6 is included in the slewing drive (vibrator) of, for example, a pile driver, and is different from Figure 1 Compared to the slewing drive of the excavator shown in , which is represented by the three drawn electric motors, which drive the slewing gear 6 , this can result in higher power requirements and / or regenerative power peaks.

[0038] Figure 2 Another system for transferring regenerative electrical power according to an exemplary embodiment is shown. Figure 2, the second subsystem includes an electric motor 12 (or motor), an inverter 14 (a second inverter), a hydraulic pump 16, and a flywheel mass 26 (e.g., a flywheel) or a rotating mass. The inverter 14 is electrically connected to the power supply system and the electric motor 12 so that the electric motor 12 is provided with power. The electric motor 12 is arranged to drive the hydraulic pump 16, for example, the output shaft of the electric motor is coupled to the drive shaft of the pump. The hydraulic pump 16 is, for example, a variable displacement pump. The hydraulic pump 16 is arranged to supply pressurized hydraulic fluid to a hydraulic component 22 (not explicitly shown; for example, a hydraulic motor and / or a hydraulic cylinder) of the second subsystem via a hydraulic supply line 20 connected to the output of the pump. The input of the pump can be connected to a tank.

[0039] The flywheel mass 26 is coupled to the drive shaft of the hydraulic pump 16 and / or the output shaft of the electric motor 12. When the power source is unable to fully receive the regenerative electrical power and the regenerative power is transferred to the second subsystem, the flywheel mass 26 can be accelerated (by means of the additional torque provided by the electric motor 12 when receiving the additional power) so that the kinetic energy of the flywheel mass 26 increases and can later (after receiving the regenerative electrical power in the second subsystem) be used to (partially) drive the pump 16 and decelerate the flywheel mass accordingly. In the above example (150 kW in 0.1-0.2 seconds), a suitable moment of inertia of the flywheel mass 26 can be, for example, 2 kg·m / s 2 Furthermore, a deceleration valve and / or an unloading valve can be used to suppress an increase in the volume flow to the tank (see above), since this increase in the volume flow can only be partially compensated by adjusting the displacement of the pump 16 .

[0040] Figure 3 Another system for transferring regenerative electrical power according to an exemplary embodiment is shown. Figure 3 In the embodiment of the present invention, the second subsystem includes a capacitor 28 for buffering electric power. For example, the capacitor 28 is a supercapacitor. The terminals of the capacitor 28 are electrically connected to the power supply system. In the connection of the power supply 8 with the conductive line of the power supply system, a switch 30 is arranged in parallel with a diode so that when the switch 30 is disconnected, the flow of current to the power supply is blocked (corresponding to the flow of current when the power supply is charged), and when the switch 30 is closed, the flow of current to the power supply can be allowed. When the power supply 8 cannot fully receive the regenerated electric power, the switch 30 can be disconnected so that the regenerated electric power is transferred to the capacitor 28, and the electric energy is temporarily stored in the capacitor 30. The capacity of the capacitor 28 (e.g., a supercapacitor) can be, for example, 30mF. During loading (the switch 30 is disconnected), its voltage can, for example, increase from 700V to 1000V.

[0041] It should be understood that Figure 1 , 2 The embodiments shown in and 3 can be combined. For example, in Figure 1 In the system, such as Figure 2The flywheel mass shown in can be additionally provided in the second subsystem. In this example, the second subsystem can be controlled to firstly use the received power to accelerate the flywheel mass as much as possible (e.g., as little as possible without interfering with the function of the components supplying hydraulic fluid in the second subsystem or as much as technically feasible), and secondly to open the deceleration valve (or any other valve adopted in a similar manner) to allow a throttled flow of hydraulic fluid to the tank, so that throttling losses occur. In another example, Figure 1 or Figure 2 The system may also be provided with a capacitor which may be connected to the power supply in a switchable manner, such as Figure 3 In this example, received regenerative power having a high but short peak value (in watts) may be transferred to the capacitor, while received regenerative power having a lower peak value but a relatively higher amount of energy (in watt-hours) may be transferred to the retarder valve or flywheel mass. In addition, Figure 1 , 2 All three embodiments of 3 and 3 can also be combined.

[0042] Figure 4 A flow chart illustrating the steps of a method for transferring regenerative electric power according to an exemplary embodiment is shown. Figures 1 to 3 A description of the steps of the present invention includes a detailed description of at least part of the following steps, which description will not be repeated.

[0043] In step 100, electric power is regenerated (recovered) by the first subsystem. The regenerated electric power may be detected and / or calculated (e.g., using measurements and / or models). Step 100 is not part of the method for transferring regenerated electric power, but is shown to fully show the process of regenerating and transferring regenerated power. If step 100 is included in the method, a regeneration method (recovery method) for a system as previously described is obtained.

[0044] In optional step 110, it is determined whether the power source cannot fully receive the regenerated electric power. This determination can be based on one or more of the detection of at least one predefined operation (i.e., operating state or specific working process) of the working machine, the charge level of the battery of the power source, and the amount of regenerated electric power.

[0045] In step 120 (when the power source cannot fully receive the regenerative electric power), the second subsystem of the working machine is controlled to receive at least a portion of the regenerative electric power. A specific example of this process is described in the above reference. Figures 1 to 3 Described.

[0046] In case the second subsystem comprises a hydraulic component, in optional step 130 the hydraulic component is controlled such that an increase in pressure and / or volume flow is taken into account during receipt of regenerative power in the second subsystem.

[0047] As mentioned above, step 120 and / or step 130 may also include activating power dissipators other than hydraulics, such as using capacitors (especially supercapacitors) to store short power pulses, or using rotating masses to receive regenerative power peaks by accelerating electric motors.

Claims

1. A method for transferring electric power regenerated by an electrically driven first subsystem of a working machine (2) having a power supply (8), in, When the power source (8) cannot fully receive the regenerative electric power, the second subsystem of the working machine is controlled to receive at least a portion of the regenerative electric power (120).

2. The method according to claim 1, wherein: At least a portion of the regenerative electrical power received by the second subsystem exceeds an actual power requirement of the second subsystem.

3. A method according to any one of the preceding claims, wherein: The second subsystem is a hydraulic subsystem, which includes an electric motor (12) driving a hydraulic pump (16), wherein the pressure and / or volume flow of the hydraulic fluid output by the hydraulic pump (16) is changed.

4. The method according to claim 3, wherein: The second subsystem comprises a deceleration valve (18) and / or an unloading valve, in particular connected to a hydraulic supply line (20) and a hydraulic fluid tank; wherein the deceleration valve (18) and / or the unloading valve is controlled to open so that throttling losses occur, in particular so that the throttling losses correspond to a power received during at least a portion of the receipt of the regenerative electrical power that exceeds an actual power requirement of the second subsystem.

5. The method according to claim 3 or 4, wherein: In response to detecting at least one predefined first operation of the working machine, a deceleration valve (18) and / or an unloading valve is opened before at least a portion of the regenerated electric power is received by the second subsystem.

6. The method according to any one of claims 3 to 5, wherein: The second subsystem comprises a hydraulic component (22) supplied with hydraulic fluid; wherein an increase (130) in pressure and / or volume flow of the hydraulic fluid is taken into account in the control of the hydraulic component, in particular so that the forces and / or torques and / or movements caused by the hydraulic component (22) remain unchanged.

7. The method according to any one of claims 3 to 6, wherein: The flywheel mass (26) is coupled to the output shaft of the electric motor (12) and / or the drive shaft of the hydraulic pump; wherein the flywheel mass (26) is accelerated during at least a portion of the receipt of the regenerative electrical power, in particular so that power received during at least a portion of the receipt of the regenerative electrical power that exceeds the actual power requirement of the second subsystem is transmitted to the flywheel mass (26).

8. A method according to any one of the preceding claims, wherein: The power source (8) is unable to fully receive the regenerative electric power (110) based on one or more of detection of at least one predefined second operation of the working machine, a charge level of a battery of the power source, and an amount of regenerative electric power.

9. A method according to any one of the preceding claims, wherein: The second subsystem comprises a capacitor (28), in particular a supercapacitor; wherein the capacitor is charged during at least a portion of the reception of the regenerative electrical power.

10. A method according to any one of the preceding claims, wherein: At least a portion of the regenerated electric power is used in the second subsystem as at least one of hydraulic power, electric power charged in the supercapacitor, and kinetic power generated by accelerating the rotating mass.

11. A system for transferring electric power regenerated by an electric drive first subsystem of a working machine having a power supply (8), comprising a second subsystem of the working machine configured to receive at least a portion of the regenerated electrical power; and A controller configured to perform the method of any preceding claim.

12. The system according to claim 11, wherein: The second subsystem includes an electric motor (12) and a hydraulic pump (16), wherein the electric motor drives the hydraulic pump and the hydraulic pump is configured to supply pressurized hydraulic fluid to a hydraulic supply line (20) of the second subsystem; wherein the second subsystem also includes a deceleration valve (18) and / or an unloading valve connected between the supply line and the hydraulic fluid tank.

13. The system according to claim 11 or 12, wherein: The second subsystem comprises an electric motor (12) and a hydraulic pump (16), wherein the electric motor drives the hydraulic pump and the hydraulic pump is configured to supply pressurized hydraulic fluid to a hydraulic supply line (20) of the second subsystem; wherein the second subsystem further comprises a flywheel mass (26) coupled to an output shaft of the electric motor and / or coupled to a drive shaft of the hydraulic pump.

14. The system according to claim 12 or 13, wherein: The second subsystem includes a hydraulic component (22) connected to the hydraulic supply line (20).

15. A system according to any one of claims 11 to 14, wherein: The second subsystem comprises a capacitor (28), in particular a supercapacitor.

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