Method for controlling an electric drive of an electrohydraulic system when an overload occurs

Abstract

The invention relates to a method for controlling an electric drive of an electrohydraulic system when an overload occurs, wherein the electric drive (4, 6) drives a hydraulic pump (2) which delivers pressure medium in the electrohydraulic system and is controlled at a target speed (20), and wherein a demand fulfillment speed (22) is given according to a demand of hydraulic consumers of the electrohydraulic system, wherein, when an overload of the electric drive is detected (110), it is determined (120) whether an actual speed (34) of the electric drive has fallen below a speed threshold value (48), and wherein, after the overload of the electric drive has ended (140), a restart is carried out (150) if it has been determined that the actual speed of the electric drive has fallen below the speed threshold value, wherein the restart (150) includesto increase the target speed (20) over a start-up period (42) with a time length greater than zero to the request fulfillment speed (22) or to a speed that is greater than or equal to the request fulfillment speed minus a predetermined first speed difference (52).

Description

The present invention relates to a method for controlling an electric drive of an electrohydraulic system when an overload occurs, and to a computing unit and a computer program for carrying it out.BACKGROUND OF THE INVENTIONWorking machines, in particular mobile working machines, can have an electrohydraulic system with which the movement of machine components is hydraulically effected. One possibility for making the use of such working hydraulics more efficient for electrified and, in particular, battery-operated mobile work machines is the provision of a volume flow according to requirements. For this purpose, in particular, the rotational speed of the electric drive can also be varied, since electric drives typically have high rotational speed dynamics and therefore a rapid adaptation of the volume flow to a variable demand can take place.Disclosure of the InventionAccording to the invention, a method for controlling an electric drive of an electrohydraulic system when an overload occurs, and a computing unit and a computer program for carrying it out are proposed having the features of the independent patent claims. Advantageous embodiments are the subject matter of the dependent claims and of the following description.The invention makes use of the measure of, in an electrohydraulic system in which an electric drive is actuated at a setpoint rotational speed, wherein a demand-fulfilment rotational speed is given in accordance with a demand of hydraulic consumers of the electrohydraulic system, determining, in the event of an overload of the electric drive, whether an actual rotational speed of the electric drive has fallen below a rotational speed threshold value, and, if the actual rotational speed of the electric drive has fallen below the rotational speed threshold value, carrying out a restart after the overload of the electric drive has ended, wherein the restart includes increasing the setpoint rotational speed over a start-up time period with a temporal length greater than zero to the demand-fulfilment rotational speed or to a rotational speed which is greater than or equal to the demand-fulfilment rotational speed minus a predetermined first rotational speed difference.This procedure achieves a controlled and predictable restart of hydraulic consumers (e.g. hydraulic cylinders, hydraulic motor) and of machine components moved by the hydraulic consumers. In particular, uncontrolled or sudden movements of the driven machine components are avoided, which can occur if a rotational speed increase with high dynamics occurs, which would result if the setpoint rotational speed were increased to the demand-satisfying rotational speed immediately after the overload has ceased, i.e. not over the starting time period.The restart (or delayed start) represents a start process or restart process which includes at least one step (rotational speed increase over a start time period). The time length of the start-up time period can be directly predefined or can result indirectly. The latter is the case, for example, when the speed increase with a specific gradient (approximately given in 1 / s 2 or. 1 / min 2) so that the time length is obtained as the quotient of the final rotational speed (of the increase) minus the initial rotational speed (of the increase) by the slope.The restart is triggered in particular at the end of the overload, i.e. the execution of the restart or the restart process begins at the end of the overload.The electric drive comprises an inverter and an electric machine. An overload (or an overload situation or overload situation) of the electric drive can occur in various situations. For example, the required torque, which is caused by the pump pressure, i.e. the pressure of the pressure medium at the pump output, can be higher than a torque which can be applied by the electric drive. The electric drive is therefore not capable of applying the required torque. The torque requested depends in particular on the load moved by the hydraulic consumers. A thermal overload of the electric drive can also occur, wherein elements of the inverter, in particular semiconductor switching elements, and / or elements of the electric machine, in particular windings, become too warm or have an excessively high temperature, for example when a high torque and / or a high power are required over a longer period of time. In order to prevent overheating and possible damage to the electric drive, provision can then be made to reduce the retrievable torque or the retrievable power (so-called "derating"), so that the electric drive cools down.Whether an overload is present or not can be established in particular on the basis of data of the inverter and / or an inverter controller. Whether an overload of the inverter is present can be determined, for example, by comparing the maximum possible torque of the drive / motor (which is a parameterizable value in the control unit of the inverter) and the currently measured torque (determinable via the measurement of the phase currents and present in the inverter controller). The thermal loads (e.g. determinable with temperature sensors) for motor and inverter can also be used or evaluated for this purpose. If these or at least one of them exceed an upper threshold value of e.g. 100% (e.g. an upper temperature limit), the maximum possible torque decreases, wherein this can then be compared with the current engine torque. If necessary, the state of overload can also be determined by regulating the rotational speed if the manipulated variable is at its maximum, but the rotational speed can no longer be increased or it falls even further down to zero.The inverter controller carries out a control and / or regulation of the electric drive or of its inverter, so that the setpoint rotational speed is reached. In this case, a rotational speed and / or a torque and / or a drive current can be regulated or controlled. In the event of overload, the override can be configured in particular to reduce a setpoint value, for example for the rotational speed, in order to prevent damage. In this case, for example in the event of a longer or high overload, a setpoint value of zero or below the rotational speed threshold value can be reached.The consumer requirement, i.e. the requirement of hydraulic consumers of the electrohydraulic system, is given in particular by a volume flow requirement and / or a pressure requirement (to the pump pressure). For example, a volume flow requirement for a given displacement volume of the hydraulic pump causes a minimum rotational speed which the hydraulic pump must at least have in order to meet the volume flow requirement. The rotational speed can likewise be dependent on a required pump pressure, which is caused, for example, by a load pressure. Overall, such requirements result in the preset rotational speed, which is determined, for example, by a control of the hydraulic system or a higher-order working machine control unit. The load demand is typically given by inputs, such as by a joystick, from an operator of the work machine in which the hydraulic system is deployed. The load demand, and thus also the demand fulfilment speed, generally varies with time. An overload situation can accordingly also be ended when the load requirements are reduced.The first rotational speed difference, as well as the second rotational speed difference mentioned further below, can be, for example, less than 0.2 or 0.1 or 0.05 times a nominal rotational speed of the electric drive or times a typical or average rotational speed (during operation of the electrohydraulic system).According to one embodiment, if there is no overload of the electric drive and while no restart is being carried out, the setpoint rotational speed is set to the request-fulfilment rotational speed. This corresponds to normal operation at a rotational speed of the electric drive which is given by the pressure medium requirement corresponding to the requested volume flow and / or pressure.According to one embodiment, data of the inverter is captured and / or received and evaluated in order to determine whether an overload of the electric drive exists, and / or measured pressure values of the electrohydraulic system are used in order to determine whether an overload of the electric drive exists. From pressure measurement values, e.g. a pump pressure or load pressure, it is possible to infer the torque, which is caused by the hydraulic pressure at the hydraulic pump, given a known displacement of the hydraulic pump. This can be compared with a torque which can be applied by the electric drive at a given rotational speed and which is known, for example, from data sheets, in order to determine whether an overload is present. In addition, the control unit of the inverter always supplies the currently maximum possible torque of the electric drive and can compare this with the currently present torque of the electric motor. This can be determined, for example, by knowing the current (and measured) phase currents of a three-phase machine.According to one embodiment, the time length of the starting time period is greater than or equal to a predetermined minimum length. The minimum length can be selected suitably, in particular for a respective working machine.According to one configuration, the increase of the setpoint rotational speed during the restart takes place with a predetermined gradient and / or a predetermined average gradient. The predetermined gradient or average gradient can be selected, for example, (i.e., small enough) such that the associated volume flow increase does not lead to abrupt movements of machine components.According to one embodiment, the start-up time period has a predetermined time length. Here, the increase of the setpoint rotational speed takes place over the predetermined time length from an initial value (e.g. zero) to a final value, wherein in particular the final value is dependent on the current request-satisfying rotational speed, in particular corresponds to this.According to one embodiment, in particular in the absence of the overload, the setpoint rotational speed is determined as the minimum from the requirement-satisfying rotational speed and the sum of the actual rotational speed and a second rotational speed difference. In this case, a controller (e.g. implemented in the inverter controller) which controls or regulates the rotational speed of the electric drive in accordance with the setpoint rotational speed (i.e. adjusts the actual rotational speed to the setpoint rotational speed) has, in particular, an integral component. During the restart, the sum (actual rotational speed plus second rotational speed difference) is selected during the minimum selection, so that ultimately, in particular due to the integral component of the rotational speed control, a rotational speed ramp is generated or a rotational speed increase takes place over a time period, i.e. the starting time period (the length of which is given indirectly here). The slope of the speed increase may be affected by the second speed difference (and may still be load dependent).According to one embodiment, the setpoint rotational speed is set to zero or a value less than the rotational speed threshold value until the overload of the electric drive no longer exists if it is determined that the actual rotational speed of the electric drive has fallen below the rotational speed threshold value. This can prevent the electric drive from being actuated at a high setpoint rotational speed, for example at the requirement-satisfying rotational speed, before the start of the restart in the event of an overload still existing, which can result in high electric currents, for example.According to one embodiment, the demand-fulfilment rotational speed is directly predetermined or is determined from a demand-fulfilment volume flow. For a given demand-fill volume flow, the demand-fill rotational speed can be determined from the known displacement volume of the hydraulic pump, wherein leakage of the hydraulic pump can optionally be taken into account. The displacement as usual denotes the volume of pressure medium delivered by the hydraulic pump per revolution.According to one embodiment, the restarting further includes waiting a predetermined time after the end of the overload until the increase of the target rotational speed is started. In particular, if the overload occurs again during the predetermined time, it is not started up at first, but rather the restart is started again after this renewed overload no longer exists. This may be expedient in order to exclude starting if the overload does not exist for a very short time. At the renewed start of the predelayed starting, in particular the waiting for the predetermined time is again included.According to one embodiment, the rotational speed threshold value corresponds at most to a fraction of a nominal rotational speed of the electric drive or an average rotational speed of the electric drive during operation of the electrohydraulic system. In particular, the fraction is at most 0.2. for example, the fraction may be at most 0.1 or at most 0.05.According to one embodiment, the hydraulic pump is a fixed displacement pump or a variable displacement pump with a hydraulically mechanical delivery flow regulator. The application of the invention to such hydraulic pumps is expedient since in these the torque acting on the hydraulic pump on account of the hydraulic pressure cannot be varied by changing the displacement or a change of the displacement can at least not take place in an independent manner, so that influencing the rotational speed is an effective means.A computing unit according to the invention, e.g. a control device of a mobile work machine, is configured, in particular by programming, to carry out a method according to the invention.The implementation of a 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 is also advantageous since this causes particularly low costs, in particular if an executing control device is also used for further tasks and is therefore present in any case. Suitable data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs, among others. Download of a program via computer networks (Internet, intranet, etc.) is also possible.Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.It is understood that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the present invention.The invention is schematically illustrated in the drawing on the basis of exemplary embodiments and is described in detail below with reference to the drawing.DESCRIPTION OF THE FIGURESFIG. 1 shows an example electro-hydraulic system of a work machine. FIG. 2 shows another example electro-hydraulic system of a work machine. FIG. 3 shows an exemplary temporal profile of an actual rotational speed in the case of a temporary overload when carrying out a method according to the invention. FIG. 4 shows a flow chart according to one embodiment of the method according to the invention. Figure 5 illustrates one possible signal flow for implementing the invention. FIG. 6 shows, by way of example, a part of a controlled system of a speed control with which a speed ramp can be implemented.Detailed Description of the DrawingsFIG. 1 shows an exemplary electrohydraulic system of a working machine in which hydraulic consumers 10, e.g. hydraulic cylinders, are supplied with pressure medium (i.e. hydraulic fluid, typically a hydraulic oil) from a hydraulic pump 2 via a valve arrangement 8 or a valve block. The valve arrangement 8 comprises one or more valves which, based on actuation by a control unit, control volumetric flows of pressure medium to and from the hydraulic consumers 10. The hydraulic pump 2 is driven by an electric drive 4, 6. The electric drive comprises an electric machine 4 which is coupled to the hydraulic pump 2, for example by means of a shaft, and an inverter 6 or converter which provides alternating electric currents for windings of the electric machine 4. The inverter 6 is supplied with electrical (DC) voltage or energy, for example, via an intermediate circuit.The system is shown by way of example as an LS-regulated system (LS: load pressure), in which both the pump pressure is measured by a sensor 14 and the load pressure is measured by a sensor 12 and corresponding measured values are transmitted to a control unit 16. A setpoint rotational speed 20 (or a setpoint rotational speed value) can be predefined by the control unit 16 to the electric drive, i.e. to the inverter 6. A further control unit may be a higher-order working machine control unit 18, which, for example, sends a request-fulfilment rotational speed 22 (or a request-fulfilment rotational speed value) to the control unit 16, which is based, for example, on an input of an operator of the working machine in which the electrohydraulic system is used. The system does not necessarily have to be designed as an LS system or operated under LS regulation. A volume flow specification or also a simple speed specification with or without the possibility of pump pressure limitation is also conceivable. For the supply of systems with a simple speed specification without pump pressure limitations, the sensors 12 and 14 are not required.The hydraulic pump 2 is here a fixed displacement pump, i.e. it has a non-variable displacement, so that a counter-torque is always generated at the pump, which is dependent only on the load or pump pressure. Depending on the design of the system or the dimensioning of the system components, when the maximum pressure of the hydraulic pump 2 is reached (for example by a consumer 10 being driven into an end stop), a counter torque can be generated which can be greater than the drive torque which can be applied by the electric machine 4, in particular if, for example, the phase current has to be limited on the basis of a thermal overload of the inverter 6 and / or of the electric machine 4.Instead of a fixed displacement pump, the hydraulic pump 2 can also be designed as a variable displacement pump (not shown), i.e. have a displacement volume that can be adjusted according to a pivot angle. The variable displacement pump may be equipped with a pivot angle sensor (not shown). The control device 16 can implement the regulation of the pump pressure in the case of LS regulation, but can also implement any other form of regulation, for example pure pressure regulation, volume flow regulation, pivot angle regulation, torque regulation. Similarly, the speed specification, which is specified by the superordinate work machine control device 18, is also sent by the control device 16 to the inverter or the inverter controller. In this type of system with the possibility of adjusting the pivot angle of the hydraulic pump 2, an overload of the electric machine can theoretically be avoided. Nevertheless, in this case too, in the event of unfavourable parameterization of the corresponding controllers or as a result of latencies in the system, situations may occur in which it is not possible to react to the overload rapidly enough and the electric machine also loses rotational speed.FIG. 2 shows another example electro-hydraulic system of a work machine. This corresponds in large parts to that of FIG. 1, so that only differences will be discussed below and reference is otherwise made to the description of FIG. 1.In FIG. 2, the hydraulic pump 2 is a variable displacement pump with a hydraulic-mechanical delivery flow regulator 28, wherein the load pressure and the pump pressure are tapped hydraulically and the hydraulic pump is adjusted by the delivery flow regulator 28, so that a preset pressure difference is maintained between these pressures. In such systems, there is the possibility of an intervention in the load torque only indirectly via a possible increase in the rotational speed in LS control. If this possibility is exhausted, this pump behaves basically like a fixed displacement pump and here too the load or overload situations already described can occur, in which situations the rotational speed can come to a standstill until a reduction in the rotational speed occurs.FIG. 3 shows an exemplary temporal profile of an actual rotational speed 34 in the case of a temporary overload when carrying out a method according to the invention. FIG. 3 shows a diagram formed by a time axis 30 and a rotational speed axis 32. The profile over a plurality of time segments 36, 38, 40, 42, 44 is shown.In addition to the actual rotational speed 34, a request-satisfying rotational speed 46 is drawn in, which is determined or predefined or results by a higher-order controller (e.g. the work machine controller 18 of FIGS. 1 and 2 ) of the hydraulic system, for example based on a joystick input by the operator. From the requirement-fulfilment rotational speed, a desired rotational speed (not shown in FIG. 3 ) is determined with which the electric drive or the inverter is actuated. In particular, the setpoint rotational speed usually corresponds to the requirement-fulfilment rotational speed. For example, the setpoint rotational speed is transmitted to an inverter controller, which implements a control and / or regulation, so that the electric drive (i.e. the electric machine) and thus the hydraulic pump coupled to it reaches or reaches as far as possible the setpoint rotational speed. In the event of an overload, the setpoint rotational speed cannot be reached. The inverter controller generates control signals for switching elements of the inverter, for example.In the period 36, the target rotational speed is determined to be equal to the request satisfaction rotational speed 46. There is no overload, so that the actual rotational speed 34 is equal to the setpoint rotational speed and thus equal to the request-fulfilment rotational speed.In the time segment 38, the situation of an overload described with reference to FIGS. 1 and 2 occurs. Here, the target rotational speed is further determined to be equal to the request satisfying rotational speed 46. Due to the overload, the setpoint rotational speed is not reached by the electric drive, however, so that a collapse or a rapid decrease of the actual rotational speed 34 occurs, which decreases to zero.As a result of the decrease in the rotational speed, the delivery volume flow of the hydraulic pump decreases and thus the movement speed of the hydraulic consumers supplied therewith decreases until a possible standstill occurs. If the demand-satisfying rotational speed continues to exist (for example with the joystick still deflected), a rapid increase in rotational speed (for example with a positive slope which corresponds in terms of amount to the negative slope in the interval 38) can occur on account of the high rotational speed dynamics of electrical drives, and therefore a sudden or jerklike resumption of the movement of the hydraulic consumers or of machine components moved by them can occur as soon as the overload no longer exists. However, this very dynamic introduction of movement can be undesirable, since uncontrolled movements can also occur here, especially if the operator does not consider such a dynamic approach of the consumers.In order to overcome this problem, according to the invention a restart or delayed start-up is provided. If it is determined in the presence of an overload that the actual rotational speed 34 has fallen below a rotational speed threshold value 48, a controlled restart takes place or this is triggered if or after the overload no longer exists. Due to the condition that the actual rotational speed has fallen below the rotational speed threshold value, short-term overloads can be filtered out, so that no restart is triggered if the overload is canceled after a short time and, accordingly, no substantial deviation of the actual rotational speed from the setpoint rotational speed occurs. The rotational speed threshold value 48 can therefore also be higher than outlined in the figure.In FIG. 3, the overload exists substantially during periods 38 and 40. In period 40, actual speed 34 remains substantially at zero (i.e., is less than, and more particularly equal to, speed threshold 48 due to the overload). During the time interval 40, the setpoint speed of the speed regulator can optionally be set to zero or to a value that is less than the speed threshold value. At the end of the time interval 40, i.e. at the end of the overload, the restart begins, which takes place during the time interval 42, which is also referred to as the start-up time interval. In this case, the setpoint rotational speed is increased starting from an initial value which is equal to zero or equal to a value which is less than the rotational speed threshold value (e.g. if the setpoint rotational speed has already been set to such a value during the time segment 40 as described above), over a time period which has a time length which is greater than zero, until a final value is reached which is equal to the request-fulfilment rotational speed or is at least equal to or greater than the request-fulfilment rotational speed minus a predetermined (first) rotational speed difference. This rotational speed, i.e. the request-fulfilment rotational speed minus the predetermined first rotational speed difference 52, is also drawn in FIG. 3 and can be referred to as reduced request-fulfilment rotational speed. Since the overload is no longer present, the actual rotational speed 34 follows the setpoint rotational speed.At the end of the restart, the setpoint rotational speed is set equal to the request-fulfilment rotational speed again, i.e. in the time segment 44, the setpoint rotational speed is equal to the request-fulfilment rotational speed 46 and, since there is no overload, the actual rotational speed 34 is likewise equal to the request-fulfilment rotational speed 46 (possibly apart from transition segments which are caused, for example, by latencies, for example a control delay).Since the setpoint rotational speed is increased over a finite period of time, the rotational speed of the electric drive is likewise increased relatively slowly over the period of time, so that the above-mentioned problem of uncontrolled or jerking behavior of the hydraulic consumers as a result of direct actuation of the electric drive with the requirement-satisfying rotational speed and the relatively rapid increase in rotational speed associated therewith is avoided.The time length of the time period can be directly fixed or indirectly determined. For example, the time length can be predetermined (e.g. in seconds or fractional seconds) and the increase can be linear or substantially linear, i.e. with a constant gradient from the initial value to the final value. A slope or an average slope with which the increase is to take place can also be predefined, so that the time length results indirectly. A further possibility which is implemented within the scope of a controlled system is illustrated in FIG. 6.FIG. 4 shows a flow chart according to one embodiment of the method according to the invention.In step 100, the target speed is determined to be equal to the demand-satisfying speed, and step 100 is performed during periods where no overload is present and no restart is performed (corresponding to normal smooth operation).In step 110, it is determined that there is an overload, e.g. based on inverter signals or corresponding signals of the inverter controller or by the inverter controller.In step 120, it is determined whether the actual speed of the electric drive has fallen below a speed threshold. For this purpose, (at least after the overload has been detected), the actual rotational speed which is transmitted by the electric drive and / or by the inverter controller can be continuously compared (for example on a time grid) with the rotational speed threshold value.In an optional step 130, if it has been determined that the actual rotational speed of the electric drive has fallen below the rotational speed threshold value, the setpoint rotational speed is set to zero or set to a value less than or equal to the rotational speed threshold value.In step 140, it is determined that the overload is no longer present or that its end has been reached.In step 150, restarting is carried out when or after the overload of the electric drive no longer exists. The restart is triggered at the end of the overload or begins at the end of the overload. Restarting includes increasing the desired speed to the demand-satisfying speed or to a speed greater than or equal to the demand-satisfying speed minus a predetermined first speed difference over a starting time period having a time length greater than zero.When the target rotational speed has reached the request-satisfying rotational speed or the rotational speed greater than or equal to the request-satisfying rotational speed minus the predetermined first rotational speed difference, the normal operation is continued, i.e., the procedure jumps to step 100.FIG. 5 shows a possible signal flow for implementing the invention, with which in particular the profile of the actual rotational speed shown in FIG. 3 can be achieved. The signal flow shown represents only a portion of a control and control strategy for the operation of an electrified pump drive.Thus, starting from input variables 60 into a functional module 62, a setpoint rotational speed 20 for the electric machine is generated. For this purpose, the input variables 60 can comprise a direct rotational speed specification, referred to as demand-fulfilment rotational speed, or a volume flow specification, referred to as demand-fulfilment volume flow, from which the setpoint rotational speed is determined, wherein in the case of a demand-fulfilment volume flow the known displacement volume of the hydraulic pump can be taken into account. The determination of the setpoint rotational speed 20 takes place depending on whether an overload is present and whether a restart or delayed start is carried out. In time segments in which no overload is present and no predelayed starting is carried out, the setpoint rotational speed 20 can be determined as being equal to the demand-fulfilment rotational speed or from the demand-fulfilment volume flow substantially as the quotient of the demand-fulfilment volume flow by the displacement. This quotient can likewise be regarded as a requirement-fulfilment rotational speed, i.e. the requirement-fulfilment rotational speed can be determined from this by means of the known displacement when a requirement-fulfilment volumetric flow is given. In addition, the demand-fulfilment rotational speed can also be determined from a corresponding characteristic diagram of the pump, which is present, for example, as a function of the required effective volume flow and the currently prevailing pump pressure. As a result, leaks in the pump can be taken into account in the determination of the demand-fulfilment rotational speed, i.e. the demand-fulfilment volumetric flow represents an indirect specification of the demand-fulfilment rotational speed.The reference variable, i.e. the setpoint rotational speed 20, is transferred to a functional module 64, which converts the setpoint rotational speed of the electric machine into one or more control variables 65 or control signals. These can be either a torque signal or the output signals of the field-oriented control for operating a permanently excited synchronous machine used by way of example. The functional module 45 may be, for example, part of the software running on the processor of an inverter controller of the inverter.A functional module 66 receives signals 68 of the inverter necessary for the control / regulation of the electrified pump drive. This also includes, for example, the signal of the actual rotational speed of the electric machine and thus also of the pump. The signals 68 received by the functional module can be supplied at least partially themselves and / or in further processed form as signal 70 both to the functional module 62 and to a functional module 72. In the functional module 72, it is determined whether an overload of the drive is present (from the signals 68, which are processed in the functional module 66 or are forwarded directly to the functional module 72, e.g. thermal load on inverter and electric machine) and whether the actual rotational speed has fallen below the rotational speed threshold value. This rotational speed threshold value can be any reasonable value between zero and close to zero, which characterizes a standstill of the electric drive. If both requirements are fulfilled, a corresponding signal 74 indicating the overload and the standstill is transferred to the functional module 62 by the functional module 72. The signal 74 may be, for example, a flag or a parameter. Likewise, it is detected in the functional module 72 whether the overload (e.g. thermal overload) is still present. If the overload is no longer present, the corresponding signal 74 can be reset to its original value. The function module 62 may perform the delayed (re)untilizing based on the signal 74 indicating whether or not there is an overload state. In this case, as soon as the overload state is ended, the setpoint rotational speed 20 is increased until the actual value of the rotational speed (i.e. the actual rotational speed) has reached the request-fulfilment rotational speed or a reduced rotational speed (or a reduced rotational speed value) in the vicinity of the request-fulfilment rotational speed.FIG. 6 shows, by way of example, a part of a controlled system of a speed control with which a speed increase can be implemented, i.e. with which delayed starting of the electric drive can be realized after the overload has ceased. A part of the controlled system of the speed control of the electrified pump drive is depicted here.In this case, the actual predefined value 46, i.e. the request-fulfilment rotational speed, is compared within a minimum selection block (min function) 80 with a value which results from an addition 82 of the actual rotational speed 34 and a predetermined rotational speed difference 81. The result of the minimum selection, i.e., the smaller value, is used as the target rotational speed 20. The actual control deviation is formed by a subtraction 84 of the actual rotational speed 34 and is used for the speed control by a controller 86 (shown in simplified form), wherein, for example, one or more control variables 65 are determined. If the value of the sum of the actual rotational speed 34 and rotational speed difference 81 is greater than the request-fulfilment rotational speed 46, the request-fulfilment rotational speed is selected as a minimum and the control functions in its actual sense, i.e. the desired rotational speed 20 is equal to the request-fulfilment rotational speed 46. As a result, the actual values are canceled in subtraction 84, and only rotational speed difference 81 is transferred to controller 86. In particular, either the controller 86 or the further section I have components (integral components), so that a ramp-like increase of the rotational speed takes place until the state in which the actual rotational speed becomes so great that the demand-satisfying rotational speed 46 is selected again in the minimum selection block 80 and the regulation of its correspondingly "normal" functionality follows. The addition 82, the minimum selection block 80 and the second rotational speed difference 81 are implemented, for example, in the functional module 62 of FIG. 5 and the elements 84, 86 are implemented, for example, in the functional module 64 of FIG. 5. Depending on the magnitude of the second rotational speed difference, the increase of the rotational speed takes place more or less quickly. The second speed difference may be equal to or different from the first speed difference.In order to avoid impermissible heating of the motor and / or of the inverter at the moment of overload (rotational speed zero or close to zero) and thereby keep the thermal load low, in the case of the detection of the overload the maximum possible actuating torque can be limited if this has not already taken place if the overload has taken place from the state of a thermal overload. In this state, the controller still sets the rotational speed difference 81, wherein the rotating field of the electric machine reaches its standstill frequency. In this case, the phases and therefore also the transistors / diodes in the inverter are loaded asymmetrically and heat up as a result more quickly.

Claims

Method for controlling an electric drive of an electrohydraulic system in the event of an overload, wherein the electric drive (4, 6) drives a hydraulic pump (2) which delivers pressure medium in the electrohydraulic system and is actuated at a setpoint rotational speed (20), and wherein a demand-fulfilment rotational speed (22) is given in accordance with a demand of hydraulic loads of the electrohydraulic system, comprising: determining (110) whether an overload of the electric drive exists; determining (120) whether an actual rotational speed (34) of the electric drive has fallen below a rotational speed threshold value (48) if an overload of the electric drive is determined (110); and carrying out (150) a restart if it is determined that the overload of the electric drive has ended (140) and if it has been determined that the actual rotational speed of the electric drive has fallen below the rotational speed threshold value; wherein restarting (150) includes increasing the setpoint rotational speed (20) over a starting time period (42) with a temporal length greater than zero to the request-satisfying rotational speed (22) or to a rotational speed that is greater than or equal to the request-satisfying rotational speed minus a predetermined first rotational speed difference (52).Method according to claim 1, comprising, if there is no overload of the electric drive (4, 6) and while no restart is being performed, setting (100) the target rotational speed (20) equal to the demand-fulfilment rotational speed (22).Method according to any of the preceding claims, comprising detecting and / or receiving du / or evaluating data of an inverter (6) of the electric drive to determine whether there is an overload of the electric drive; and / or using pressure measurement values of the electro-hydraulic system to determine whether there is an overload of the electric drive.Method according to one of the preceding claims, wherein the time length of the start-up time period (42) is greater than or equal to a predetermined minimum length.Method according to one of the preceding claims, wherein the increase of the setpoint rotational speed (20) during the restart (150) takes place at a predetermined gradient and / or a predetermined average gradient.Method according to one of the preceding claims, wherein the start-up time period (42) has a predetermined time length.Method according to one of the preceding claims, comprising, in particular in the absence of the overload, determining the setpoint rotational speed (20) as the minimum of the requirement-satisfying rotational speed (22) and the sum of the actual rotational speed (34) and a second rotational speed difference (81).Method according to one of the preceding claims, comprising, if it is determined that the actual rotational speed of the electric drive has fallen below the rotational speed threshold value, setting the setpoint rotational speed to zero or to a value less than the rotational speed threshold value until the overload of the electric drive no longer exists.Method according to one of the preceding claims, wherein the demand-fulfilment rotational speed (22) is directly predetermined or is determined from a demand-fulfilment volume flow.The method of any preceding claim, wherein restarting (150) further includes waiting a predetermined time after the end of the overload until the target speed (20) is started to be increased; in particular, wherein when the overload occurs again during the predetermined time, restarting is started again after this renewed overload fails.Method according to any one of the preceding claims, wherein the threshold speed value (48) corresponds to at most a fraction of a nominal speed of the electric drive (4, 6) or an average speed of the electric drive during operation of the electro-hydraulic system; in particular wherein the fraction is at most 0.2.Method according to one of the preceding claims, wherein the hydraulic pump (2) is a fixed displacement pump; or wherein the hydraulic pump is a variable displacement pump which has in particular a hydraulic-mechanical delivery flow regulator (28).A computing unit (16) comprising a processor configured to perform the method of any preceding claim.A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claims 1 to 12.Computer-readable data medium on which the computer program according to Claim 14 is stored.

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