EP4363259A1

EP4363259A1 - Method for operating an electrified drive train for a working machine, electrified drive train for a working machine and working machine

Info

Publication number: EP4363259A1
Application number: EP22734966.9A
Authority: EP
Inventors: Jürgen LEGNER
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 2021-06-28
Filing date: 2022-06-23
Publication date: 2024-05-08
Also published as: CN117580725A; WO2023274836A1; DE102021206653B3; US20240294162A1

Abstract

The invention relates to a method for operating an electrified drive train for a working machine, wherein an electric motor of the drive train is drive-coupled to at least one wheel of the working machine, wherein the working machine, when travelling downhill, experiences a downhill force which causes (100) an acceleration torque on the working machine, wherein a generator torque of the electric motor counters the acceleration torque during generator operation, wherein a maximum generator torque is dependent on an actual motor rotation speed of the electric motor and wherein, when the working machine accelerates on account of the downhill force (101), an automatic braking intervention is performed (102). The method according to the invention is distinguished in that, owing to the braking intervention, the actual motor rotation speed is reduced until the maximum generator torque at a reduced actual motor rotation speed is at least equal to the acceleration torque (104). The invention also relates to a corresponding electrified drive train for a working machine as well as to a corresponding working machine.

Description

Method for operating an electrified powertrain for a work machine, electrified powertrain for a work machine and work machine

The present invention relates to a method for operating an electrified drive train for a work machine according to the preamble of claim 1, an electrified drive train for a work machine and a corresponding work machine.

Electrically driven machines, such as wheel loaders, skid steer loaders, telehandlers, dump trucks and excavators are known in the prior art. Such work machines are either driven purely electrically, i.e. they only have an electric battery as energy storage or a fuel cell for generating electricity from hydrogen. Or they are diesel-electrically driven, which means that the required energy is provided by a diesel-driven generator and, if necessary, by an electrical buffer storage device, such as an appropriately dimensioned capacitor or a comparatively small battery. In all cases, the mechanical power required for the travel drive and the working drive is provided by one or more electric motors. Furthermore, it is known to use the electric motors of electric drives during braking processes in generator mode to recuperate electric power. In addition, a mechanical friction brake is always provided so that a sufficiently large braking power can be provided at any time for safety reasons.

In this context, EP 0962597 A2 describes a battery-operated working machine which has two electric motors for the traction drive and a further electric motor for the working drive.

WO 2008/128674 A1 discloses a work machine with a hybrid drive train, comprising an internal combustion engine and an electric machine. An electrical energy storage device is provided for supplying energy to the electric machine. which can be recuperatively charged by operating the electric motor in generator mode when the working machine brakes.

The applicant's as yet unpublished DE 102020201497.3 discloses a method for operating an electrified drive train for a work machine. A slope descent of the working machine can be recognized by means for detecting a gradient in conjunction with means for detecting a movement of the working machine. If the working machine exceeds a threshold speed when descending a slope, the electric motor is put into recuperation mode in order to prevent further acceleration of the working machine due to the driving force driving down the slope.

The likewise still unpublished DE 102020203594.6 of the applicant also describes a method for operating an electrified drive train for a working machine. A braking torque generated by the electric motor in generator mode results in a speed reduction of the at least one wheel via a driving coupling of an electric motor of the drive train with at least one wheel of the working machine. If, for example, when descending a slope, the working machine accelerates unintentionally despite the braking torque, an additional automatic brake intervention takes place.

However, the known electric drive trains for working machines have the disadvantage that when descending a slope at a comparatively high speed, they often cannot generate a sufficiently large recuperation torque and therefore no sufficiently large braking torque to prevent further unwanted acceleration of the working machine due to the downhill force in a speed range that is inadmissibly high for the working machine. The reason for this insufficient recuperation torque lies in the inherent behavior of an electric motor, which in its upper speed range and thus at high driving speeds of the working machine only has a comparatively low torque, which in turn only allows a comparatively low braking or recuperation torque . Thus, in such a driving situation is a constant reference to an operating or Parking brake required to prevent further unintentional acceleration of the work machine when descending a slope. However, since the service or parking brake is not usually designed for permanent or prolonged actuation, the braking system can be damaged if the service or parking brake is actuated for a correspondingly long time.

It is an object of the invention to propose an improved method for operating an electrified drive train for a working machine.

According to the invention, this object is achieved by the method for operating an electrified drive train for a working machine according to claim 1 . Advantageous refinements and developments of the invention emerge from the dependent claims.

The invention relates to a method for operating an electrified drive train for a working machine, with an electric motor of the drive train being coupled to at least one wheel of the working machine for driving purposes, with the working machine experiencing a downhill force when descending a slope, which causes an acceleration torque on the working machine, wherein a generator torque of the electric motor in generator mode counteracts the acceleration torque, with a maximum generator torque depending on an actual engine speed of the electric motor and with automatic braking intervention taking place when the working machine accelerates due to the downhill slope force. The method according to the invention is characterized in that the actual engine speed is reduced by the brake intervention until the maximum generator torque at a reduced actual engine speed is at least as great as the acceleration torque.

The invention thus describes a method by which an electrified drive train can be operated. The drive train is suitable for driving an Ar beitsmaschine. Since working machines are usually operated most of the time under high drive loads and, in particular, have to perform comparatively high work rates in absolute terms, there is a difference the drive train according to the invention in its design, for example, from a passenger car drive train, which is typically operated in a utilization range of 5% to 10% of the maximum output and in particular comparatively low absolute work performance.

An electric motor of the drive train is drivingly coupled to at least one wheel of the work machine, for example via a gear or a detachable hitch, in particular via a multi-plate clutch. It follows from the driving coupling between the electric motor and the at least one wheel that an acceleration torque generated by the electric motor leads to an increase in the speed of at least one wheel and thus to an acceleration of the work machine. The increase in the speed of the at least one wheel corresponds to the increase in the speed of the electric motor, taking into account a transmission ratio between the electric motor and the at least one wheel. Conversely, an acceleration torque of the at least one wheel, which can result, for example, from the downhill force when the driven machine is going down a slope, also increases an increase in speed of the electric motor.

Provision is preferably made for the driving coupling of the electric motor to all wheels of the work machine to exist.

The electric motor can also be operated in generator mode to recuperate electrical power. In this operating mode, also known as recuperation mode, the electric motor converts the kinetic energy of the working machine into electrical energy, which is fed to a battery storage unit in order to charge it.

This electrical energy can be taken from the battery storage later if required, in order to supply the electric motor or other electrical consumers. In addition, it can also be provided that the battery storage device can be charged with external electrical energy via a charging cable or another suitable charging device, for example an induction charging device. The use of the electric motor for recuperation also reduces the wear and tear of a mechanical friction brake, since a generator torque acts as a braking torque during generator operation. In particular, as a result, the mechanical friction brake can be comparatively less powerful, less expensive and not designed for continuous operation.

However, a maximum generator torque that can be provided by the electric motor—and thus a maximum braking torque that can be provided by the electric motor—depends significantly on the current engine speed, with the maximum generator torque decreasing as the engine speed increases. At high engine speeds, the electric motor can therefore only generate a comparatively low maximum generator torque. Conversely, the electric motor can generate a comparatively large maximum generator torque at low engine speeds.

In order to prevent the working machine from accelerating further and further when descending a slope due to the constantly decreasing generator torque as the engine speed increases and possibly resulting in a dangerous situation due to the vehicle speed being too high or the electric motor being damaged due to the engine speed being too high, it is also provided that in this case, an automated brake intervention takes place, in particular with the mechanical friction brake.

For this purpose, an undesired acceleration of the working machine is advantageously initially detected during generator operation of the electric motor. Unintentional acceleration is understood to mean an acceleration not initiated by a driver of the working machine, such as occurs due to the descent or the downhill force acting to accelerate the working machine, provided that the downhill force or the resulting acceleration torque is the maximum at the current engine speed applicable generator torque exceeds.

The unwanted acceleration can, for example, like any other acceleration of the working machine based on the monitoring and temporal differentiation of a wheel speed, engine speed or other speed he recorded in a transmission Working machine are recognized. It is also conceivable that the acceleration is detected, for example, via a satellite navigation system.

According to the invention, it is now provided that the actual engine speed is reduced by the brake intervention until the maximum generator torque at a reduced actual engine speed is at least as great as the acceleration torque. The braking intervention counteracts the acceleration or the downhill force and supports the generator torque of the electric motor. The sum of a braking torque of the braking intervention of the mechanical friction brake and the generator torque of the electric motor is greater than the acceleration torque, so that the working machine decelerates when descending a slope. The deceleration of the working machine in turn leads to a reduction in the engine speed of the electric motor and thus to an increase in the maximum generator torque that can be applied. The maximum generator torque that can be generated continues to increase as the engine speed decreases.

This results in the advantage that the maximum generator torque increased by the brake intervention is again sufficiently large at the reduced actual engine speed to completely compensate for the acceleration due to the downhill force, so that the machine no longer accelerates. The braking intervention by the mechanical friction brake can thus be ended and the wear on the mechanical friction brake can be reduced. From now on, the speed of the working machine can be reliably controlled solely by the generator torque of the electric motor when descending a slope.

According to a preferred embodiment of the invention, it is provided that the reduced actual engine speed is specified as the new setpoint engine speed. This means that the actual engine speed of the electric motor is initially reduced by the brake intervention until the maximum generator torque is large enough to prevent further acceleration of the machine due to the downhill driving force. This reduced actual engine speed is now in a Mo torsteuergerät, such as an inverter of the electric motor, specified as the new target engine speed to re-accelerate the machine and This avoids a reduction in the generator torque when descending a slope.

In particular, the driver of the work machine can now no longer increase the setpoint engine speed while descending a slope until the descent is complete.

The end of the descent can be detected, for example, by an inclination sensor.

According to a further preferred embodiment of the invention, it is provided that the reduced setpoint engine speed is increased again as the downhill force decreases, up to the original setpoint engine speed. Thus, the Ge speed of the work machine is automatically increased again when the descent is completed, until finally the target engine speed specified by the driver before the procedural process according to the invention is carried out can be specified again as the target engine speed. The reduction of the actual engine speed and the target engine speed to increase the generator torque is therefore limited to the duration of the descent.

According to a particularly preferred embodiment of the invention, it is provided that the target engine speed is increased when the maximum generator torque is greater than the acceleration torque by a predetermined threshold value when the actual engine speed is reached. This means that the target engine speed can be increased again before the end of the descent if the maximum generator torque that can be provided is around the predefinable threshold value - which represents an additional generator torque in the form of a safety buffer if the incline of the slope and thus the downhill force short-term increase men - is greater than the acceleration torque, for example due to a Re duration of the gradient of the slope. In particular, due to the definable threshold value acting as a safety buffer, there is then no further unwanted acceleration of the working machine to be feared. According to a further preferred embodiment of the invention, it is provided that the brake intervention is increased until the actual engine speed is reduced. This ensures that the braking intervention causes a sufficiently strong braking torque to actually lead to a reduction in the actual engine speed and thus increase the generator torque.

According to a further preferred embodiment of the invention, it is provided that a required braking torque of the braking intervention is determined using a generator torque characteristic. The generator torque characteristic shows a maximum generator torque that can be provided as a function of the actual engine speed of the electric motor. Thus, the required braking torque of the brake intervention can be easily determined based on the current actual speed of the electric motor, which is preferably detected by a speed sensor, and based on a change in the actual speed, which corresponds to an acceleration of the working machine. The braking torque required is the difference between the total braking torque required and the maximum generator torque that can be provided.

According to a further preferred embodiment of the invention, provision is made for the braking intervention to be regulated continuously. This means that a braking torque caused by the brake intervention can be set or regulated steplessly and thus optimally adapted to the greatest possible extent to the required overall braking torque.

According to a further preferred embodiment of the invention, it is provided that the braking intervention is ended when the maximum generator torque is greater than the acceleration torque when the actual engine speed is reached. In this case, it is no longer necessary to use the mechanical friction brake to generate an additional braking effect. Further acceleration of the working machine can now be avoided solely through the generator torque. This also reduces the wear on the mechanical friction brake. The invention also relates to an electrified drive train for a working machine, comprising an electric motor, a control unit for controlling a motor speed of the electric motor, at least one drive wheel and a hydraulically actuated and continuously controllable mechanical friction brake. The electrified drive train according to the invention is characterized in that the drive train is designed to carry out the method according to the invention.

This also results in the advantages already described in connection with the method according to the invention for the drive train according to the invention.

The electrified drive train according to the invention includes all means and devices for carrying out the method according to the invention.

Finally, the invention also relates to a working machine comprising a drive train according to the invention. This results in the advantages already described in connection with the drive train according to the invention, also for the work machine according to the invention.

The work machine is preferably a wheel loader. But it can also be a compact loader, telehandler, dumper, excavator or tractor.

The invention is explained below by way of example with reference to embodiments shown in the figures.

Show it:

figs 1 and 2 as an example a maximum engine torque and a maximum generator torque each depending on an actual engine speed of an electric motor and

3 shows an example and a schematic of a possible embodiment of the method according to the invention in the form of a flow chart. Identical objects, functional units and comparable components are denoted by the same reference symbols across the figures. These objects, functional units and comparable components are identical in terms of their technical features, unless the description explicitly or implicitly states otherwise.

figs 1 and 2 show, by way of example, a maximum engine torque 10 and a maximum generator torque 20, each as a function of an actual engine speed of an electric motor. As can be seen, the maximum engine torque 10 and the maximum generator torque 20 decrease as the actual engine speed increases, which represents a behavior that is typical for electric motors.

The characteristic curve 10 shows the engine torque 10 over the engine speed for a certain accelerator pedal position. The characteristic curve 20 shows the generator torque 20 in the generator operation genes over the engine speed. The dash-dotted line describes an example of a necessary braking torque when driving down a certain downhill stretch. The incline of the downhill stretch, the vehicle mass and the rolling resistance of the vehicle tires are unknown, for example.

At point 1 (FIG. 1), a driving condition at maximum speed of the working machine on the level with a corresponding accelerator pedal position is shown as an example. The engine torque 10 is in balance with the driving resistance.

If, for example, the working machine drives down a slope whose gradient generates a greater downhill force than the maximum generator torque 20 can compensate for at the current actual engine speed, the working machine will accelerate and the electric motor runs the risk of being subjected to excessive speeds and get damaged as a result. The maximum genera tormoment at 20 of the target engine speed is shown in point 2 (Fig. 1).

Exceeding a permissible maximum speed of the electric motor is detected, for example, when point 2 (FIG. 1) is exceeded. It is characteristic of electric drives that the generator torque 20 decreases as the actual engine speed continues to rise, which leads to an ever faster acceleration of the working machine.

In order to prevent the maximum permissible speed of the electric motor from being exceeded, an additional braking torque or an additional braking torque is required. A balance of forces between the acceleration torque resulting from the slope force and the braking torque is given in point 3 (Fig. 1) and exceeding the maximum permissible speed is prevented here.

If the service or parking brake, which is designed as a mechanical friction brake, for example, is applied when the maximum permissible speed of the electric motor is exceeded, its braking torque is added to the generator torque 20. The braking effect is increased until the actual engine speed falls. The braking effect component of the service or parking brake is higher than the actually required value by a percentage for decelerating the moving mass of the working machine.

The braking torque can be determined from the technical properties of the service or parking brake and the current actuation. The momentary component of the loading service or parking brake corresponds to the difference between the two points 2 and 3 (Fig. 1).

The generator characteristic 20 is known and thus also the actual engine speed at which the generator torque corresponds to the value of point 5 (FIG. 2).

An electronic control unit, for example an inverter assigned to the electric motor, now lowers the target engine speed in point 6 (FIG. 1) accordingly, so that the maximum generator torque 20 reaches the value of point 5 (FIG. 2).

With constant actuation of the service or parking brake, the working machine is further decelerated. If the new target engine speed is reached in point 7, the current generator torque 20 falls to the value of point 4 (FIG. 2). At this moment it is ensured that the maximum generator torque 20 without additional braking effect of the service or parking brake is sufficient to prevent unwanted acceleration of the working machine. The service or parking brake can now be deactivated in a controlled manner. The current generator torque 20 increases, but remains below the speed-dependent maximum generator torque 20.

If the current generator torque 20 falls below a threshold value below the maximum generator torque 20, the previously reduced setpoint engine speed can be increased again until the original setpoint engine speed is reached again. This ends the execution of the method.

FIG. 3 shows, by way of example and schematically, a possible embodiment of the method according to the invention in the form of a flowchart. The method according to the invention is carried out in an electrified drive train for a working machine. The drive train includes an electric motor, a control unit for regulating a motor speed of the electric motor, at least one drive wheel and a hydraulically actuated and continuously controllable mechanical friction brake. There is a drive coupling between the electric motor and at least one wheel of the work machine.

In a first method step 100, the work machine moves from a level onto a sloping ground and thus transitions into a downhill slope. Accordingly, it experiences a downhill force that acts on the working machine in the form of an acceleration moment. Since the downhill force acts in an accelerating manner on the working machine and there is a driving coupling between the at least one wheel of the working machine and the electric motor, a generator torque of the electric motor in generator mode counteracts the downhill driving force and thus the acceleration of the working machine. The maximum male generator torque 20 is dependent on an actual engine speed of the elec romotors.

In the following step 101, it is determined that the acceleration torque caused by the downhill slope force is greater than the maximum from the electric motor available generator torque 20, so that the machine is accelerated and the actual engine speed increases accordingly.

In order to prevent this unwanted acceleration of the work machine, in step 102 there is an automated brake intervention. The brake intervention is continuously increased in step 103 until the actual engine speed is reduced and the work machine is decelerated accordingly. In step 104, the brake intervention is continued until the maximum generator torque 20 at a reduced actual engine speed is greater than the acceleration torque generated by the downhill slope force. When this has been achieved, ie when the maximum generator torque 20 that can be provided is greater than the acceleration torque, the reduced actual engine speed is specified in step 105 as the new setpoint speed. At the same time, in step 106, the brake intervention is ended.

If it is recognized that the working machine is again on a level surface and no downhill force is acting on the working machine, in step 107 the original desired engine speed from step 100 is specified as the new desired engine speed.

Reference sign Engine torque Generator torque Beginning of descent Determination that caused by the downhill force

Accelerating torque is greater than the maximum generator torque that can be provided by the electric motor Automated braking intervention Continuous reinforcement of the braking intervention Continuation of the braking intervention until the maximum generator torque at a reduced actual engine speed is greater than the acceleration torque generated by the downhill gradient Specification of the reduced actual engine speed as the new target speed Termination of the brake intervention Specification of the original setpoint engine speed as the new setpoint speed

Claims

patent claims

1. A method for operating an electrified drive train for a working machine, with a driving coupling of an electric motor of the drive train with at least one wheel of the working machine. , wherein a generator torque of the electric motor counteracts the acceleration torque in generator mode, wherein a maximum generator torque is dependent on an actual engine speed of the electric motor and wherein when the working machine accelerates due to the downhill force (101), an automated brake intervention takes place (102), characterized that the actual engine speed is reduced by the brake intervention until the maximum generator torque at a reduced actual engine speed is at least as great as the acceleration torque (104).

2. The method according to claim 1, characterized in that the reduced actual engine speed is specified as the new setpoint engine speed (105).

3. The method according to at least one of claims 1 and 2, characterized in that the reduced setpoint engine speed is increased again with decreasing downhill slope force up to the original setpoint engine speed

(107).

4. The method according to claim 3, characterized in that the target engine speed is increased when the maximum generator torque at an actual engine speed reached is greater than the acceleration torque by a predetermined threshold value.

5. The method according to at least one of claims 1 to 4, characterized in that the braking intervention is increased until the actual engine speed is reduced (103).

6. The method according to at least one of claims 1 to 5, characterized in that a required braking torque of the brake intervention is determined using a generator torque characteristic.

7. The method according to at least one of claims 1 to 6, characterized in that the braking intervention is continuously regulated.

8. The method according to at least one of claims 1 to 7, characterized in that the brake intervention is terminated when the maximum generator torque is greater than the acceleration torque when the actual engine speed is reached (106).

9. Electrified drive train for a working machine, comprising an electric motor, a control unit for controlling a motor speed of the electric motor, at least one drive wheel and a hydraulically actuable and continuously controllable friction brake, characterized in that the drive train is designed to carry out the method according to the invention.

10. Work machine, comprising a drive train according to claim 9.