Powertrain, method for controlling a powertrain, and vehicle

Abstract

The present application relates to a powertrain, a method for controlling a powertrain and a vehicle, in particular a powertrain (110) for a vehicle (100), comprising:-a first electric machine (2) for driving a driveshaft (3),-a power take-off device (4) to be driven by the first electric machine,-a power take-off clutch (dog clutch, 5), the invention relates to a power take-off device (1) comprising a power take-off clutch (2) for mechanically engaging or disengaging the power take-off device, a control unit (6) for controlling the rotational speed of the first electric machine and for controlling the power take-off clutch, a transmission (7) comprising shafts (8, 9) for drivingly connecting the first electric machine for driving the power take-off device and the transmission shaft, the control unit is adapted to first control the first electric machine to brake the rotational speed of the power take-off device to zero and then control the power take-off clutch to a disengaged position to disengage the power take-off device at zero rotational speed, depending on an operating demand for disengagement of the power take-off device.

Description

Technical Field

[0001] The present invention relates to a power transmission system, a method for controlling a power transmission system, a control unit, and a vehicle.

[0002] The present invention is applicable to heavy vehicles such as trucks, buses, and construction equipment. Although the present invention will be described with respect to trucks, the present invention is not limited to this type of vehicle, but can also be used in other vehicles such as buses, trailers, wheel loaders, excavators, etc. Background Art

[0003] Electrically or partially electrically operated heavy vehicles are becoming increasingly common. For heavy vehicles, it may be necessary to provide a power take-off (PTO) to drive an auxiliary system (PTO device) using an electric motor of the power transmission system.

[0004] In such a power transmission system, a transmission may be provided to transmit torque from the electric motor to an output shaft of a driven axle for propelling the vehicle. If there is a second drive unit, the output shaft can be shared with the second drive unit to drive, for example, the wheels of the vehicle to propel the vehicle.

[0005] US20110219900A1 discloses an example of a power transmission system unit having an electric motor, wherein the transmission is provided with a clutch (K PTO ) for engaging / disengaging the PTO device. The rotational speed of the transmission-side connection point of the auxiliary / PTO device is affected by means of a transmission brake or the electric motor.

[0006] There is a continuous effort to improve the power transmission system of electrically operated vehicles and to ensure safer and more efficient operation of the vehicle. Summary of the Invention

[0007] The main object of the present invention is to provide an improved power transmission system for a vehicle including a PTO device in at least some aspects. In particular, the object is to provide such an improved power transmission system that uses an electric motor to propel the vehicle and drive the PTO device when the PTO device is engaged.

[0008] According to a first aspect of the present invention, the above object is achieved by a power transmission system for a vehicle according to claim 1. The power transmission system includes: - a first drive unit adapted to drive a propeller shaft, - the first drive unit is a first electric motor, - a power output device adapted to be driven by the first electric motor, - A power take-off clutch adapted to be positioned in an engaged position or a disengaged position to mechanically engage or disengage a power take-off device relative to a first electric machine. - A control unit configured to control the rotational speed of the first electric machine and to control the power take-off clutch to one of the engaged position or the disengaged position. - A transmission including a shaft for drivingly connecting the first electric machine to drive the power take-off device and a transmission shaft. And wherein, in accordance with an operating requirement for disengaging the power take-off device, the control unit is adapted to first control the first electric machine to brake the rotational speed of the power take-off device to zero, and then the control unit is adapted to control the power take-off clutch to the disengaged position to disengage the power take-off device at zero rotational speed.

[0009] In a first aspect of the power transmission system, before performing the disengagement of the PTO device, the first electric machine first stops the engaged rotating PTO device. In this way, the PTO device is ready to be re-engaged at any time because zero PTO rotational speed has been ensured before the disengagement of the PTO. This has great benefits because otherwise, for example, if there is no rotational speed sensor on the PTO device side (auxiliary system side) of the PTO clutch, the rotational speed of the PTO device would be unknown.

[0010] By expressions such as "braking the rotational speed of the power take-off device to zero", "zero PTO rotational speed" or "zero rotational speed of the power take-off device" throughout this application means that the rotational speed of the PTO device has reached full rotational stop or near rotational stop.

[0011] "Mechanically engage or disengage" means that a first rotating part and a second rotating part in a disengaged state have no mechanical connection for transferring torque from the first rotating part to the second rotating part. An example of a connecting device for achieving such a mechanical connection can be a coupling sleeve on a shaft adapted to be positioned in a first axial position and a second axial position, in the first axial position, a gear engages with the shaft, in the second axial position, the gear disengages from the shaft, and thus, can rotate freely relative to the shaft. Therefore, the coupling sleeve moves axially on the shaft to rotationally engage and disengage the gear relative to the shaft.

[0012] Optionally, the power take-off clutch is a dog clutch.

[0013] The following may occur: For example, the operator requests the disengagement of the PTO device, and shortly thereafter, requests the re-engagement of the PTO device. This may occur if the operator inadvertently requests disengagement, for example, and suddenly realizes the erroneous disengagement request. If the PTO device is rotating during disengagement, there is a risk that the PTO device is still rotating when re-engagement occurs, which may cause severe wear of the PTO clutch in the case where the PTO clutch is a jaw clutch, or in the worst case, even cause clutch damage. If the PTO clutch belongs to the jaw clutch type (claw clutch type), the powertrain according to the first aspect is thus particularly beneficial.

[0014] In another case, for example, the control unit is programmed to temporarily disengage the PTO device to alternatively provide additional torque from the first motor to propel the vehicle. Such control for temporarily disengaging the PTO device may occur, for example, when vehicle acceleration is requested during a relatively short period of time. In a situation that is disadvantageous to the present invention, if the electric motor still supplies torque to the PTO device during disengagement (for example, for braking the PTO device), and thereby, the torque on the PTO clutch may result in an increased disengagement time, or in the worst case, as long as torque is applied to the PTO clutch and if the disengagement device (for example, a springback mechanism or a clutch actuator) is not strong enough to be able to disengage, it cannot be disengaged, then this situation may lead to difficulty in disengaging the PTO clutch. And furthermore, here, when re-engaging, if the PTO device is still rotating, this may cause severe wear of the PTO clutch, or in the worst case, even cause clutch damage. The problem can be solved by the first aspect of the present invention by first stopping the rotation of the PTO device before disengaging the PTO clutch. Another advantage regarding the first aspect is that it is easier (requires less force) to disengage the PTO clutch when the PTO device is not rotating because no torque is applied to the PTO clutch.

[0015] Optionally, the transmission includes a clutch actuator that is adapted to be controlled by the control unit and thereby disengage or engage the PTO clutch. The clutch actuator can be, for example, an electric, pneumatic, or hydraulic actuator or other actuator according to known techniques, and the control unit can control the clutch actuator according to known techniques by providing a control medium (for example, electricity, air, fluid) to move the actuator between an engaged state and a disengaged state. The actuator is mechanically connected to the PTO clutch in a known manner to set the PTO clutch to an engaged state or a disengaged state.

[0016] Optionally, the transmission further includes: - A first input shaft that is drivingly connected to the first motor, - An output shaft that is drivingly connected to a drive shaft, - A second shaft to which a power take-off device is drivingly connected or is connected via a power take-off clutch, - A gear pair for drivingly connecting the second shaft to one of the first input shaft or the output shaft.

[0017] The configuration of the transmission can be a solution with or without a countershaft. In fact, the present invention can be implemented using many different transmission configurations according to the known art.

[0018] "Drivingly connected" or "drivenly connected" between two rotating parts means herein that torque can be transmitted between said parts and the rotational speeds of said parts are proportional.

[0019] When two shafts are drivingly connected, torque can be transmitted from one shaft to the other directly through a clutch or a coupling or other rotational fixed connection or through, for example, a gear pair.

[0020] When two gears in a gear pair are drivingly connected, torque can be transmitted between said gears. This can be achieved by: said gears being in meshing engagement, or the first gear being in meshing engagement with the second gear, the second gear being in turn in meshing connection with the third gear, or the first gear being in meshing engagement with the second gear, the second gear being rigidly connected to the third gear, the third gear being in turn in meshing connection with the fourth gear. Thus, for drivingly connected, the two gears do not necessarily have to be in meshing engagement. It is sufficient that the rotation of one of the gears necessarily causes the rotation of the other gear.

[0021] Meshing engagement means permanent meshing engagement.

[0022] Optionally, the transmission further includes: - An additional gear pair for drivingly connecting the first input shaft to the second shaft and for drivingly connecting the second shaft to the output shaft, - The second shaft is a countershaft.

[0023] This embodiment specifically defines that the transmission includes at least one countershaft. A transmission configuration with a countershaft can provide a more compact transmission design and increase the possibility of additional gear ratios. As already mentioned, the present invention can be implemented using many different transmission configurations known in the art.

[0024] Optionally, the power transmission system includes a second drive unit adapted to drive the drive shaft via a second input shaft and a gear pair.

[0025] The second drive unit can be any type of drive unit according to known techniques, which is adapted to provide additional propulsion torque to propel the vehicle. According to known techniques, the transmission for the first electric machine and the second drive unit can be adapted to provide torque to the drive shaft either in parallel or one at a time.

[0026] Optionally, the second drive unit is one of an electric machine, an internal combustion engine, or a gas turbine engine.

[0027] According to a second aspect of the present invention, at least the main object is also achieved by a vehicle including a powertrain according to the first aspect. The vehicle can be, for example, a fully electric vehicle or a hybrid vehicle, which further includes at least one drive unit in the form of a combustion engine in addition to one or two power units in the form of electric machines. The advantages and beneficial features of the vehicle according to the present invention become apparent from the above description of the first aspect of the present invention.

[0028] According to a third aspect of the present invention, there is provided a method for controlling a powertrain according to the first aspect. The method includes: - registering a request to disengage the power take-off device, - braking the rotational speed of the power take-off device, - registering the zero rotational speed of the power take-off device, - disengaging the power take-off device.

[0029] The advantages and beneficial features of the method according to the present invention become apparent from the above description of the first aspect of the present invention. Preferably, the control unit registers the zero rotational speed of the power take-off device by registering that the first electric machine has reached zero rotational speed.

[0030] Optionally, the powertrain can further include a rotational speed sensor adapted to measure the rotational speed of the power take-off device, wherein the method includes: - if a faulty rotational speed signal from the rotational speed sensor is registered, - then initiating a failsafe mode, wherein the failsafe mode includes: - if a request to disengage the power take-off device is registered, - then first braking the rotational speed of the power take-off device and - after registering the zero rotational speed of the power take-off device, disengaging the power take-off device.

[0031] Preferably, the control unit registers the zero rotational speed of the power take-off device by registering that the first electric machine has reached zero rotational speed. Thus, the first electric machine can be used to register the rotational speed of the PTO device.

[0032] If the powertrain includes a rotational speed sensor adapted to measure the rotational speed of the power output device, and for some reason, such as malfunction, the rotational speed sensor cannot provide a sufficiently good rotational speed signal, a failsafe mode is initiated. The failsafe mode includes the method steps according to the third aspect that provides the advantages mentioned above. Thus, the present invention can be implemented using a powertrain that includes a rotational speed sensor adapted to measure the rotational speed of the power output device.

[0033] Optionally, the method includes the steps of: before the step of braking the rotational speed of the power output device, first disengaging the first electric machine from the drive shaft. By disengaging the first electric machine from the drive shaft before the braking of the first electric machine begins, the propulsion of the vehicle will be less affected.

[0034] According to a fourth aspect of the present invention, there is provided a control unit configured to execute the method according to the third aspect and the optional method. The control unit can control the clutch actuator to disengage and engage the PTO clutch in response to, for example, an operator request or after the control unit records a specific vehicle condition. The control unit further includes an electronic circuitry for automatically controlling at least the clutch actuator and being capable of communicating with other systems and control units of the vehicle.

[0035] Further advantages and advantageous features of the present invention are disclosed in the following description and the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the present invention cited as examples will be described in more detail hereinafter with reference to the drawings.

[0037] In the drawings:

[0038] Figure 1 A vehicle according to an embodiment of the present invention is schematically shown,

[0039] Figures 2a to 2c A powertrain according to different embodiments of the present invention is schematically shown.

[0040] Figure 3a and Figure 3b is a flowchart showing methods according to different embodiments of the present invention.

[0041] The drawings illustrate illustrative embodiments of the present invention and are therefore not necessarily drawn to scale. It should be understood that the embodiments shown and described are illustrative and the present invention is not limited to these embodiments. It should also be noted that some details in the drawings may be exaggerated for better description and illustration of the present invention. Throughout the specification, unless otherwise indicated, the same reference numerals refer to the same elements. DETAILED DESCRIPTION

[0042] Figure 1 Vehicle 100 in the form of a truck according to an embodiment of the present invention is shown. Vehicle 100 includes a powertrain 110 having a first electric machine 2 for propulsion of vehicle 100. The powertrain 110 further includes a second drive unit 10 which is also used for propulsion of the vehicle and can be, for example, one of an electric machine, an internal combustion engine or a gas turbine engine. Each of the electric machine 2 and the second drive unit 10 is drivingly connected to a drive shaft 3 via a transmission 7. The transmission 7 is arranged to transfer torque from the electric machine 2 and the second drive unit 10 to the drive shaft 3, thereby connecting the powertrain 110 to a driven axle that drives the driven wheels 16 of vehicle 100. According to the known art, the transmission 7 can have different configurations of shafts and gears to provide different gears with different gear ratios.

[0043] Vehicle 100 further includes a power take-off (PTO) device 4 which is arranged to be driven by at least the first electric machine 2 via the transmission 7. The PTO device 4 can be of different kinds known in the art and can have a large moment of inertia, which means that if the rotational speed of the PTO device 4 needs to be changed rapidly, a relatively large amount of torque is required. This also means that when disengaged and if not actively braked, it can take a relatively long time for the PTO device to stop rotating. The first electric machine 2 and the second drive unit 10 (if it is an electric machine) are configured to be powered by an electrical energy storage system (not shown) of vehicle 100.

[0044] The vehicle and the powertrain can have many different configurations. For example, the drive units 2 and 10 and the transmission 7 do not need to be arranged at the front of the vehicle as Figure 1 disclosed, but can be combined with, for example, the rear axle of the vehicle.

[0045] Figure 2a Disclosed is a powertrain 110 of a vehicle 100 according to a first embodiment of the present invention, the powertrain including a first electric machine 2 which is adapted to drive a drive shaft 3 and also drive a PTO device 4 when requested. Further, a PTO clutch 5 of the dog clutch type is adapted to be positioned in an engaged position or a disengaged position to mechanically engage or disengage the PTO device 4 relative to the first electric machine 2. A control unit 6 is adapted to control the rotational speed of the first electric machine 2 by transmitting a signal to the first electric machine 2 via a wire 13 and in accordance with a request from an input signal via a wire 12. The control unit 6 actuates a clutch actuator 15 by controlling a medium such as air, hydraulic fluid or electricity (depending on the type of actuator) via a wire 16. The clutch actuator 15 is mechanically connected to an axially movable part of the PTO clutch so as to position the PTO clutch 5 in an engaged state or a disengaged state, and thereby engage or disengage the PTO device 4.

[0046] The transmission 7 includes at least an input shaft (not disclosed) drivingly connected to the first electric machine 2, an output shaft 8 drivingly connected to the first electric machine 2 to drive the PTO device 4 and the transmission shaft 3, and a second shaft 9. The transmission shaft 3 is drivingly connected through the output shaft 8. Different shafts of the transmission 7 can be drivingly connected through gear pairs, and different gear ratios are provided in a known manner when engaged in different combinations. Preferably, the second shaft 9 can be a countershaft. The configuration of the transmission 7 can be implemented in many different ways according to known techniques, and thus will not be further described in this specification. When the PTO device 4 is engaged, the control unit 6 can control the rotational speed of the PTO device 4 by controlling the rotational speed of the first electric machine 2.

[0047] Therefore, the first electric machine 2 can drive the PTO device 4 at the requested rotational speed. According to the operation requirement for disengaging the PTO device through the wire 12, the control unit 6 is adapted to first control the first electric machine 2 to brake the rotational speed of the PTO device 4 to zero, and then, the control unit 6 is adapted to control the PTO clutch 5 to the disengaged position to disengage the PTO device 4 at zero rotational speed. Preferably, the first electric machine 2 is first disengaged from the transmission shaft 3 before the step of braking the rotational speed of the PTO device 4 starts. The rotational speed of the PTO device 4 is recorded by the control unit 6 through the wire 13 by a signal indicating that the first electric machine 2 has reached zero rotational speed.

[0048] Figure 2b Disclosed is a powertrain 110 of a vehicle 100 according to a second embodiment of the present invention. This embodiment is similar to the first embodiment according to Figure 2a but differs in that this embodiment is additionally provided with a second drive unit 10 which is also adapted to drive the transmission shaft 3, but alternatively drives through a second input shaft (not disclosed) and one or more additional gear pairs. The second drive unit 10 is controlled by the control unit 6 via a wire 14 in the disclosed example. Preferably, the second drive unit 10 is a second electric machine, but can also be an internal combustion engine or a gas turbine engine or other types of propulsion units known in the art.

[0049] Figure 2c Disclosed is a powertrain 110 of a vehicle 100 according to a third embodiment of the present invention. This embodiment is similar to the one according to Figure 2bSecond embodiment, except that in this embodiment, a rotational speed sensor 11 is additionally provided, which is adapted to measure the rotational speed of the PTO device 4 on the PTO device side of the PTO clutch 5. The rotational speed value of the PTO device 4 is transmitted from the rotational speed sensor 11 to the control unit 6 via the electric wire 17. In this embodiment, the control unit 6 is adapted to record a fault signal indicating a faulty rotational speed sensor 11. The fault signal may include, for example, the absence of a signal transmitted from the rotational speed sensor, or the signal from the rotational speed sensor 11 is recorded as being damaged or significantly incorrect, etc. If the control unit 6 records a fault signal, the control unit is adapted to initiate a fail-safe mode. The fail-safe mode includes the following steps: If the control unit 6 records a request to disengage the PTO device 4, the control unit 6 first brakes the rotational speed of the PTO device 4, and then, when the control unit records zero rotational speed of the PTO device 4, the control unit 6 is adapted to disengage the PTO device 4. The rotational speed of the PTO device 4 is recorded by the control unit 6 by recording a signal that the first motor 2 has reached zero rotational speed via the electric wire 13.

[0050] In all exemplary embodiments of the present invention, different shafts of the transmission are preferably mounted to the transmission housing of the transmission 7 using bearings (not disclosed).

[0051] Preferably, the transmission 7 is an automated mechanical transmission (AMT).

[0052] Figures 2a to 2c The electric wires 12, 13, 14, 16, 17 indicated as dashed lines in may alternatively be communication devices using a known transmission bus (such as a CAN bus) and / or wireless communication devices.

[0053] In all exemplary embodiments of the present invention, when the control unit 6 controls the first motor 2 to brake the PTO device 4, the braking energy can be converted into electrical energy by the motor 2, and this electrical energy can be used, for example, to charge an electrical energy storage system (not disclosed).

[0054] Of course, features from different embodiments can be combined such that, for example, Figure 2a the embodiment including only one motor 2 can be equipped with the rotational speed sensor 15 from the Figure 2c embodiment, thus including the method steps of the fail-safe mode of the embodiment using Figure 2c .

[0055] For all disclosed embodiments of the present invention, during the re-engagement of the PTO device 4, the motor 2 can be controlled to rotate slowly to avoid a tooth-to-tooth situation in the PTO clutch 5 of the jaw clutch type. In this way, a smoother and safer re-engagement can be achieved. The tooth-to-tooth situation is not desirable because the jaw clutch can remain in the disengaged state at least temporarily.

[0056] Figure 3a A method for controlling a powertrain 110 according to an embodiment of the present invention is shown. The method includes the following steps: S1: Record a request for disengaging the PTO device 4, S2: Brake the rotational speed of the PTO device 4, S3: Record the zero rotational speed of the PTO device 4, S4: Disengage the PTO device 4.

[0057] Figure 3b is shown according to Figure 2c Another method for controlling a powertrain 110 of an embodiment having a rotational speed sensor is shown. Here, the method includes the following steps: S10: If a faulty rotational speed signal from the rotational speed sensor 11 is recorded, S11: Then initiate a fail-safe mode, wherein the fail-safe mode includes the following steps: S12: If a request for disengaging the PTO device 4 is recorded, S13: Then first brake the rotational speed of the PTO device 4, and S14: After recording the zero rotational speed of the PTO device 4, disengage the power take-off device 4.

[0058] In step S14, the control unit 6 records the zero rotational speed of the PTO device 4 by recording a signal that the first motor 2 has reached zero rotational speed via the electric wire 13.

[0059] The control unit 6 may be an electronic device configured to execute the Figure 3a or Figure 3b methods shown. For this purpose, it may include components for controlling the first motor 2, components for controlling the second drive unit 10 (if any), and components for controlling the transmission 7, including at least gear engagement devices (such as an actuator for a coupling sleeve, not shown) and a clutch actuator 15 of the PTO clutch 5. The control unit 6 may be configured to control the powertrain 110 and the PTO clutch 5 depending on a request for disengaging the PTO device 4.

[0060] The control unit 6 may include a microprocessor, a microcontroller, a programmable digital signal processor, or another programmable device. Thus, the control unit 6 includes electronic circuitry and connections (not shown) as well as processing circuitry (not shown) such that the control unit 6 can communicate with different parts of the vehicle 100 or with different control units of the vehicle 100, such as communicating with various sensors, systems, and control units, in particular communicating with one or more electronic control units (ECUs) that control an electrical system or subsystem in the vehicle 100, such as an energy storage system control unit of an electrical energy storage system (not shown) of the vehicle 100.

[0061] The control unit 6 may include modules in the form of hardware or software, or partly in the form of hardware or software, and communicate using a known transmission bus (such as a CAN bus) and / or wireless communication capabilities. The processing circuitry may be a general-purpose processor or a specific processor. The control unit 6 may include a non-transitory memory for storing computer program code and data. Thus, those skilled in the art will realize that the control unit 6 can be embodied in many different configurations.

[0062] Although shown herein as a single control unit, the control unit 6 may be formed by several different control units configured to communicate with each other, such as a separate control unit for controlling at least the first electric machine 2 and for controlling the transmission 7. Thus, the software for performing the control of at least the first electric machine 2 and for controlling the transmission 7 may be distributed among several control units.

[0063] It should be understood that the present invention is not limited to the embodiments shown above and in the drawings; rather, those skilled in the art will recognize that many changes and modifications can be made within the scope of the appended claims.

Claims

1. A powertrain (110) for a vehicle (100), comprising: - A first drive unit adapted to drive a transmission shaft (3), - The first drive unit is a first electric machine (2), - A power output device (4) adapted to be driven by the first electric machine, - A power output clutch (5) adapted to be positioned in an engaged position or a disengaged position to mechanically engage or disengage the power output device relative to the first electric machine, - A control unit (6) for controlling the rotational speed of the first electric machine and for controlling the power output clutch to one of the engaged position or the disengaged position, - A transmission (7) comprising shafts (8, 9) for drivingly connecting the first electric machine to drive the power output device and the transmission shaft, and wherein, in accordance with an operating requirement for disengaging the power output device, the control unit is adapted to first control the first electric machine to brake the rotational speed of the power output device to zero, and then the control unit is adapted to control the power output clutch to the disengaged position to disengage the power output device at zero rotational speed.

2. The powertrain according to claim 1, wherein the power output clutch (5) is a jaw clutch.

3. The powertrain according to one of the preceding claims, wherein the transmission comprises a clutch actuator (15) adapted to be controlled by the control unit (6) and thereby disengage or engage the PTO clutch (5).

4. The powertrain according to one of the preceding claims, wherein the transmission comprises: - A first input shaft drivingly connected to the first electric machine, - An output shaft (8) drivingly connected to the transmission shaft (3), - A second shaft (9) to which the power output device can be drivingly connected or connected via the power output clutch, - A gear pair for drivingly connecting the second shaft to one of the first input shaft or the output shaft.

5. The powertrain according to the preceding claim, wherein the transmission further comprises: - An additional gear pair for drivingly connecting the first input shaft to the second shaft and for drivingly connecting the second shaft to the output shaft, - The second shaft (9) is a countershaft.

6. The powertrain according to one of the preceding claims, wherein a second drive unit (10) is adapted to drive the transmission shaft (3) via a second input shaft and a gear pair.

7. The powertrain according to the preceding claim, wherein the second drive unit (10) is one of an electric machine or an internal combustion engine or a gas turbine engine.

8. A vehicle (100) comprising a powertrain (110) according to any one of the preceding claims.

9. A method for controlling a power transmission system (110) according to any one of claims 1 to 6, the method comprising: - Recording a request to disengage the power output device (4), - Braking the rotational speed of the power output device, - Recording the zero rotational speed of the power output device, - Disengaging the power output device.

10. A method for controlling a power transmission system according to any one of claims 1 to 6, the power transmission system further comprising a rotational speed sensor (11) adapted to measure the rotational speed of the power output device (4), the method comprising: - If a faulty rotational speed signal from the rotational speed sensor is recorded, - Then initiating a fail-safe mode, Wherein the fail-safe mode comprises: - If a request to disengage the power output device (4) is recorded, - Then first braking the rotational speed of the power output device (4), and - After recording the zero rotational speed of the power output device (4), disengaging the power output device (4).

11. The method according to one of claims 9 and 10, wherein the control unit (6) records the zero rotational speed of the power output device (4) by recording that the first electric machine (2) has reached zero rotational speed.

12. The method according to one of claims 9 and 11, wherein before the step of braking the rotational speed of the power output device (4), the first electric machine (2) is first disengaged from the transmission shaft (3).

13. A control unit (6) configured to perform the method according to one of claims 9 to 11.

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