SE541481C2

SE541481C2 - A method and an arrangement for controlling a gearbox for a vehicle standstill

Info

Publication number: SE541481C2
Application number: SE1750162A
Authority: SE
Inventors: Tomas Selling
Original assignee: Scania Cv Ab
Priority date: 2017-02-17
Filing date: 2017-02-17
Publication date: 2019-10-15
Also published as: DE102018000734A1; SE1750162A1

Abstract

The invention relates to a method for controlling a gearbox (4) when a clutch (11) cannot be disengaged. The gearbox (4) comprising an input shaft (200); an output shaft (300); a first gearbox unit (4A) connected to the input shaft (200); a second gearbox unit (4B) connectable to the first gearbox unit (4A); and a third gearbox unit (4C) connectable to the second gearbox unit (4B) and the output shaft (300). The third gearbox unit (4C) comprises a coupling device (40) for selectively disconnecting the third gearbox unit (4C) from the output shaft (300). The method comprising: controlling (s101) the coupling device (40) to disconnect the third gearbox unit (4C) from the output shaft (300); ensuring (s102) that no torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B); ensuring (s103) that a gear in the second gearbox unit (4B) is engaged; ensuring (s104) that torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B); and controlling (s105) the coupling device (40) to connect the third gearbox unit (4C) to the output shaft (300).

Description

A method and an arrangement for controlling a gearbox for a vehicle standstill TECHNICAL FIELD The present invention relates to a method for controlling a gearbox, a control arrangement, a powertrain comprising such a control arrangement, a vehicle, a computer program and a computer-readable medium. The present invention more specifically relates to a method for controlling a gearbox of a vehicle powertrain at standstill, when a clutch between a propulsion unit of the powertrain and the gearbox cannot be disengaged.

BACKGROUND Powertrains may comprise a clutch arranged between the propulsion unit and the gearbox. The clutch is controllable between an engaged state, in which torque can be transferred from the propulsion unit to the gearbox, and a disengaged state in which no torque can be transferred from the propulsion unit to the gearbox. Typically, the clutch is controlled to the disengaged state when changing a gear so that the gearbox can be controlled without affecting the propulsion unit. For various reasons problems with the clutch may occur, which may result in that the clutch cannot be disengaged. This is specifically problematic if the vehicle is standing still with no gear engaged. In such situation, a so called limp home mode enabling the vehicle to be propelled to a workshop would be advantageous.

Such a limp home mode may comprise to engage a start gear before activating the propulsion unit. After that, the start motor may be activated and the start motor thus has to transfer torque to the driving wheels via the gearbox and also activate the propulsion unit. This way, it will be possible to drive the vehicle to a workshop or similar. It may, however, be very difficult to propel the vehicle and start the propulsion unit by means of the start motor, especially when the vehicle is heavily loaded. The start motor will also be subject to great loads.

Another way of achieving a limp home mode when the clutch cannot disengage may comprise to first activate the propulsion unit and then try to force engagement of the lowest gear. This solution is, however, only possible when the gearbox is unsynchronized. In an unsynchronized gearbox a gear is engaged by means of a sleeve with splines engaging with a gear wheel on a main shaft. To engage a gear when the clutch is engaged and the propulsion unit is activated the sleeve must be engaged with the gear wheel when there is a large difference in rotational speed between the main shaft and the gear wheel. The main shaft of the gearbox will be still since the vehicle is standing still but the input shaft of the gearbox and thus the gear wheel will start rotating essentially with the engine speed when the propulsion unit is activated. The difference in rotational speed between the main shaft and the crawler gear wheel may thus be about 100 rpm. With such a difference in rotational speed it is very difficult to manoeuvre the sleeve fast enough to fit in a gap between the gear wheel teeth. This solution also entails a risk of increased wear. For synchronized gearboxes this limp home solution will not be possible since the synchronization device will prevent the gear from being engaged when there is a difference in rotational speed.

SUMMARY OF THE INVENTION Despite known solutions in the field, it would be desirable to develop a method for controlling a gearbox at standstill, when the clutch cannot disengage, which overcomes or at least alleviates the drawbacks mentioned above.

An object of the present invention is therefore to achieve a new and advantageous method for controlling a gearbox of a vehicle powertrain at standstill, when a clutch between a propulsion unit of the powertrain and the gearbox cannot be disengaged, which achieves a limp home mode in an easier and more efficient way, with reduced wear of the gearbox components.

Another object of the invention is to achieve a new and advantageous control arrangement, a powertrain, a vehicle, a computer program and a computerreadable medium which achieve a limp home mode in an easier and more efficient way, with reduced wear of the gearbox components.

The herein mentioned objects are achieved by a method for controlling a gearbox of a vehicle powertrain at standstill, when a clutch between a propulsion unit of the powertrain and the gearbox cannot be disengaged, a control arrangement, a vehicle, a computer program and a computer-readable medium according to the independent claims.

Hence, according to an aspect of the present invention a method is provided for controlling a gearbox of a vehicle powertrain at standstill, when a clutch between a propulsion unit of the powertrain and the gearbox cannot be disengaged. The gearbox is arranged to selectively transfer torque between the propulsion unit and at least one driving wheel, the gearbox comprising: an input shaft connected to the clutch; an output shaft connected to the at least one driving wheel; a first gearbox unit connected to the input shaft; a second gearbox unit arranged downstream of the first gearbox unit and connectable to the first gearbox unit; and a third gearbox unit arranged downstream of the second gearbox unit connectable to the second gearbox unit and the output shaft, wherein the third gearbox unit comprises a coupling device for selectively disconnecting the third gearbox unit from the output shaft. The method comprising: - controlling the coupling device to disconnect the third gearbox unit from the output shaft; - ensuring that no torque is transferred from the first gearbox unit to the second gearbox unit; - ensuring that a gear in the second gearbox unit is engaged; - ensuring that torque is transferred from the first gearbox unit to the second gearbox unit; and - controlling the coupling device to connect the third gearbox unit to the output shaft.

When a vehicle is at standstill with no gear engaged in the gearbox and the clutch for some reason cannot be disengaged, a gear must in some way be engaged in order to achieve a limp home mode and thus in order to be able to propel the vehicle to a workshop. The clutch not being able to disengage means that it cannot open and thus that the propulsion unit and the gearbox are connected through the clutch. At standstill the output shaft connected to the driving wheels will be still. If the propulsion unit is activated the output shaft will remain still, even though the clutch is engaged and the output shaft is connected to the third gearbox unit, since no gear is engaged in the gearbox. If a gear is engaged, however, the start motor will not only have to activate the propulsion unit but it will also initially have to propel the vehicle. By disconnecting the third gearbox unit from the output shaft according to the invention the start motor will be used to start the propulsion unit, the first gearbox unit and the second gearbox unit only, not to propel the vehicle. The start motor will thereby be subject to less loads and the life of the start motor may thereby be increased.

The method may comprise to start/activate the propulsion unit prior to controlling the coupling device to disconnect the third gearbox unit from the output shaft. Alternatively, ensuring that torque is transferred from the first gearbox unit to the second gearbox unit may comprise to start/activate the propulsion unit.

It may be assumed that the gearbox is in neutral when the method according to the invention is initiated. Alternatively, the method may comprise to ensure that the second gearbox unit is in neutral, i.e. that no gear is engaged in the second gearbox unit. When the second gearbox unit is in neutral no torque can be transferred from the second gearbox unit to the third gearbox unit. The method may comprise to ensure that the second gearbox unit is in neutral after the step of controlling the coupling device to disconnect the third gearbox unit from the output shaft, and prior to the step of ensuring that no torque is transferred from the first gearbox unit to the second gearbox unit.

The first gearbox unit may constitute a split gearbox, the second gearbox unit may constitute a conventional main gearbox that can be set to a number of different forward gear ratios, and the third gearbox unit may constitute a range gearbox. The second gearbox unit is arranged downstream of the first gearbox unit and the third gearbox unit is arranged downstream of the second gearbox unit. It is to be understood that downstream relates to the position in relation to the propulsion unit and thus in relation to the transfer of torque through the powertrain.

According to an embodiment of the invention, ensuring that no torque is transferred from the first gearbox unit to the second gearbox unit comprises to control the first gearbox unit to neutral. The first gearbox may be engaged or it may be in neutral. When the first gearbox unit is engaged torque can be transferred from the first gearbox unit to the second gearbox unit and when it is in neutral torque cannot be transferred from the first gearbox unit to the second gearbox unit. How the torque may be transferred from the first gearbox unit to the second gearbox unit will be further described below. The step of controlling the first gearbox to neutral may be performed essentially simultaneously as the step of controlling the coupling device to disconnect the third gearbox unit from the output shaft.

According to an embodiment of the invention the second gearbox unit comprises a lay shaft and a main shaft, both connectable to the first gearbox unit, wherein ensuring that a gear in the second gearbox unit is engaged comprises to ensure that the speed of the lay shaft is decreased prior to engaging a gear in the second gearbox unit. The lay shaft may be arranged in parallel with the main shaft. The third gearbox unit may be connectable to the main shaft of the second gearbox unit.

The second gearbox unit may comprise gear wheels that are rotatably fixed to the lay shaft. The main shaft may comprise corresponding gear wheels which rotate freely in relation to the main shaft, but which can be selectively locked for rotation with the main shaft in order to engage a gear. Engaging a gear in the second gearbox unit comprises manoeuvring a sleeve, arranged to rotate with the main shaft, to a position where the gearwheel on the main shaft is engaged with the sleeve. Each pair of gear wheels on the lay shaft and the main shaft represents a gear ratio. When engaging a gear in the second gearbox unit the main shaft and the gear wheel on the main shaft should have synchronized rotational speeds to enable manoeuvring of the sleeve. The gear wheel on the main shaft is rotated by the corresponding gear wheel on the lay shaft and thus has the same rotational speed as the lay shaft. The main shaft and the lay shaft should thus have essentially synchronized rotational speeds when engaging a gear. If the second gearbox unit is connected to the first gearbox unit the rotational speeds of the lay shaft and the main shaft cannot be synchronized without affecting the first gearbox unit, the input shaft and thus the propulsion unit. Thus, by ensuring that no torque can be transferred from the first gearbox unit to the second gearbox unit, the second gearbox unit can be controlled to achieve similar rotational speeds of the lay shaft and the main shaft without affecting the propulsion unit. Since the vehicle is at standstill the main shaft connected to the third gearbox unit is typically still. The lay shaft may, however, rotate after having been connected to the first gearbox unit and the rotational speed of the lay shaft should therefore be decreased in order to be able to engage a gear in the second gearbox unit.

The method may comprise to ensure that the lay shaft is still prior to engaging a gear in the second gearbox unit. That the lay shaft is still, means that the lay shaft is essentially still and thus that the rotational speed of the lay shaft is essentially zero.

According to an embodiment of the invention it is ensured that the speed of the lay shaft is decreased by controlling a lay shaft brake mechanism. A lay shaft brake mechanism may be a brake arranged at the lay shaft in order to be able to actively control the rotational speed of the lay shaft. The method may thus comprise to activate said lay shaft brake mechanism so that the rotational speed of the lay shaft is decreased. Alternatively, the rotational speed of the lay shaft is decreased by the internal resistance in the gearbox.

According to an embodiment of the invention ensuring that torque is transferred from the first gearbox unit to the second gearbox unit comprises to engage the first gearbox unit. It is to be understood that engaging the first gearbox unit means to engage a gear in the first gearbox unit. After a gear has been engaged in the second gearbox unit the first gearbox unit is engaged, whereby the lay shaft and the main shaft of the second gearbox unit start rotating. The first gearbox unit suitably comprises a gear wheel rotating freely in relation to the input shaft but which can be selectively locked for rotation with the input shaft through a split sleeve. The lay shaft comprises a corresponding gear wheel rotatably fixed to the lay shaft. By locking the gear wheel on the input shaft by means of the split sleeve torque can be transferred from the input shaft to the lay shaft. The split sleeve may be provided with a split synchronising device. The split sleeve can further be used to connect the input shaft and thus the first gearbox unit to a gear wheel on the main shaft directly. The gear wheel on the main shaft may rotate freely in relation to the main shaft but can be selectively locked for rotation with the main shaft. The gear wheel on the main shaft is engaged with a corresponding gear wheel on the lay shaft. Thus, by controlling the split sleeve, such that the input shaft is directly connected to a gear wheel on the main shaft, torque can be transferred from the input shaft to the lay shaft or from the input shaft to the main shaft directly. The first gearbox unit may thus be engaged by being connected to the second gearbox unit, such that torque may be transferred directly from the input shaft to the main shaft or such that torque may be transferred from the input shaft to the lay shaft and from the lay shaft to the main shaft. The first gearbox unit may thus be arranged to provide, for each gear of the second gearbox unit, two gear steps with different gear ratios. The first gearbox unit may thus be controlled to engage a low-split gear or a high-split gear. The low-split gear has the highest gear ratio and the highsplit gear has the lowest gear ratio. Controlling the first gearbox unit to engage the low-split gear may involve controlling the split sleeve to connect the input shaft with the gear wheel on the main shaft and engaging the high-split gear may involve controlling the split sleeve to lock the gear wheel on the input shaft. Alternatively, depending on the number of teeth on the respective gear wheels, engaging the low-split gear may involve controlling the split sleeve to lock the gear wheel on the input shaft and engaging the high-split gear may involve controlling the split sleeve to connect the input shaft with the gear wheel on the main shaft. Ensuring that torque is transferred from the first gearbox unit to the second gearbox unit suitably comprises to engage the low-split gear in the first gearbox unit.

According to an embodiment of the invention the third gearbox unit comprises a planetary gear arranged to provide a high range gear and a low range gear, and wherein disconnecting the third gearbox unit from the output shaft comprises to disconnect the planetary gear from the output shaft. In a first gear position corresponding to the low range gear a lower gear ratio than 1:1 is provided in the planetary gear. In a second gear position corresponding to the high range gear the gear ratio is 1:1 in the planetary gear. The third gearbox unit is thus used to double the number of gear ratios that are available from the second gearbox unit, and also, as in this case, the number of gears available from the combination of the first gearbox unit and the second gearbox unit. In the situation associated with the method according to the invention, the third gearbox unit is suitably in the low range gear position. The method may comprise to ensure that the third gearbox unit is in the low range gear position before engaging the first gearbox unit. Alternatively, the third gearbox unit may be in a reverse gear position.

The planetary gear may comprise three components, which are rotatably arranged relative to each other, namely a sun gear wheel, a planet wheel carrier with planet gear wheels and a ring gear wheel. A number of planet gear wheels are rotatably arranged with bearings on the planet wheel carrier. The sun gear wheel is rotatably connected to the main shaft of the second gearbox unit and the planet gear wheels engage the sun gear wheel. The ring gear wheel surrounds and engages the planet gear wheels. The main shaft may be connected to the sun gear wheel by means of a splines connection. The coupling device is suitably arranged to selectively connect/disconnect the planet wheel carrier and the output shaft of the gearbox. Thus, the step of disconnecting the third gearbox unit from the output shaft may comprise to control the coupling device, such that the planet wheel carrier is disconnected from the output shaft.

According to an embodiment of the invention controlling the coupling device suitably comprises to control a shift fork to axially displace a coupling sleeve. The coupling device in the third gearbox unit may thus comprise an axially displaced coupling sleeve. The coupling sleeve may be arranged to, in a first position, connect the planet wheel carrier to the output shaft of the gearbox. In a second position the coupling sleeve may be arranged to disconnect said gearbox from said output shaft and thus the at least one drive wheel of said vehicle. The coupling sleeve may also in a third position couple the ring gear wheel with the output shaft and thus achieve a reverse gear. The coupling sleeve may, on an inner surface, comprise splines which are arranged to cooperate with corresponding cooperating splines arranged on the ring gear wheel of the planetary gear, the planet wheel carrier and the output shaft, respectively. The axial displacement of the coupling sleeve is provided with a shift fork of the coupling device. The shift fork may be arranged in engagement with an outside circumferential groove in the coupling sleeve. The shift fork may be influenced by an actuator. The actuator may be a pneumatic or hydraulic cylinder.

The coupling device is suitably unsynchronized. The coupling device is thus not controlled by means of a synchronization device and the components that are to be engaged by means of the coupling sleeve should thus be controlled in some other way to achieve similar rotational speeds. By controlling the gearbox to synchronous speed between the two components to be engaged, the axial displacement of the coupling sleeve, in order to connect the components, is facilitated. When the components should be disengaged the gearbox may be controlled, such that torque balance occurs between the components, whereby the coupling sleeve does not transfer torque. It then becomes possible to move the coupling sleeve axially in order to disengage the components from each other.

With regard to the present method, when a gear has been engaged in the second gearbox unit and the first gearbox unit has been engaged the gearbox will rotate from the input shaft to the planet wheel carrier in the third gearbox unit. The output shaft will, however, be still since the output shaft is not connected to the planet wheel carrier. The step of controlling the coupling device to connect the third gearbox unit to the output shaft may thus comprise to the control the coupling sleeve to connect the planet wheel carrier to the output shaft. The coupling sleeve is suitably only connected to the output shaft in the second position, where the third gearbox unit is disconnected from the output shaft. Since the output shaft is still and the planet wheel carrier is rotating there is a difference in rotational speed. However, since the third gearbox is in the low range gear position the difference in rotational speed will be much lower than in prior art solutions. With a relatively low rotational speed difference, it will be easier to axially displace the coupling sleeve and force engagement with the rotating planet wheel carrier. Thus, by connecting components with different rotational speed in the third gearbox unit instead of in the second gearbox unit as described in prior art solutions, a limp home mode is achieved more easily and with less risk of damaging the splines/teeth of the coupling sleeve and the planet wheel carrier.

According to an embodiment of the invention it is ensured that the lowest gear in the second gearbox unit is engaged. The gear engaged in the second gearbox unit may be the so called crawler gear. By engaging the lowest gear (the gear with the highest gear ratio) in the second gearbox unit, the difference in rotational speed between the planet wheel carrier in the third gearbox unit and the output shaft will be decreased and connecting the third gearbox unit to the output shaft will thereby become easier. In the case where the lowest gear is engaged in the second gearbox unit and the third gearbox unit is in the low range gear position or in a reverse gear position, the difference in rotational speed between the planet wheel carrier and the output shaft may be around 25 rpm. Compared to the difference of 100 rpm as described in prior art solutions the inventive method will facilitate the manoeuvring of the coupling sleeve considerably.

The method may be performed by a control arrangement (which e.g. may comprise one or more control units) of a vehicle powertrain. Such control arrangement may be adapted to control the gearbox, the clutch and the propulsion unit.

The method may be initiated by an operator of a vehicle. The method may be performed when an operator manoeuvres a manoeuvring means, such as a push button, lever or similar. Suitably, the operator manoeuvers the manoeuvring means when it has been identified that the clutch cannot disengage. Alternatively, the method is automatically performed when it has been identified that the clutch cannot be disengaged. The method may also comprise to inform the operator of the vehicle that a limp home mode has been activated.

Each of the gears in the second gearbox unit may be used for a plurality of the total number of gears provided by the gearbox as a whole. For example, a first gear of the second gearbox unit may be used for first and second gear of the gearbox, low and high split, low range, and also for seventh and eighth gear, low and high split, high range, in a manner well known to a person skilled in the art.

According to an aspect of the invention a control arrangement is provided for controlling a gearbox of a vehicle powertrain at standstill, when a clutch between a propulsion unit of the powertrain and the gearbox cannot be disengaged. The gearbox is arranged to selectively transfer torque between the propulsion unit and at least one driving wheel. The gearbox comprising: an input shaft connected to the clutch; an output shaft connected to the at least one driving wheel; a first gearbox unit connected to the input shaft; a second gearbox unit arranged downstream of the first gearbox unit connectable to the first gearbox unit; and a third gearbox unit arranged downstream of the second gearbox unit connectable to the second gearbox unit and the output shaft, wherein the third gearbox unit comprises a coupling device for selectively disconnecting the third gearbox unit from the output shaft. The control arrangement comprising: - means for controlling the coupling device to disconnect the third gearbox unit from the output shaft; - means for ensuring that no torque is transferred from the first gearbox unit to the second gearbox unit; - means for ensuring that a gear in the second gearbox unit is engaged; - means for ensuring that torque is transferred from the first gearbox unit to the second gearbox unit; and - means for controlling the coupling device to connect the third gearbox unit to the output shaft.

It will be appreciated that all the embodiments described for the method aspect of the invention are also applicable to the control arrangement aspect of the invention. That is, the control arrangement may be configured to perform any one of the steps of the method according to various embodiments described herein.

The means for controlling the coupling device to disconnect the third gearbox unit from the output shaft; the means for ensuring that no torque is transferred from the first gearbox unit to the second gearbox unit; the means for ensuring that a gear in the second gearbox unit is engaged; the means for ensuring that torque is transferred from the first gearbox unit to the second gearbox unit; and the means for controlling the coupling device to connect the third gearbox unit to the output shaft, may e.g. be different software modules/portions in the control arrangement, program code or similar.

According to an aspect of the invention a vehicle powertrain is provided, the powertrain comprising: a propulsion unit; a gearbox; and a clutch arranged between the propulsion unit and the gearbox, wherein the gearbox is arranged to selectively transfer torque between the propulsion unit and at least one driving wheel. The gearbox comprising: an input shaft connected to the clutch; an output shaft connected to the at least one driving wheel; a first gearbox unit connected to the input shaft; a second gearbox unit arranged downstream of the first gearbox unit connectable to the first gearbox unit; and a third gearbox unit arranged downstream of the second gearbox unit connectable to the second gearbox unit and the output shaft, wherein the third gearbox unit comprises a coupling device for selectively disconnecting the third gearbox unit from the output shaft. The powertrain further comprises a control arrangement comprising: means for controlling the coupling device to disconnect the third gearbox unit from the output shaft; means for ensuring that no torque is transferred from the first gearbox unit to the second gearbox unit; means for ensuring that a gear in the second gearbox unit is engaged; means for ensuring that torque is transferred from the first gearbox unit to the second gearbox unit; and means for controlling the coupling device to connect the third gearbox unit to the output shaft.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following details, and also by putting the invention into practice. Whereas embodiments of the invention are described below, it should be noted that it is not restricted to the specific details described. Specialists having access to the teachings herein will recognise further applications, modifications and incorporations within other fields, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS For fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Figure 1 schematically illustrates a vehicle according to an embodiment of the invention; Figure 2 schematically illustrates a vehicle powertrain according to an embodiment of the invention; Figure 3 schematically illustrates a vehicle powertrain according to an embodiment of the invention; Figure 4 illustrates a flow chart for a method for controlling a gearbox according to an embodiment of the invention; and Figure 5 schematically illustrates a control arrangement or computer according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 schematically illustrates a vehiclel according to an embodiment of the invention. The vehicle 1 includes a powertrain 3 comprising a propulsion unit 2, such as an internal combustion engine, a gearbox 4 and clutch (not shown) arranged between the propulsion unit 2 and the gearbox 4. The gearbox 4 is connected to the driving wheels 8 of the vehicle 1 via an output shaft of the gearbox 4. The gearbox 4 of the present invention may comprise a first gearbox unit 4A, a second gearbox unit 4B arranged downstream of the first gearbox unit 4A, and a third gearbox unit 4C arranged downstream of the second gearbox unit 4B. Herein the term upstream refers to a position closer to the propulsion unit 2, and the term downstream refers to a position closer to the driving wheels 8.

Fig. 2 schematically illustrates a powertrain 3 according to an embodiment of the invention. The powertrain 3 comprises a propulsion unit 2, a gearbox 4 and a clutch 11 arranged between the propulsion unit 2 and the gearbox 4. The gearbox 4 comprises an input shaft 200 connected to the clutch 11 and an output shaft 300 connected to the driving wheels 8. The gearbox 4 further comprises a first gearbox unit 4A connected to the input shaft 200; a second gearbox unit 4B arranged downstream of the first gearbox unit 4A and connectable to the first gearbox unit 4A; and a third gearbox unit 4C arranged downstream of the second gearbox unit 4B and connectable to the second gearbox unit 4B and the output shaft 300. The first gearbox unit 4A may constitute a split gearbox. The second gearbox unit 4B may constitute a conventional main gearbox that can be set to a number of different forward gear ratios. The third gearbox unit 4C may constitute a range gearbox. The third gearbox unit 4C comprises a coupling device (not shown) for selectively disconnecting the third gearbox unit 4C from the output shaft 300. The third gearbox unit 4C is discussed further with reference to Figure 3.

The second gearbox unit 4B comprises a lay shaft 202 with gear wheels 203A, 204A, 205A that are rotatably fixed to the lay shaft 202. For example, gear wheel 203A may represent the first gear, gear wheel 204A the second gear and gear wheel 205A the third gear. The second gearbox unit 4B also comprises a main shaft 206 with corresponding gear wheels 203B, 204B, 205B which rotate freely in relation to the main shaft 206, but which can be selectively locked for rotation with the main shaft 206 in order to engage a gear. When the gear wheels 203B, 204B, 205B rotate freely in relation to the main shaft 206, the second gearbox unit 4B is in neutral. The gear wheels 203B, 204B, 205B on the main shaft 206 are suitably locked by means of sleeves 207, 208, 210. For example, the first gear in the second gearbox unit 4B can be engaged by manoeuvring a first sleeve 207, arranged to rotate with the main shaft 206, to a position where the gear wheel 203B is engaged, i.e. to the left in the figure. The gear wheel 203B will thereby rotate with the main shaft 206 and the lay shaft 202 will thereby be connected to the main shaft 206 via gear wheel 203A. Each pair of gear wheels on the lay shaft 202 and main shaft 206 represents a gear ratio. The second gear in the second gearbox unit 4B may be engaged by disengaging the first sleeve 207 from gearwheel 203B and instead moving a second sleeve 208 to a position to the right in the figure where, instead, gear wheel 204B is engaged. The gear wheel 204B is thereby brought into rotation with the main shaft 206. Correspondingly, the third gear in the second gearbox unit 4B may be engaged by manoeuvring the second sleeve 208 to a position to the left in the figure where, instead, gear wheel 205B is engaged. Each of the first through third gears in the second gearbox unit 4B is used for a plurality of the total number of gears provided by the gearbox 4 as a whole. The second gearbox unit 4B may further comprise a reverse gear (not shown) and a crawler gear (not shown).

The lay shaft 202 further comprises an additional gear wheel 209A that, similar to the above, is rotatably fixed to the lay shaft 202. The first gearbox unit 4A comprises a corresponding gear wheel 209B rotating freely in relation to the input shaft 200, but which can be selectively locked for rotation with the input shaft 200 through a split sleeve 210. When the split sleeve 210 locks the gear wheel 209B on the input shaft 200, torque can be transferred to the lay shaft 202 via the corresponding gear wheel 209A on the lay shaft 202. The split sleeve 210 may be provided with a split synchronising unit. The split sleeve 210 can further be used to connect the input shaft 200 to the gear wheel 205B of the second gearbox unit 4B directly. This way, depending on whether the gear wheel 205B on the main shaft 206 is rotating freely in relation to the main shaft 206 or if it is locked on the main shaft 206, torque can be transferred to the lay shaft 202 via the corresponding gear wheel 205A on the lay shaft 202 or torque can be transferred from the input shaft 200 directly to the main shaft 206. The gear wheel pair 209A-B and the split sleeve 210 can thereby be used to provide two different split gear ratios for each gear of the second gearbox unit 4B. The first gearbox unit 4A may thus be controlled to engage a high-split gear or a low-split gear. For example, engaging the low-split gear may comprise to connect the input shaft 200 with the gear wheel 205B on the main shaft 206 by means of the split sleeve 210. When e.g. the first gear is engaged in the second gearbox unit 4B, the split sleeve 210 may be arranged to engage gear wheel 205B. This way, the input shaft 200 is directly connected to gear wheel 205B, which via gear wheel 205A establishes a first gear ratio between the input shaft 200 and the lay shaft 202. The gear wheel 205B, however, is not locked to the main shaft 206, but the lay shaft 202 may be connected to the main shaft 206 through gear wheel pair 203A-B. To engage the second gear, gear wheel pair 209A-B is instead engaged, resulting in a second gear ratio between an input shaft 201 and lay shaft 202. The gear wheel 203B is still engaged by the first main sleeve 207 according to the above, thereby extending the range of each gear. This split can be performed for each gear of the second gearbox unit 4B.

The lay shaft 202 may be provided with a shaft brake mechanism 212, such as a lay shaft brake, in order to control the rotational speed of the lay shaft 202.

Figure 3 schematically illustrates a powertrain 3 according to an embodiment of the invention. The powertrain 3 may be configured as described in Figure 1 and in Figure 2. The powertrain 3 thus comprises a propulsion unit 2, a clutch 11 and a gearbox 4. The gearbox 4 comprises an input shaft 200 connected to the clutch 11 and an output shaft 300 connected to the driving wheels 8. The gearbox 4 further comprises a first gearbox unit 4A connected to the input shaft 200; a second gearbox unit 4B arranged downstream of the first gearbox unit 4A and connectable to the first gearbox unit 4A; and a third gearbox unit 4C arranged downstream of the second gearbox unit 4B and connectable to the second gearbox unit 4B and the output shaft 300. The third gearbox unit 4C comprises a planetary gear 14 which has a low and a high gear. The third gearbox unit 4C may thus be controlled between a high range gear position and a low range gear position. In the low range gear position a lower gear ratio than 1:1 is provided in the planetary gear 14. In the high range gear position the gear ratio is 1:1 in the planetary gear 14. Figure 3 shows the third gearbox unit 4C in the low range gear position.

The third gearbox unit 4C may be accommodated in a gearbox housing 12 and comprises an input shaft, which may be the main shaft 206 of the second gearbox unit 4B. The planetary gear 14 comprises three main components which are rotatably arranged in relation to each other, namely a sun gear wheel 18, a planet wheel carrier 20 and a ring gear wheel 22. A number of planet gear wheels 24 are rotatably arranged with bearings on the planet wheel carrier 20. With knowledge of the number of teeth 32 of sun gear wheel 18 and the ring gear wheel 22, the relative gear ratio of the three components can be determined. The sun gear wheel 18 is rotatably connected to the main shaft 206 of the second gearbox unit 4B and the planet gear wheels 24 engage the sun gear wheel 18. The ring gear wheel 22 surrounds and engages the planet gear wheels 24. The teeth 32 of the sun gear wheel 18, the planet gear wheels 24 and the ring gear wheel 22 may be bevelled, so that they have an angle relative to a common axis of rotation 30 of the sun gear wheel 18, the planet gear carrier 20 and the ring gear wheel 22. The main shaft 206 of the second gearbox unit 4B may be connected to a shaft 38 of the sun gear wheel 18 by means of a splines connection 34.

The third gearbox unit 4C may comprise a first axially displaceable coupling sleeve 42. The first coupling sleeve 42 may be arranged to, in a first position, connect the gearbox housing 12 with the ring gear wheel 22. This first position corresponds to the low range gear position. The first coupling sleeve 42 may be arranged to, in a second position, disconnect the gearbox housing 12 from the ring gear wheel 22 and to connect the main shaft 206 and the planet wheel carrier 20, such that the main shaft 206 and the planet wheel carrier 20 rotate with similar rotational speeds.

The third gearbox unit 4C also comprises a coupling device 40 for selectively disconnecting the third gearbox unit 4C from the output shaft 300. The coupling device 40 may comprise a second axially displaceable coupling sleeve 43, which in a first position connects the planet wheel carrier 20 with the output shaft 300, and in a second position disconnects the planet wheel carrier 20 from the output shaft 300. Thus, in the first position the second axially displaceable coupling sleeve 43 may be arranged to connect said gearbox 4 to the driving wheels 8 of said vehicle 1. In the second position, the second coupling sleeve 43 is only connected to the output shaft 300 and not to the ring gear wheel 22 or the planet wheel carrier 20. In Figure 3, the coupling device 40 is in the second position where the planet wheel carrier 20, and thus the third gearbox unit 4C, is disconnected from the output shaft 300. Thus, in this figure, no torque may be transferred from the gearbox 4 to the output shaft 300 and the driving wheels 8.

The second axially displaceable coupling sleeve 43 may also, in a third position, connect the ring gear wheel 22 with the output shaft 300 and thus achieve a reverse gear. In the third position, corresponding to the reverse gear, the first coupling sleeve 42 is suitably arranged to connect the planet wheel carrier 20 with the gearbox housing 12.

The first coupling sleeve 42 may be provided with first splines 50 on an inner periphery of the sleeve 42 and second splines 51 on an outer periphery of the sleeve 42. The first splines 50 arranged on the inner periphery of the sleeve 42 may interact with corresponding first cooperating splines 50’ arranged on the shaft 38 of the sun gear wheel 18. The corresponding first cooperating splines 50’ disposed on the shaft 38 of the sun gear wheel 18 may be provided on the periphery of a first sprocket 46, which is mounted on the shaft 38 of the sun gear wheel 18. The first splines 50 on the inner periphery of the sleeve 42 may also be arranged to cooperate with corresponding first cooperating splines 50” arranged on the planet wheel carrier 20. The corresponding first cooperating splines 50” disposed on the planet wheel carrier 20 may be provided on the periphery of a second sprocket 44, which is mounted on the planet wheel carrier 20. The second splines 51 arranged on the an outer periphery of the sleeve 42 may interact with corresponding second cooperating splines 51’ arranged on a projection 52, which is fixedly connected to the gearbox housing 12.

The second axially displaceable coupling sleeve 43 of the coupling device 40 may comprise third splines 59 on an inner surface, which splines 59 are arranged to cooperate with corresponding third cooperating splines 59’, 59”, 59’” arranged on the ring gear wheel 22, the planet wheel carrier 20 and the output shaft 300, respectively. The corresponding third cooperating splines 59” arranged on the planet wheel carrier 20 may be formed on the periphery of a third sprocket 49, which is mounted on the planet wheel carrier 20. The corresponding third cooperating splines 59’” provided on the output shaft 300 may be formed on the periphery of a fourth sprocket 53, which is mounted on the output shaft 300.

The axial displacement of the first and second coupling sleeves 42, 43 may be provided with a first and second shift fork 60, 61 arranged in an outside circumferential groove 62 in the respective coupling sleeve 42, 43. The first shift fork 60 may be actuated by a first actuator 66 and the second shift fork 61 may be actuated by a second actuator 67. The first and second actuator 66, 67 may be a pneumatic or hydraulic cylinder.

The powertrain 3 may further comprise a control arrangement, such as an electronic control unit 70, connected to the gearbox 4, the propulsion unit 2 and the clutch 11. A computer 72 may be connected to the control unit 70. The control unit 70 may be a transmission control unit, engine control unit or any other control unit of a vehicle. The control arrangement 70 may comprise a plurality of different control units. The control unit 70 may comprise means for controlling the coupling device 40 to disconnect the third gearbox unit 4C from the output shaft 300. The control unit 70 may be adapted to control the gearbox 4 when the clutch 11 cannot be disengaged, such that a limp home mode is achieved. The control unit 70 may comprise means for ensuring that no torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B. The control unit 70 may comprise means for ensuring that a gear in the second gearbox unit 4B is engaged. The control unit 70 may comprise means for ensuring that torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B. The control unit 70 may comprise means for controlling the coupling device 40 to connect the third gearbox unit 4C to the output shaft 300.

Figure 4 illustrates a flow chart for a method for controlling a gearbox 4 of a vehicle powertrain 3 at standstill, when a clutch 11 between a propulsion unit 2 of the powertrain 3 and the gearbox 4 cannot be disengaged according to an embodiment of the invention. The vehicle powertrain 3 and the gearbox 4 may be configured as disclosed in Figure 2 and 3. The method comprises: controlling s101 the coupling device 40 to disconnect the third gearbox unit 4C from the output shaft 300; ensuring s102 that no torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B; ensuring s103 that a gear in the second gearbox unit 4B is engaged; ensuring s104 that torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B; and controlling s105 the coupling device 40 to connect the third gearbox unit 4C to the output shaft 300.

The method may be performed by means of the control unit 70 of the powertrain 3. The method may be referred to as a method for controlling a gearbox 4 for achieving a limp home mode when the clutch 11 cannot disengage.

Controlling s101 the coupling device 40 to disconnect the third gearbox unit 4C from the output shaft 300 may comprise to disconnect the planetary gear 14 of the third gearbox unit 4C from the output shaft 300. Controlling s101 the coupling device 40 to disconnect the third gearbox unit 4C from the output shaft 300 may comprise to control the coupling device 40, such that the planet wheel carrier 20 is disconnected from the output shaft 300.

Controlling s101 the coupling device 40 to disconnect the third gearbox unit 4C from the output shaft 300 may comprise to control a shift fork 61 to axially displace a coupling sleeve 43. The shift fork 61 may be controlled to displace the coupling sleeve 43, such that the planet wheel carrier 20 is disconnected from the output shaft 300.

When the method is initiated the third gearbox unit 4C is suitably in the low range gear position. The method may alternatively comprise to ensure that the third gearbox unit 4C is in the low range position before ensuring s104 that torque can be transferred from the first gearbox unit 4A to the second gearbox unit 4B. Alternatively, the third gearbox unit 4C may be in a reverse gear position.

The method may also comprise to ensure that the second gearbox unit 4B is in neutral, i.e. that no gear is engaged in the second gearbox unit 4B. The method may comprise to ensure that the second gearbox unit 4B is in neutral prior to ensuring s102 that no torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B.

Ensuring s102 that no torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B may comprise to control the first gearbox unit 4A to neutral. Ensuring s102 that no torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B may thus comprise to control the split sleeve 210, such that the input shaft 200 is neither connected to the gear wheel 205B on the main shaft 206 nor to the gear wheel 209A on the lay shaft 202 via gear wheel 209B on the input shaft 200.

Engaging a gear in the second gearbox unit 4B may comprise to control a sleeve 207, 208, 210 arranged to rotate with the main shaft 206, to a position where a gear wheel 203B, 204B, 205B, on the main shaft 206 is engaged with the sleeve 207, 208, 210. When engaging a gear in the second gearbox 4B the main shaft 206 and the gear wheel 203B, 204B, 205B on the main shaft 206 should have synchronized rotational speeds to enable manoeuvring of the sleeve 207, 208, 210. The gear wheel 203B, 204B, 205B on the main shaft 206 is rotated by the corresponding gear wheel 203A, 204A, 205A on the lay shaft 202 and thus has the same rotational speed as the lay shaft 202. The main shaft 206 and the lay shaft 202 should thus be controlled, such that the main shaft 206 and the gear wheel 203B, 204B, 205B on the main shaft 206 have essentially synchronized rotational speeds when engaging a gear. Since the vehicle 1 is at standstill the main shaft 206 connected to the third gearbox unit 4C is typically still. The lay shaft 202 may, however, rotate after having been connected to the first gearbox unit 4A. Ensuring s103 that a gear in the second gearbox unit 4B is engaged may thus comprise to ensure that the speed of the lay shaft 202 is decreased prior to engaging a gear in the second gearbox unit 4B. Ensuring s103 that a gear in the second gearbox unit 4B is engaged may comprise to ensure that the lay shaft 202 is essentially still prior to engaging a gear in the second gearbox unit 4B. This way, the rotational speeds of the lay shaft 202 and the main shaft 206 are synchronized and a gear in the second gearbox unit 4B can be engaged.

The speed of the lay shaft 202 may be decreased by controlling a lay shaft brake mechanism 212. The method may thus comprise to activate said lay shaft brake mechanism 212 so that the rotational speed of the lay shaft 202 is decreased. Alternatively, the rotational speed of the lay shaft 202 is decreased by the internal resistance in the gearbox 4.

Ensuring s104 that torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B may comprise to engage the first gearbox unit 4A. Ensuring s104 that torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B may thus comprise to control the split sleeve 210 to connect the first gearbox unit 4A with the second gearbox unit 4B. The first gearbox unit 4A may be controlled to engage a low-split gear or a high split-gear. The first gearbox unit 4A may be engaged by controlling the split sleeve 210 to connect the input shaft 200 and the lay shaft 202 or to connect the input shaft 200 and the main shaft 206. The split sleeve 210 may be controlled to connect the input shaft 200 with the gear wheel 205B on the main shaft 206. The gear wheel 205B may be rotating freely in relation to the main shaft 206, whereby the input shaft 200 is connected to the lay shaft 202 via the corresponding gear wheel 205A on the lay shaft 202. This may correspond to the low-split gear or the high-split gear depending on the gear ratio of the gear wheel pair 205B/205A. If the gear wheel 205B is locked on the main shaft 206 the input shaft 200 is connected directly to the main shaft 206. Alternatively, ensuring s104 that torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B may comprise to control the split sleeve 210, such that the gear wheel 209B on the input shaft 200 is locked for rotation with the input shaft 200. This way, the input shaft 200 is connected to the lay shaft 202 via the corresponding gear wheel 209A on the lay shaft 202. This may correspond to the low-split gear or the highsplit gear depending on the gear ratio of the gear wheel pair 209B/209A.

Controlling s105 the coupling device 40 to connect the third gearbox unit 4C to the output shaft 300 may comprise to the control the coupling sleeve 43 to connect the planet wheel carrier 20 to the output shaft 300. The coupling sleeve 43 of the coupling device 40 may be controlled, such that the third splines 59 of the coupling sleeve 43 engages with the corresponding third cooperating splines 59”, 59”’ on the planet wheel carrier 20 and the output shaft 300. Since the output shaft 300 is still and the planet wheel carrier 20 is rotating there is a difference in rotational speed. The coupling sleeve 43 is thus forced into engagement with the planet wheel carrier 20. However, since the third gearbox unit 4C is in the low range gear position the difference in rotational speed will be much lower than described in prior art solutions. With a relatively low rotational speed difference, it will be easier to axially displace the coupling sleeve 43 and force engagement with the rotating planet wheel carrier 20.

Ensuring s103 that a gear in the second gearbox unit 4B is engaged may comprise to engage the lowest gear in the second gearbox unit 4B. The method may comprise to engage the so called crawler gear in the second gearbox unit 4B. By engaging the lowest gear (the gear with the highest gear ratio) in the second gearbox unit 4B, the difference in rotational speed between the planet wheel carrier 20 in the third gearbox unit 4C and the output shaft 300 will be decreased and connecting the third gearbox unit 4C to the output shaft 300 will thereby become easier.

Ensuring s104 that torque is transferred from the first gearbox unit 4A to the second gearbox unit 4B may comprise to engage the low-split gear in the first gearbox unit 4A. The low-split gear is the lowest gear in the first gearbox unit 4A and thus the gear with the highest gear ratio. By engaging the low-split gear in the first gearbox unit 4A, the difference in rotational speed between the planet wheel carrier 20 in the third gearbox unit 4C and the output shaft 300 will be decreased and connecting the third gearbox unit 4C to the output shaft 300 will thereby become easier.

The method may be initiated by an operator of a vehicle 1. The method may be performed when an operator manoeuvres a manoeuvring means, such as a push button, lever or similar. Suitably, the operator manoeuvers the manoeuvring means when it has been identified that the clutch 11 cannot disengage. Alternatively, the method is automatically performed when it has been identified that the clutch 11 cannot be disengaged. The method may also comprise to inform the operator of the vehicle 1 that a limp home mode has been activated.

Figure 5 is a diagram of a version of a device 500. The control unit 70 and/or computer 72 described with reference to Figure 2 and 3 may in a version comprise the device 500. The term “link” refers herein to a communication link which may be a physical connection such as an optoelectronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link. The device 500 comprises a non-volatile memory 520, a data processing unit 510 and a read/write memory 550. The non-volatile memory 520 has a first memory element 530 in which a computer programme, e.g. an operating system, is stored for controlling the function of the device 500. The device 500 further comprises a bus controller, a serial communication port, I/O means, an A/D converter, a time and date input and transfer unit, an event counter and an interruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540.

There is provided a computer programme P which comprises routines for controlling a gearbox of a vehicle powertrain. The computer programme P comprises routines for controlling the coupling device to disconnect the third gearbox unit from the output shaft. The computer programme P comprises routines for ensuring that no torque is transferred from the first gearbox unit to the second gearbox unit. The computer programme P comprises routines for ensuring that a gear in the second gearbox unit is engaged. The computer programme P comprises routines for ensuring that torque is transferred from the first gearbox unit to the second gearbox unit. The computer programme P comprises routines for controlling the coupling sleeve to connect the third gearbox unit to the output shaft.

The programme P may be stored in an executable form or in a compressed form in a memory 560 and/or in a read/write memory 550.

Where the data processing unit 510 is described as performing a certain function, it means that the data processing unit 510 effects a certain part of the programme stored in the memory 560 or a certain part of the programme stored in the read/write memory 550.

The data processing device 510 can communicate with a data port 599 via a data bus 515. The non-volatile memory 520 is intended for communication with the data processing unit 510 via a data bus 512. The separate memory 560 is intended to communicate with the data processing unit 510 via a data bus 511. The read/write memory 550 is adapted to communicating with the data processing unit 510 via a data bus 514.

When data are received on the data port 599, they are stored temporarily in the second memory element 540. When input data received have been temporarily stored, the data processing unit 510 is prepared to effect code execution as described above.

Parts of the methods herein described may be effected by the device 500 by means of the data processing unit 510 which runs the programme stored in the memory 560 or the read/write memory 550. When the device 500 runs the programme, methods herein described are executed.

The foregoing description of the preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for specialists to understand the invention for various embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for controlling a gearbox (4) of a vehicle powertrain (3) at standstill, when a clutch (11) between a propulsion unit (2) of the powertrain (3) and the gearbox (4) cannot be disengaged, the gearbox (4) being arranged to selectively transfer torque between the propulsion unit (2) and at least one driving wheel (8), the gearbox (4) comprising: an input shaft (200) connected to the clutch (11); an output shaft (300) connected to the at least one driving wheel (8); a first gearbox unit (4A) connected to the input shaft (200); a second gearbox unit (4B) arranged downstream of the first gearbox unit (4A) and connectable to the first gearbox unit (4A); and a third gearbox unit (4C) arranged downstream of the second gearbox unit (4B) and connectable to the second gearbox unit (4B) and the output shaft (300), wherein the third gearbox unit (4C) comprises a coupling device (40) for selectively disconnecting the third gearbox unit (4C) from the output shaft (300), the method comprising: - controlling (s101) the coupling device (40) to disconnect the third gearbox unit (4C) from the output shaft (300); - ensuring (s102) that no torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B); - ensuring (s103) that a gear in the second gearbox unit (4B) is engaged; - ensuring (s104) that torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B); and - controlling (s105) the coupling device (40) to connect the third gearbox unit (4C) to the output shaft (300).

2. The method according to claim 1, wherein ensuring (s102) that no torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B) comprises to control the first gearbox unit (4A) to neutral.

3. The method according to claim 1 or 2, wherein the second gearbox unit (4B) comprises a lay shaft (202) and a main shaft (206), both connectable to the first gearbox unit (4A), and wherein ensuring (s103) that a gear in the second gearbox unit (4B) is engaged comprises to ensure that the speed of the lay shaft (202) is decreased prior to engaging a gear in the second gearbox unit (4B).

4. The method according to claim 3, wherein it is ensured that the lay shaft (202) is still prior to engaging a gear in the second gearbox unit (4B).

5. The method according to claim 3 or 4, wherein it is ensured that the speed of the lay shaft (202) is decreased by controlling a lay shaft brake mechanism (212).

6. The method according to any one of the preceding claims, wherein it is ensured (s103) that the lowest gear in the second gearbox unit (4B) is engaged.

7. The method according to any one of the preceding claims, wherein ensuring (s104) that torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B) comprises to engage the first gearbox unit (4A).

8. The method according to any one of the preceding claims, wherein controlling (s105) the coupling device (40) comprises to control a shift fork (61) to axially displace a coupling sleeve (43).

9. The method according to any one of the preceding claims, wherein the third gearbox unit (4C) comprises a planetary gear (14) arranged to provide a high range gear and a low range gear, and wherein disconnecting the third gearbox unit (4C) from the output shaft (300) comprises to disconnect the planetary gear (14) from the output shaft (300).

10. A computer program (P), wherein said computer program comprises programme code for causing a control arrangement (70; 500) or a computer (72; 500) connected to the control arrangement (70; 500) to perform the method according to any one of the preceding claims.

11. A computer-readable medium comprising instructions, which when executed by a control arrangement (70; 500) or a computer (72; 500) connected to the control arrangement (70; 500), cause the control arrangement (70; 500) or the computer (72; 500) to perform the method according to any one of claims 1-9.

12. A control arrangement (70) adapted to control a gearbox (4) of a vehicle powertrain (3) at standstill, when a clutch (11) between a propulsion unit (2) of the powertrain (3) and the gearbox (4) cannot be disengaged, the gearbox (4) being arranged to selectively transfer torque between the propulsion unit (2) and at least one driving wheel (8), the gearbox (4) comprising: an input shaft (200) connected to the clutch (11); an output shaft (300) connected to the at least one driving wheel (8); a first gearbox unit (4A) connected to the input shaft (200); a second gearbox unit (4B) arranged downstream of the first gearbox unit (4A) and connectable to the first gearbox unit (4A); and a third gearbox unit (4C) arranged downstream of the second gearbox unit (4B) and connectable to the second gearbox unit (4B) and the output shaft (300), wherein the third gearbox unit (4C) comprises a coupling device (40) for selectively disconnecting the third gearbox unit (4C) from the output shaft (300), the control arrangement (70) comprising: - means for controlling the coupling device (40) to disconnect the third gearbox unit (4C) from the output shaft (300) when the vehicle powertrain is at standstill and the clutch cannot be disengaged; - means for ensuring that no torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B) when the vehicle powertrain is at standstill and the clutch cannot be disengaged; - means for ensuring that a gear in the second gearbox unit (4B) is engaged when the vehicle powertrain is at standstill and the clutch cannot be disengaged; - means for ensuring that torque is transferred from the first gearbox unit (4A) to the second gearbox unit (4B) when the vehicle powertrain is at standstill and the clutch cannot be disengaged; and - means for controlling the coupling device (40) to connect the third gearbox unit (4C) to the output shaft (300) when the vehicle powertrain is at standstill and the clutch cannot be disengaged.

13. A vehicle powertrain (3), comprising: a propulsion unit (2); a gearbox (4); and a clutch (11) arranged between the propulsion unit (2) and the gearbox (4), wherein the gearbox (4) is arranged to selectively transfer torque between the propulsion unit (2) and at least one driving wheel (8), the gearbox (4) comprising: an input shaft (202) connected to the clutch (11); an output shaft (300) connected to the at least one driving wheel (8); a first gearbox unit (4A) connected to the input shaft (200); a second gearbox unit (4B) arranged downstream of the first gearbox unit (4A) connectable to the first gearbox unit (4A); and a third gearbox unit (4C) arranged downstream of the second gearbox unit (4B) connectable to the second gearbox unit (4B) and the output shaft (300), wherein the third gearbox unit (4C) comprises a coupling device (4) for selectively disconnecting the third gearbox unit (4C) from the output shaft (300), wherein the powertrain (3) further comprises a control arrangement (70) according to claim 12.

14. A vehicle (1) comprising a powertrain according to claim 13.