Method and system for controlling a vehicle

Abstract

A method for driving a vehicle when the vehicle starts on a running surface, the vehicle having at least one driven wheel for driving the vehicle and comprising an electric motor configured to generate a torque for driving the driven wheels, the method comprising the steps of: a) selection by a driver of the vehicle of a preferred direction in which the vehicle is to start, b) using the electric motor to transmit a torque that drives the driven wheels in a first direction, c) Determining a representative value for the adhesion of the driven wheels to the running surface, and d) reversing the direction of rotation of the electric motor to transmit a torque that drives the driven wheels in the opposite direction to the first direction, if the representative value for the adhesion of the driven wheels to the running surface meets a first criterion, where the representative value for the adhesion to the tread is a speed of the driven wheels, the first criterion comprises a determination of whether the rotational speed of the driven wheels has reached a first rotational speed and / or whether a rotational speed difference between the driven wheels and at least one non-driven wheel provided on the vehicle exceeds a first value, steps b) and c) are repeated at increased torque, and preventing the vehicle from being moved by more than a predetermined amount in a direction opposite to the preferred direction in which the vehicle is intended to start.

Description

Field of the invention

[0001] The present invention relates to a system for controlling a vehicle during start-up under adverse tread conditions. In particular, the present invention relates to a method according to the features of the preamble of claim 1. The invention also relates to a system according to claim 8 and a vehicle according to claim 9. Background of the invention

[0002] It is well known that vehicles in general, and trucks in particular, can get stuck when starting from a standstill in situations with unfavorable road or tread conditions. Such unfavorable tread conditions can include snow or soft treads such as sand, earth, etc.

[0003] When attempting to move a vehicle in such conditions, in situations where one or more of the driven wheels is stuck in a depression or rut, the engine torque applied to move the wheels will typically cause the wheels to spin before they clear the depression / rut. This deepens the depression / rut and makes it even more difficult to extricate the vehicle.

[0004] When such situations occur, one or more "tried and tested" methods are often used to free the vehicle. For example, the vehicle's air suspension system can be used and / or the tag axle can be raised to increase the load on the driven wheels, thereby making it easier for the wheels to maintain grip on the tread. Another method often used in the case of such cavities / ruts is a rocking method, whereby the driver attempts to free the vehicle by increasing the power applied to the driven wheels, preferably with a relatively high gear engaged, until the wheels spin freely. The driver then immediately stops accelerating and, if applicable, disengages the clutch, allowing the vehicle to roll backward. This process is then repeated when movement in the opposite direction ceases.Continuous rocking of the vehicle in this manner can result in a force large enough to move the vehicle out of the cavity / rut.

[0005] The probability of success with such a rocking method varies from driver to driver and with the driver's experience. For this reason, systems have been developed in which the rocking function is automated. An example of such a function is described in document WO 2004 / 098940 A1 (Volvo Lastvagnar AB). This document describes the automatic control of a vehicle drive system for use when starting off under adverse tread conditions. The drive force is applied via the vehicle's driveline to at least one driven wheel until a condition such as wheel spin is detected, whereupon the application of the drive force is stopped and effected in the opposite direction.

[0006] The function described in the above-mentioned document has the advantage that the driving force for application to the vehicle's driven wheels for rocking purposes can act not only in one direction, but in both directions, in an attempt to improve the chances of being able to drive the vehicle away. However, the described solution has the disadvantage that changing the direction in which the driving force is applied involves first disconnecting the vehicle's driveline and then changing gears to apply the driving force in the opposite direction. Although such a method works well in principle, disconnecting the driveline, subsequently changing gears, and reconnecting the driveline are relatively time-consuming processes. As a result, the change in the direction in which the driving force is applied often occurs too late to take full advantage of it.

[0007] There is therefore a need, at least in certain situations, for an improved method for freeing a stuck vehicle.

[0008] For example, a vehicle with a hybrid drive is known from US 2004 / 0 263 099 A1. In this case, each wheel can be driven by an electric motor in two directions, and an algorithm and corresponding control system allow these directions to be changed immediately when skidding is detected. This allows the vehicle to swing up, for example, to free the tires from a snow-covered surface.

[0009] In addition, US 2007 / 0 074 918 A1 discloses a possibility of swinging a vehicle with electrically driven wheels by moving the vehicle back and forth.

[0010] From DE 10 2006 061 391 A1, a slip control system for a vehicle is known in which a spinning wheel is detected and the torque of the drive motor is then reduced.

[0011] DE 11 2004 001 387 T5 relates to a vehicle control method in which a spinning wheel is braked by a mechanical braking device. If a predetermined temperature increase is detected during braking, braking is terminated. This prevents overheating of the mechanical braking device.

[0012] DE 100 26 102 A1 relates to a method for detecting uphill or downhill driving of a motor vehicle, wherein wheel slip is detected by comparing the individual wheel speed values ​​with a reference value.

[0013] DE 10 2005 023 247 A1 discloses a method for controlling the driving operation of a motor vehicle, in which rotation sensors provided in the drive train detect certain driving conditions, generating control commands or warning signals based on these. Instead of passive rotational speed sensors, active rotational speed sensors are used. A so-called rocking-free control function is also disclosed, whereby a reverse movement of the vehicle is automatically initiated when it is determined that the vehicle is stationary. Summary of the invention

[0014] It is an object of the present invention to propose a method for controlling a vehicle during start-up that solves the problem described above. This object is achieved by a method according to claim 1 and a system according to claim 8.

[0015] The present invention relates to a method for propelling a vehicle during start-up, wherein the vehicle has at least one driven wheel for propelling the vehicle and comprises an electric motor configured to generate a drive torque acting on the driven wheel. The electric motor is used to apply a drive torque to the driven wheel in a first direction, a representative value for the grip of the wheel on the tread is determined, and the direction of rotation of the motor is reversed to apply a drive torque to the wheel in a direction opposite to the first direction if the representative value for the grip of the wheel on the tread meets a first criterion.

[0016] The present invention has the advantage that, by using an electric motor to generate the drive torque, for example, to effect the rocking function described above, it is possible, unlike the prior art, to reverse the torque acting on the wheel very quickly when at least one of the vehicle's driven wheels loses its grip on the running surface, which can be determined, for example, by detecting wheel spin / a sharp increase in the speed of the wheel(s). In this case, the spinning wheel(s) can be quickly stopped and the drive torque applied in the opposite direction to assist the vehicle's movement in the opposite direction, thereby increasing the rocking, resulting in a higher probability of freeing the vehicle.The present invention therefore makes it possible for a driver to extricate a vehicle from a situation that would otherwise require extrication or some other form of assistance, such as attempting to place something under the wheels. The invention has the further advantage of eliminating the need for additional new components in vehicles with existing electric motor systems.

[0017] The steps described above, which involve reversing the direction of the moment, may be repeated until the process is stopped, for example, by a driver of the vehicle or until the vehicle's control system determines that the vehicle has been freed and has actually started moving.

[0018] According to one embodiment, the electric motor is connected to the driven wheel via a transmission. The present invention has the advantage that even when the motor is connected to the wheel via the transmission, the transmission can still be operated in both directions, meaning that the same gear can be used when carrying out the method according to the present invention, thus eliminating the time required to disengage one gear and engage another.

[0019] The invention is suitable for parallel hybrid vehicles and other vehicle types that can be powered by an electric motor. For example, the invention can be used in series hybrid vehicles and in vehicles in which one or more electric motors directly drive the vehicle's drive wheels without intermediate gears, such as in pure electric vehicles.

[0020] Further features of the present invention and advantages thereof will become apparent from the following detailed description of exemplary embodiments and the accompanying figures. Short description of the characters Fig. 1 shows a drive train of a hybrid vehicle with which the present invention can be advantageously used. Fig. Figure 2 illustrates an example of a method according to the present invention. Fig. 3 illustrates an example of a rocking method according to the present invention. Detailed description of exemplary embodiments

[0021] As described above, with automated rocker functions, it is difficult to disengage an automatic clutch at the right time, i.e., precisely when the wheels begin to spin. When the vehicle's driven wheels begin to spin, traction with the tread is lost, and if the wheels have passed through a cavity / rut on the way out, the vehicle will move in the opposite direction, i.e., back into the cavity or rut. In this situation, it is desirable to be able to quickly engage a gear for moving in the opposite direction, so that the gravitational effect that occurs as the wheels roll back down into the cavity can be taken advantage of and used to exit the cavity in an opposite direction.The separation of the engine output shaft from the gearbox input shaft, which is involved in a gear change, means that such a solution takes a lot of time and makes optimal function impossible.

[0022] In contrast, the present invention does not cause this problem. The invention described below will first be described with reference to a parallel hybrid system, and the rocking function according to the present invention makes it possible to disconnect an internal combustion engine, for example by means of a conventional clutch, from the input shaft of the transmission, so that only the electric motor is used to drive the driven wheels during the rocking process. The direction and speed of an electric motor are controlled by the frequency of the voltage supplying it. The use of an electric motor therefore offers the advantage that, unlike an internal combustion engine, the electric motor can rotate in both directions.According to the present invention, this achieves that, by reversing the direction of rotation of the electric motor, in principle any desired gear can be engaged, since the driven wheels can be driven in both directions with the same engaged gear. According to the present invention, therefore, there is no need for a gear change and thus no need to waste time on such a gear change. The present invention therefore makes it possible, at least in principle, to switch the drive torque directly from forward to reverse (or vice versa).

[0023] Fig. 1 illustrates a powertrain in a hybrid vehicle 100 according to a first embodiment of the present invention. There are various types of hybrid vehicles, and the illustrated vehicle is a parallel hybrid vehicle.

[0024] The drive train of the Fig. The parallel hybrid vehicle shown in Figure 1 comprises an internal combustion engine 101, which is connected in a conventional manner to a transmission 103 via an output shaft 102 of the internal combustion engine 101 and a friction element (clutch) 106. The vehicle further comprises drive shafts 104, 105, which are connected to the driven wheels 113, 114 of the vehicle and which, as in a conventional internal combustion engine system, are driven by an output shaft 107 of the transmission via an axle drive, which may, for example, be a conventional differential 108.

[0025] Unlike conventional vehicles, the Fig. 1 also includes an electric motor 110 connected to an input shaft 109 of the transmission 103 "downstream" of the clutch 106. The parallel hybrid vehicle can therefore simultaneously transmit power to the drive wheels 113, 114 from two separate drive sources, i.e., both the internal combustion engine 101 and the electric motor 110. Alternatively, the vehicle can be driven by one or the other drive source, i.e., either the internal combustion engine 101 or the electric motor 110.

[0026] As those skilled in the art will appreciate, there are various types of electric motors that can be used in hybrid vehicles. Accordingly, the electric motor 110 can be of any desired, suitable type. The motor used as an example in the description below is a three-phase motor, and the power supply is a three-phase supply. Three-phase motors can be so-called asynchronous or synchronous types, with synchronous motors having the advantage of allowing exact determination of the speed. In vehicle applications, it is naturally preferable to be able to vary the speed of the driven wheels, and thus that of the electric motor, in addition to the changes achieved by conventional gear changes via the transmission. Therefore, the motor must conventionally be speed-controlled. The speed of the electric motor is generally controlled by the frequency of the supply voltage driving the motor.The motor's speed is directly proportional to this frequency. Speed ​​control of an electric motor therefore involves changing the voltage supplied to the motor. In the case of an AC motor, this means that the frequency of the AC power supply must be variable.

[0027] The Fig. The motor 110 shown in Figure 1 is therefore driven by a three-phase variable frequency supply generated by using a power electronics device 111 which, in the case of hybrid vehicles, can also be used for various other functions not described in detail here, in a known manner.

[0028] The power electronics device 111 cooperates with an energy storage device 112, for example one or more batteries, supercapacitors, etc., wherein the energy storage device must be chargeable in various ways, for example by regenerative braking by means of the electric motor 110 and / or by plugging into an external energy source, for example a conventional electricity grid.

[0029] The illustrated power electronics device 111 may be of the type conventionally used in hybrid vehicles and will therefore not be described in detail. In general, however, it can be stated that in the case of AC motors, the power electronics device 111 converts the direct current from the energy storage device 112 into alternating current. The conversion is effected by a converter, which may, for example, comprise a number of IGBTs (Insulated Gate Bipolar Transistors), which, through suitable switching, can provide a three-phase voltage of a preferred and variable amplitude and frequency for driving the electric motor 110 and thus the driven wheels 113, 114 of the vehicle.

[0030] The electric motor 110 can be used at speeds of a vehicle in a range from 0 to maximum speed by controlling the frequency supplied to the motor 110 from 0 Hz to a frequency that results in the maximum speed of the vehicle (or of the internal combustion engine in the case of joint operation with the internal combustion engine).

[0031] Fig. Figure 1 also illustrates a control unit 115 that can be used to control the method according to the present invention. The control unit 115 forms part of the vehicle's control system. Vehicle control systems in modern vehicles conventionally comprise a communication bus system formed by one or more communication buses for connecting to various electronic control units and components arranged in the vehicle. Signals to / from, for example, an electric motor control unit / power electronics and other vehicle functions used in the method according to the invention can, for example, be routed to / from the control unit 115 through suitable communication buses. The vehicle control unit 115 can, for example, be an existing control unit. In this case, the present invention can be integrated easily and at low cost.

[0032] An embodiment according to the present invention will be described below with reference to Fig. 2, in which a rocking method is generally designated 200. The method begins with step 201, which determines whether the rocking function is activated. This can be determined in various ways. For example, after determining that the vehicle is stuck, the driver can activate the function using a button or lever, or by another means, such as a function that can be selected via a display.

[0033] The function can also be configured to be activated automatically, for example, if the vehicle control system detects wheel spin. However, for safety reasons, it may be preferable for the function to be activated manually. The invention will therefore be described below with reference to manual activation.

[0034] If it has been determined that a rocking function should be initiated, the method continues with step 202, in which it is determined whether the function should actually be executed. This determination is not necessary, but preferred to prevent "independent execution" of the vehicle immediately after activation of the rocking function. For example, execution can be subordinate to the driver's actuation of the accelerator pedal or the indication by another means, such as by means of a compliant button or lever, so that the rocking function is actively initiated. This can also be combined with the driver selecting a preferred direction (forward or reverse) in which the vehicle should preferably start off, in order to support the desired continuous drive.

[0035] This is desirable, for example, in situations where continuous operation is only possible in one direction, for example, due to obstacles in front of or behind the vehicle. The driver can, for example, indicate the preferred direction of travel by using the gear selector to select a forward or reverse direction. Whatever their choice, the control system then selects an appropriate gear in the transmission, which is subsequently used during the rocking function.

[0036] If it is detected in step 202 that the rocking function is actually being executed, the method continues with step 203 (for safety reasons, the rocking function can advantageously be designed to stop immediately when the driver releases the accelerator pedal and / or presses the brake). In step 203, a suitable gear is selected for use during the rocking function. This gear will normally be one of the lower gears of the transmission. Whether this gear is intended to drive the vehicle forward or reverse makes no difference to the method, but can be crucial for the continued movement of the vehicle once it has been freed from the cavity / rut.For example, it may be advantageous that the gear selected by the rocking function can also be used to continuously propel the vehicle, so that there is no risk of re-stalling due to an unavoidable interruption of drive torque that occurs when changing gears and which, at low speeds, can result in the vehicle stopping and leading to re-stalling.

[0037] Therefore, it may be advantageous to select a gear to propel the vehicle in the direction indicated by the driver. Another parameter to be considered when selecting the gear for use in the method according to the invention is that the gear selection must result in the desired control characteristics being achieved, i.e., the selected gear, together with the electric motor, must be capable of achieving the preferred speed of the driven wheels (this selection can be controlled at least in part based on the torque of the electric motor relative to the torque of the internal combustion engine, and the larger the electric motor, the greater the degree of freedom in gear selection).

[0038] If a suitable gear was selected in step 203, the method continues with step 204, where a torque resulting in movement in a preferred direction is generated by the electric motor. Using the power electronics (controlling the voltage and frequency of the motor's power supply), the motor speed can be controlled from standstill, while simultaneously controlling the torque provided by the motor from 0 Nm to the maximum torque. The motor's control devices make it possible for the speed and torque to be provided in an increasingly continuous or ramp-like manner.

[0039] It must be considered that as long as the applied torque is lower than the torque required to move the vehicle's driven wheels, the electric motor shaft will remain stationary, regardless of the applied frequency. A possible control method therefore involves increasing the torque provided by the motor at low speed until the shaft begins to move, after which the motor's rotation speed progressively increases once the vehicle's driven wheel(s) have begun to move.The increase in rotational speed may continue until the representation of the adhesion of the driven wheel(s) to the running surface meets a first criterion, for example when a loss of adhesion is detected during wheel spin, whereupon the method continues with step 205, or alternatively until the rocking has ceased, whereupon the method continues with step 206. In parallel with the increase in rotational speed, as in . Fig. 3, the torque can also be increased progressively. Wheel spin can be detected, for example, by determining the rotational speed of the electric motor. If the rotational speed increases rapidly, this means that the rotational speed of the driven wheel or wheels is also increasing rapidly. This is determined by comparing the rotational speed of the driven wheels with the rotational speed of the non-driven wheels. It is determined whether a difference in the rotational speed of the driven wheels and at least one non-driven wheel of the vehicle exceeds a first value.

[0040] Alternatively or additionally, wheel spin is detected by determining whether the speed of the driven wheels has reached a first speed.

[0041] Fig. Figure 3 shows a graph of torque and wheel speed / electric motor speed during an example of rocking operation. It should be noted that the Fig. 3 is very schematic and the actual process can differ significantly, for example depending on the choice of control principles and how quickly the driven wheels of the vehicle are freely rotating. The build-up process starts at time T=0 and the torque of the electric motor increases until its shaft (and thereby also the driven wheels of the vehicle) starts to rotate at T=T1. Between time T=T1 and time T=T2 the speed of the wheels increases, while at the same time the torque increases for at least part of the time range. The increase in torque is preferably torque-limited, i.e. the torque only increases up to a predetermined level in order to prevent wheelspin from occurring too quickly. The speed achieved with the corresponding torque is then maintained until time T=T3, at which time a rapid increase in speed begins.Continuously monitoring the speed of the electric motor and / or the driven wheels makes it possible, in principle, to directly detect that the speed is increasing. Continuously monitoring the speed of the electric motor and / or the driven wheels can be achieved in several ways, some of which are described below. The increase in speed that begins at time T=T3 indicates wheel spin. In this case, the direction of rotation of the motor is reversed at T=T4, step 205, in principle directly using the three-phase voltage generated by the power electronics.For example, by changing the phase sequence of the three phases driving the electric motor, it is in principle possible to instantly reverse the direction of the torque flux generated in the motor, which flux drives the motor's rotor, whereupon the spinning of the driven wheels is very quickly braked to begin rotating in the opposite direction at time T=T5. Since the torque acting on the driven wheels can, in principle, be instantly reversed, the electric motor can advantageously drive the driven wheels in the opposite direction, thereby assisting the acceleration of the vehicle in the opposite direction as it begins to slide back into the cavity / rut. The method returns to step 204 after reversing the direction of rotation.Subsequently, when wheel spin is detected again in the opposite direction (step 204), the motor torque is reversed again at T=T6, thereby achieving continuous drive of the specified wheels, which is beneficial for freeing the vehicle from the cavity / rut.

[0042] As shown in the diagram, the maximum torque applied to the driven wheels can be progressively increased as the rocking process continues.

[0043] Determining the torque provided by the electric motor, while simultaneously determining the speed of the driven wheels / engine, makes it possible to use the increase in torque along with the increase in speed to determine whether the vehicle has cleared the cavity or rut and can therefore continue its journey. This is exemplified at time T=T7, at which the speed of the driven wheels exceeds a level N1, while at the same time, despite the applied engine torque, the speed increases so slowly that propulsion continues. This can be determined, for example, by determining a derivative of the speed.If the derivative exceeds a threshold, this can be interpreted as wheelspin, whereas if the derivative is below a certain threshold, the vehicle's driven wheels can be considered to be driving the vehicle. The threshold does not need to be fixed throughout operation, but can depend, for example, on the torque provided by the electric motor at that time. The higher the torque, the greater the relative speed increase that is possible without being considered wheelspin.

[0044] When the vehicle speed has reached a certain level, for example, speed N1, the control system may decide to stop the rocking function, step 206, and resume normal travel. If this is the case and the vehicle is moving normally forward (if that direction was selected by the driver, for example, by placing the gear selector in a "D" position, or reverse if the gear selector was placed in the "R" position), the clutch is closed and the vehicle now moves forward powered by the diesel engine. The diesel engine preferentially runs during rocking operation, so it can quickly be used for continuous propulsion once the vehicle has been freed.

[0045] As described above, it can be disadvantageous to effect a gear change once the vehicle has started moving. Therefore, the control system prevents any gear change until a certain speed is reached, with the aim of preventing further stopping due to the torque gap unavoidable during gear changes. In one embodiment, the disengagement of the automatic transmission can be controlled by the driver, for example, by preventing a gear change while they are depressing the accelerator pedal, so that a gear change is only effected when they determine that the gear change can be made without risking further stopping and therefore release the accelerator pedal.

[0046] The present invention therefore offers the advantage of enabling very fast control processes as a result of operating a conventional transmission in both directions. Therefore, there is no need for time-consuming gear changes. Since the direction of the torque provided by the electric motor can, in principle, be reversed immediately when wheel spin is detected, very precise torque control is possible, thus providing a good boost function.

[0047] In the Fig.In the control method shown in Figure 3, speed control is combined with torque limiting. However, the above control can also be carried out by monitoring the speed of the driven wheels / electric motor alone. Torque limiting can still be used to ensure that the driven wheels do not spin freely at an undesirable rate. The above rocking function can also advantageously be combined with automatic application of any other devices available when the vehicle is stuck. For example, existing differential locks can be automatically engaged (or a message can be generated for the driver to prompt him / her to engage the differential locks). The automatic gear change function can also be separated as described above.

[0048] Wheel spin can be detected in several different ways. For example, a sensor can be placed on the transmission output shaft to provide signals representing the output shaft speed. Alternatively, one or more wheel speed sensors can be used (for example, all of the vehicle's wheel speed sensors can be used for the most accurate detection). A speed sensor can also be placed on the electric motor shaft. It is also possible to detect the motor speed by superimposing the high-frequency signals from its power supply. The superimposed high-frequency signal can then be used not only to detect the motor speed but also to detect its state. This enables precise detection of when the driven wheels begin to rotate.This allows for very good torque control based on the speed of the driven wheels. It may also be advantageous to use signals from some or all of the above sensors to achieve the most reliable determination possible.

[0049] It is, of course, possible that the vehicle will be released from the cavity / rut in the "wrong" direction. The software controlling the function can therefore be equipped with a locking device that prevents, for example, any movement greater than a maximum of 30 cm in the wrong direction. The function can also be combined with clear warnings and acoustic / light signals to indicate reversing, as the vehicle will also move backward during the rocking function, regardless of the direction in which it ultimately escapes.

[0050] The invention has been described above with reference to a parallel hybrid vehicle, but can equally be used with all other types of vehicles in which an electric motor can be used for propulsion. For example, the invention can be used in a series hybrid vehicle in which the internal combustion engine is used only to drive a generator, which charges an energy storage device used to drive the electric motor to drive the driven wheels. The electric motor in series hybrid solutions is supplied with motive power in a similar manner to that described above, so that the above principles are equally applicable to this type of vehicle, with the only difference being that no gear needs to be selected if, as is often the case, the electric motor in a series hybrid vehicle is directly connected to a final drive gear, for example, a differential.

[0051] The invention is also applicable in vehicles in which one or more electric motors drive the driven wheels of the vehicle directly without intermediate transmissions, i.e. pure electric vehicles.

Claims

A method for driving a vehicle when the vehicle starts moving on a running surface, the vehicle having at least one driven wheel for driving the vehicle and comprising an electric motor configured to generate a torque for driving the driven wheels, the method comprising the steps of: a) selecting a preferred direction in which the vehicle is to start moving by a driver of the vehicle, b) using the electric motor to transmit a torque that drives the driven wheels in a first direction, c) determining a representative value for the adhesion of the driven wheels to the running surface, and d) reversing the direction of rotation of the electric motor to transmit a torque that drives the driven wheels in the opposite direction to the first direction if the representative value for the adhesion of the driven wheels to the running surface meets a first criterion,wherein the representative value for the grip on the tread is a rotational speed of the driven wheels, the first criterion comprises a determination of whether the rotational speed of the driven wheels has reached a first speed and / or whether a rotational speed difference between the driven wheels and at least one non-driven wheel provided on the vehicle exceeds a first value, steps b) and c) are repeated at increased torque, and the vehicle is prevented from being moved by more than a predetermined value in a direction opposite to the preferred direction in which the vehicle is to start. The method of claim 1, wherein the first criterion comprises determining whether the driven wheels lose their grip on the running surface. Method according to claim 1 or 2, wherein the representative value for the rotational speed of the driven wheels comprises various parameter values ​​which represent the rotational speeds of the driven wheels at different points in time. Method according to claim 3, wherein the criterion comprises determining a change in the rotational speed of the driven wheels between two successive points in time. Method according to one of claims 1 to 4, wherein steps c) and d) are repeated until a second criterion is met. The method of claim 5, wherein the second criterion comprises one of the following:the method is stopped by the driver of the vehicle;a control unit provided in the vehicle detects that repeating steps c) and d) is not necessary for continuously driving the vehicle. Method according to one of claims 1 to 6, wherein the electric motor is connected to the driven wheels via a transmission. A system for propelling a vehicle when the vehicle starts moving, the vehicle having at least one driven wheel for propelling the vehicle and comprising an electric motor configured to generate torque for driving the driven wheels, the system comprising means for:- selecting, by a driver of the vehicle, a preferred direction in which the vehicle is to start moving;- using the electric motor to transmit torque for driving the driven wheels in a first direction;- determining a representative value for an adhesion of the driven wheels to the running surface, and- reversing the direction of rotation of the electric motor to transmit a torque that drives the driven wheels in a direction opposite to the first direction if the adhesion of the driven wheels to the running surface meets a first criterion,- preventing the vehicle from being moved by more than a predetermined value in a direction opposite to the preferred direction in which the vehicle is to start;wherein the representative value for the grip on the tread is a rotational speed of the driven wheels, the first criterion comprises a determination of whether the rotational speed of the driven wheels has reached a first rotational speed and / or whether a rotational speed difference between the driven wheels and at least one non-driven wheel provided on the vehicle exceeds a first value, and the system is configured to repeat the determination of a representative value and the reversal of the direction of rotation of the electric motor at increased torque.; Vehicle, characterized in that it comprises a system according to claim 8.

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