Field of the invention

The present invention relates to vehicles, and in particular to a method and system for controlling a change of gear ratio in a vehicle. The present invention also relates to a vehicle, as well as a computer program and a computer program product that implement the method according to the invention.

Background of the invention There exist various kinds of vehicle powertrains . For example, vehicle transmissions can be of an automatic kind, where a vehicle control system completely controls gear ratio changing operations. The gearboxes being used in these systems can, for example, include conventional automatic transmissions

comprising torque converters, but may also include automated manual transmissions.

Automatic gear shifting in commercial (heavy) vehicles, in particular long-haulage vehicles, often includes a control system controlled gearshift of manual gearboxes, i.e.

automatic control of gearboxes comprising a discrete number of fixed gear ratios (i.e. gears) . During operation of vehicles of this kind, the gearbox is often controlled by the vehicle control system in a manner in which the gear ratio is selected in dependence of the current driving conditions. In general, it is often desirable to propel the vehicle using as high a gear as possible (i.e. the lowest possible gear ratio given the circumstances) in order to reduce the

rotational speed of the power source, oftentimes a combustion engine, thereby reducing fuel consumption. Consequently, when selecting gear ratio, fuel consumption is often a matter of concern. However, the vehicle should, in general, also be driven using a gear ratio that is capable of propelling the vehicle in a desired manner. This is in general the case, but when the vehicle is accelerating or decelerating, it is often required that the gearbox of the vehicle is set to a gear ratio that is suitable, or capable, of providing enough power to propel the vehicle without undesired retardation, e.g. when going uphill. Oftentimes it is also desirable that change of gear is not performed in situations where a change of gear may have a negative effect on the propulsion of the vehicle.

Summary of the invention

It is an object of the present invention to provide a method and system that improves control of changes of gear ratio, in particular in situations when a change of gear ratio may adversely affect the propulsion of the vehicle, e.g. by reducing the ability of a vehicle to overcome an uphill section of road. This object is achieved by a method according to claim 1.

According to the present invention, there is provided a method for controlling change of gear ratio in a gearbox of a vehicle, said gearbox being arranged for transmitting power from a power source to at least one drive wheel of said vehicle, said gearbox comprising a plurality of selectable gear ratios, change of gear ratio being controlled by a vehicle control system, the vehicle further including a driver manoeuvrable means for requesting power from said power source, the method comprising steps of:

- detecting a reduced grip of at least one drive wheel of said vehicle ,

- determining whether driver manoeuvring of said driver manoeuvrable means for requesting power indicates a reduced grip, and - influencing a gear ratio changing operation in said gearbox if driver manoeuvring of said driver manoeuvrable means for requesting power indicates a reduced grip.

With regard to vehicles in general, gearboxes of the kind that historically has been used in vehicles having a manual

transmission, but where a change of gear is performed

automatically by means of the vehicle control system, are presently often used. This applies, e.g., both to heavy vehicles and passenger cars. Such gearboxes are usually arranged to comprise a discrete number of distinct gear ratios (gears), where the gear ratios of gearboxes in heavy vehicles often are distributed such that one or more of the lowest gear ratios (highest gears) are capable of propelling the vehicle at a constant speed when the vehicle is travelling on

substantially level ground.

When vehicles of this kind, at least when being heavily loaded, are travelling uphill, a change of gear to a higher gear ratio (lower gear) is often required in order to provide enough power to properly overtake the uphill section of road in a desired manner. In particular, when travelling uphill, there is often a minimum gear ratio below which the vehicle will lose speed due to lack of power for propelling the vehicle. This imposes requirements on the manner in which gears are selected when a change of gear is to be performed. If a change of gear is not performed at least to the highest gear (the lowest gear ratio) where this criterion is

fulfilled, the vehicle will continue to decelerate. There exists strategies for performing such selection of gear to avoid situations in which the vehicle loses speed to a too high extent, and change of gear is often performed when one or more criteria are fulfilled. For example, a change of gear to a lower gear (higher gear ratio) can be arranged to be performed when it is determined that the rotational speed of the combustion engine following the change of gear will not exceed some predetermined calibrated speed limit that is not to be exceeded. There are, however, situations which may complicate a change of gear. The present invention relates in particular to situations where it is detected that at least one drive wheel of the vehicle suffers from a reduced grip, i.e. friction between at least one drive wheel (tyre) and the surface upon which the vehicle is travelling may not be sufficient to transfer the required force to propel the vehicle. The reduced grip may e.g. be due to slippery conditions, such as due to snow, ice and/or loose gravel. Detection of reduced grip can be performed, for example, by detecting speed of rotation of the vehicle drive wheels, and when one or more wheels start spinning or rotating at speeds exceeding speeds of non-driven wheels of the vehicle, or other drive wheels, by more than a predetermined extent it can be determined that grip is reduced and one or more wheels are slipping.

A change of gear in such situations, in particular when the vehicle is going uphill, may be detrimental to the capability of the vehicle to actually climb the uphill section of road. For example, if a change of gear is performed when one or more drive wheels are spinning, or when the drive wheels of the vehicle barely exhibit sufficient grip, the interruption in propelling force on the drive wheels during a change of gear, and the associated increase in propelling force following the change of gear, may often render the grip being lost

altogether. This may have the result that the vehicle loses speed, thereby possibly requiring further changes of gear, which in the end may result in the vehicle standing still in an uphill section of slippery road. For this reason, a change of gear may be suspended, prohibited, when wheel slip is detected and one or more wheels are spinning to reduce the risk of situations of this kind arising. On the other hand, once wheel slip no longer is detected, changes of gear are again controlled according to the general criteria that prevailed prior to the detection of wheel spin. It has been realized, however, that the mere fact that the vehicle drive wheels no longer are spinning may not be sufficient to guarantee that slippery conditions no longer prevail, and hence situations of the above kind may still arise. Also, if change of gear is suspended as soon as a wheel slip is detected, changes of gear may be suspended in

situations where the reduction in grip is highly temporary and the suspended change of gear in reality is not motivated.

According to the present invention, gear ratio changing operations may be influenced in situations where a reduced grip of at least one drive wheel is detected, but only in dependence of the actually prevailing conditions.

According to the invention, when a reduced grip is detected, where this detection may comprise a step of detecting wheel slip of at least one drive wheel of said vehicle, this alone is not sufficient to suspend or otherwise influence a change of gear. Instead, according to the invention, it is also determined whether driver manoeuvring of said driver

manoeuvrable means for requesting power also indicates a reduced grip. That is, driver behaviour is taken into account to determine whether the detected wheel slip can be considered temporary, and no further action needs be taken, or whether it is likely that slippery conditions still prevail. Hence, when driver manoeuvring of the driver manoeuvrable means for requesting power also indicates a reduced grip, gear ratio changing operations are influenced. According to the invention, gear changing operations are influenced only when driver manoeuvring of said driver

manoeuvrable means for requesting power has indicated a reduced grip. That is, it can be determined from the manner in which the driver is manoeuvring the driver manoeuvrable means for requesting power whether it can be presumed that driving conditions actually are slippery, and that change of gear should be influenced accordingly. For example, if wheel slip is detected and the driver in connection to this, such as concurrently or thereafter, reduces the request for a

propelling power, e.g. by at least to some extent releasing an accelerator pedal, this can be used as a confirmation that conditions are still slippery and that a change of gear, if possible, should be avoided. At least, a change of gear to a higher gear (lower gear ratio) should be avoided in situations of this kind, since e.g. a possible loss in vehicle speed due to the slippery conditions may substantially immediately require a further change of gear to a lower gear, the gear changing operation only resulting in the vehicle ability of climbing the uphill section of road being reduced.

With regard to driver manoeuvring of the means for requesting power, the step of determining whether driver manoeuvring of said driver manoeuvrable means for requesting power indicates a reduced grip may not only comprise the step of detecting a request for reduction of power from said power source.

Instead, it can also be determined e.g. a manner in which request for power is following some pattern, such as e.g. that the driver first reduces the request for power from the power source, e.g. rapidly, to be followed by a comparatively slower increase in the request for power, which can be arranged to be interpreted as driver manoeuvring of the means for requesting power in an attempt to search a highest possible tractive force. According to embodiments of the invention, it is determined whether following a detection of a decrease in requested power from said power source, it is detected an increase in requested power from said power source to a request for less power than requested before said decrease in requested power, in which case gear ratio changing operations are influenced according to the invention.

According to the invention, gear ratio changing operations may be arranged to be influenced during at least a predetermined period of time from when a reduced grip of said at least one drive wheel no longer is detected, or from when driver

manoeuvring of the driver manoeuvrable means indicates slippery conditions. That is, change of gear can be arranged to be influenced at least for some suitable period of time, to thereby avoid situations of the vehicle again losing grip if change of gear is allowed too soon following a detection of reduced grip.

The step of influencing gear ratio changing operations can also be arranged to be terminated when driver manoeuvring of said means for requesting power no longer indicates a reduced grip, such as e.g. if the driver requests a high power from the power source, which may be taken as an indication that the driver believes the conditions to no longer be slippery.

Furthermore, according to embodiments of the invention, it is not required that wheel slip is detected while simultaneously driver manoeuvring of the means for requesting power of the power source also indicates slippery conditions. Instead, it can be determined whether the driver manoeuvring of the means for requesting power indicates slippery conditions within a predetermined period of time before and/or after the beginning and/or end of wheel spin, and if this is the case change of gear can be arranged to be influenced according to the

invention once such indication is detected.

Furthermore, the driver manoeuvrable means for requesting power from the power source may e.g. comprise an accelerator pedal or other driver manoeuvrable means such as a lever or knob for regulating the power delivered by the vehicle power source. The power source, in turn, may comprise an internal combustion engine, one or more electrical machines or a combination thereof. Change of gear can be influenced in various ways. For example, change of gear ratio can be delayed or suspended. Also, change of gear ratio to a lower gear ratio can be delayed or

suspended, while, according to embodiments of the invention, change of gear ratio to a higher gear ratio may still be allowed to occur without being influenced, i.e. only a change of gear ratio to a lower gear ratios is delayed or suspended.

Change of gear can be delayed, for example, by changing the dependency of the rotational speed of the power source when changing gear, so that propulsion on a current gear can be maintained for longer periods of time by allowing the internal combustion engine to operate at higher speeds of rotation prior to changing gear to a higher gear (lower gear ratio), or by allowing the internal combustion engine to reach lower speeds of rotation prior to changing gear to a lower gear (higher gear ratio) than the limitations prevailing when commencing influence according to the invention. For example, the vehicle may be forced to maintain propulsion on a current gear for as long as the rotational speed of the power source does not fall below a minimum speed limit. Change of gear may also be inhibited altogether for as long as continued

propulsion on a current gear is possible. According to embodiments of the invention it is also required that the driver takes at least one further action. For

example, in addition, it may be required that the driver changes load on the drive shaft, e.g. in order to increase the pressure of the drive shaft in an attempt to improve grip. In addition, or alternatively, it may be required that the driver e.g. locks a differential to thereby indicate that slippery conditions in fact are prevailing.

Further characteristics of embodiments of the present

invention and advantages thereof are indicated in the detailed description of exemplary embodiments set out below and the attached drawings.

Brief description of the drawings

Fig. 1A illustrates a powertrain of an exemplary vehicle; Fig. IB illustrates an example of a control unit in a vehicle control system;

Fig. 2 illustrates an exemplary method according to an

embodiment of the present invention;

Fig. 3 illustrates an exemplary manner of influencing change of gear ratio.

Detailed description of exemplary embodiments

Fig. 1A schematically depicts a powertrain of an exemplary vehicle 100. The powertrain comprises a power source, in the present example a combustion engine 101, which, in a

conventional manner, is connected via an output shaft of the combustion engine 101, normally via a flywheel 102, to a gearbox 103 via a clutch 106. An output shaft 107 from the gearbox 103 propels drive wheels 113, 114 via a final drive 108, such as a common differential, and drive axles 104, 105 connected to said final drive 108. The combustion engine 101 is controlled by the vehicle control system via a control unit 115. The clutch 106 and gearbox 103 are also controlled by the vehicle control system by means of a control unit 116. The gearbox 103 can be set to a plurality of distinct

(discrete) gear ratios, where a suitable gear ratio is

selected for each driving situation by means of the vehicle control system.

In general, a change of gear is performed when the combustion engine 101 speed of rotation reaches a speed where conditions for a change of gear are fulfilled. When changing gear to a lower gear (higher gear ratio) a condition for performing a change of gear may be that the speed of the combustion engine following a completed change of gear is not allowed to exceed some speed limit which often is a calibrated maximum engine speed that is not to be exceeded. This maximum calibrated engine speed can be different for different gears and/or vehicle speeds. As a consequence, a change of gear to a lower gear is, in general, performed as soon as it is determined that the speed of the combustion engine on the new gear will not exceed said speed limit. Alternatively, or in addition, the speed limits may be set for speed of rotation on a current gear .

As was discussed above, there exist situations where change of gear solely according to criteria of this kind may have a negative impact on the propulsion of the vehicle, and

embodiments of the invention provide a method for controlling change of gear ratio in a manner that e.g. may facilitate a vehicle to climb an uphill section of road when the grip between the vehicle drive wheels and the surface upon which the vehicle is travelling is reduced. An exemplary method 200 according to embodiments of the present invention is shown in fig. 2, which method can be implemented at least partly e.g. in the control unit 116 for controlling the clutch 106 and gearbox 103. As indicated above, the functions of a vehicle are, in general, controlled by a number of control units, and control systems in vehicles of the disclosed kind generally comprise a communication bus system including one or more communication buses for

connecting a number of electronic control units (ECUs), or controllers, to various components on board the vehicle. Such a control system may comprise a large number of control units, and the control of a specific function may be divided between two or more of them.

For the sake of simplicity, Fig. 1A depicts only control units 115-116, but vehicles 100 of the illustrated kind are often provided with significantly more control units, as one skilled in the art will appreciate. Control units 115-116 are arranged to communicate with one another and various components via said communication bus system and other wiring, partly

indicated by interconnecting lines in fig. 1A.

Embodiments of the present invention can be implemented in any suitable control unit in the vehicle 100 and hence not

necessarily in the control unit 115. The control of the gearbox 103 according to embodiments of the present invention will usually depend on signals being received from other control units and/or vehicle components, and it is generally the case that control units of the disclosed type are normally adapted to receive sensor signals from various parts of the vehicle 100. The control unit 116 will, for example, receive signals from the engine control unit 115, where such signals may include signals regarding engine operation as well as driver requests for power, and hence driver manoeuvring of e.g. an accelerator pedal. The control unit 116 will also deliver signals to the control unit 115, e.g. to control engine speed and delivered power during a change of gear. The control unit 116 will also receive signals, e.g. from one or more other control units, relating to a detected wheel slip.

Control units of the illustrated type are also usually adapted to deliver control signals to various parts and components of the vehicle, e.g. to control the clutch 106 and gearbox 103.

Control of this kind is often accomplished by programmed instructions. The programmed instructions typically include a computer program which, when executed in a computer or control unit, causes the computer/control unit to exercise the desired control, such as method steps according to embodiments of the present invention. The computer program usually constitutes a part of a computer program product, where said computer program product comprises a suitable storage medium 121 (see Fig. IB) with the computer program 126 stored on said storage medium 121. The computer program can be stored in a nonvolatile manner on said storage medium. The digital storage medium 121 can, for example, include any of the group

comprising: ROM (Read-Only Memory), PROM (Programmable Readonly Memory) , EPROM (Erasable PROM) , Flash memory, EEPROM (Electrically Erasable PROM), a hard disk unit etc., and be arranged in or in connection with the control unit, whereupon the computer program is executed by the control unit. The behaviour of the vehicle in a specific situation can thus be adapted by modifying the instructions of the computer program.

An exemplary control unit (the control unit 116) is shown schematically in Fig. IB, wherein the control unit can

comprise a processing unit 120, which can include, for

example, any suitable type of processor or microcomputer, such as a circuit for digital signal processing (Digital Signal Processor, DSP) or a circuit with a predetermined specific function (Application Specific Integrated Circuit, ASIC) . The processing unit 120 is connected to a memory unit 121, which provides the processing unit 120, with e.g. the stored program code 126 and/or the stored data that the processing unit 120 requires to be able to perform calculations. The processing unit 120 is also arranged so as to store partial or final results of calculations in the memory unit 121.

Furthermore, the control unit 112 is equipped with devices 122, 123, 124, 125 for receiving and transmitting input and output signals, respectively. These input and output signals can comprise waveforms, pulses or other attributes that the devices 122, 125 for receiving input signals can detect as information for processing by the processing unit 120. The devices 123, 124 for transmitting output signals are arranged so as to convert calculation results from the processing unit 120 into output signals for transfer to other parts of the vehicle control system and/or the component (s) for which the signals are intended. Each and every one of the connections to the devices for receiving and transmitting respective input and output signals can include one or more of a cable; a data bus, such as a CAN bus (Controller Area Network bus), a MOST bus (Media Oriented Systems Transport) or any other bus configuration, or of a wireless connection. Returning to the exemplary method 200 illustrated in fig. 2, the method starts in step 201, where it is determined whether a reduced grip with regard to one or more vehicle drive wheels is detected, e.g. by detecting a wheel slip, which can be determined in any suitable manner, e.g. by detecting a sudden increase in wheel speed of one or more drive wheels and/or corresponding increase in internal combustion engine speed. The method according to the invention can be arranged to be carried out at all times when a reduced grip is detected, or e.g. only when the vehicle is travelling an uphill section of road, where further criteria such as road inclination and/or current vehicle weight may influence whether change of gear is to be influenced according to the invention or not. For example, a deceleration of the vehicle may be determined, where the method according to the invention may be arranged to be carried out only when vehicle deceleration exceeds some deceleration. When vehicle deceleration is low, the impact of a change of gear is less detrimental to the propulsion of the vehicle than when vehicle deceleration is high, such as may be the case when a heavily loaded vehicle travels an uphill section of road. Hence, according to embodiments of the invention, the method may include the further step of determining a current

deceleration of the vehicle 100, where vehicle deceleration can be determined by means of any suitable means as is known per se. According to such embodiments, when it is determined that the vehicle deceleration exceeds a deceleration limit, the method may be arranged to continue according to the following while otherwise the method may be ended.

When it is determined that the vehicle 100 is subject to a reduced grip, such as by being subject to wheel slip, the method continues to step 202, where the grip of the vehicle is further evaluated by determining whether driver manoeuvring of, in this example, the accelerator pedal also indicates a reduced grip. This can be determined in various ways, e.g. by determining whether the driver, in connection with the

detection of e.g. wheel slip, reduces the request for power from the internal combustion engine. In general, when a driver notices that conditions of reduced grip prevail, a request for power from the internal combustion engine is reduced in an attempt to regain grip. Conversely, it is highly unlikely that the driver maintains a request for a high power when wheels start spinning unless the driver realises that the reduced grip is highly temporary, and that the grip immediately will recover. Hence, according to embodiments of the invention, a reduction in request for power from the internal combustion engine, e.g. detected by detecting a released accelerator pedal, can be used as indication of the grip actually being reduced and still prevailing.

According to embodiments of the invention, it is not only determined whether there is a reduction in the request for power from the internal combustion engine, but it is also determined whether the reduction is followed by an increase in the request for power, where it can be determined whether the increase at most is an increase e.g. to some percentage of the request prevailing prior to the detection of reduced grip. Such behaviour can be taken as an indication that the driver is seeking grip and trying to maximise as much as possible the force transmitted to the vehicle drive wheels. According to embodiments of the inventions, it can be determined whether the increase is followed by a further decrease in the request for power in order to determine that conditions are such that grip is still reduced and change of gear should be influenced. When it is determined in step 202 that propulsion of the vehicle is subject to a reduced grip, the method continues to step 203, where change of gear is influenced. Otherwise the method can be arranged to return to step 201. According to embodiments of the invention, gear ratio changing operations are influenced when driver manoeuvring of the accelerator pedal indicates a reduced grip within a first period of time from said detecting a reduced grip of said at least one drive wheel. That is, it is not required that driver manoeuvring of the accelerator occurs simultaneously with the detection of wheel slip, but the driver manoeuvring of the accelerator may take place within some period of time from the detection of wheel slip. For example, some suitable number of seconds may be utilised as time limit. In this way, the driver is given time to react when one or more wheels start slipping, and for as long as the driver actually reacts according to the above in a stipulated period of time change of gear is influenced according to the invention.

Hence, in step 203, control of change of gear, i.e. gear changing operations, are influenced as a result of the dual indication of reduced grip of the vehicle. Gear changing operations can be arranged to be influenced in various ways. According to embodiments of the invention, any changes of gear ratio are delayed or suspended. According to embodiments of the invention, changes of gear ratio to lower gear ratios can be delayed or suspended while changes of gear ratio to higher gear ratios may still be allowed. When it is determined, step 204, that slippery conditions no longer prevail, e.g.

indicated by driver manoeuvring of the accelerator pedal or no further indication of slippery conditions within a

predetermined period of time, influence according to the invention is discontinued and normal control of change of gear is resumed, step 205, and the method is ended in step 206.

As was mentioned above, according to embodiments of the invention, when influencing gear ratio changing operations, this may include changing a criterion regarding when a change of gear is to be performed. This criterion can be related to when a change of gear is performed in relation to the speed of rotation of the internal combustion engine, where a change of gear can be influenced such that propulsion on a current gear ratio is allowed at higher and/or lower speeds of rotation of said power source in comparison to when a reduced grip is not prevailing. This may be necessary e.g. in situations where the vehicle is decelerating due to insufficient power for

propulsion of the vehicle, or otherwise the vehicle speed is changing so that normal gear changing criteria would otherwise trigger a change of gear ratio.

This is exemplified in fig. 3. Fig. 3 exemplifies a situation where the vehicle 100 is decelerating and being forced to be propelled on a current gear for a longer period of time than normally is the case. The x-axis represents time and the y- axis represents speed of rotation n of the internal combustion engine .

The solid line 301 represents the change in combustion engine speed n for a situation where the vehicle 100 is being driven in an uphill section and in a situation where the vehicle is losing speed. The dashed line 302 represents a combustion engine speed limit n _lim that is not to be exceeded following a change of gear to a lower gear (i.e. change of gear ratio to a higher gear ratio) .

Starting from time to, in the situation being represented by solid line 301, the vehicle 100 is continuously losing speed (decelerating) and hence the rotational speed n of the

combustion engine 101 is also continuously reduced. The requirements for a change of gear in the general case are fulfilled at time ti, e.g. that the rotational speed of the internal combustion engine following the change of gear will not exceed the speed limit n _lim , and/or reaches a minimum speed ⁿ chan _g e · When change of gear is not influenced according to the present invention, a change of gear would normally be carried out at this point in time, following dashed line 303. However, when influencing gear ratio changing operations according to invention, the change of gear is not performed but delayed. This can be accomplished e.g. by lowering the maximum allowed combustion engine speed n _m following a change of gear to a second speed limit n _{lim slip} so that criteria for change of gear no longer are fulfilled at time ti. Alternatively, or in addition, the criterion can instead be set such that the vehicle is forced to continue on a current gear until the speed n of the combustion engine 101 has reduced to speed n _change _ _slip , which is lower than the combustion engine speed n _ch at which change of gear normally is carried out. In this way the criteria for performing the gear changing operation will not be fulfilled until the vehicle 100 has decelerated further and the vehicle 100 thereby being propelled for a longer period of time on the current gear.

Consequently, in the situation according to the solid line 301, when the vehicle 100 reaches time ti, no change of gear is performed since the new gear changing criteria due to detected wheel slip are not fulfilled. Instead, the vehicle 100 is controlled to continue on the current gear while vehicle speed, and thereby combustion engine speed n , may continue to reduce and change of gear is thereby at least delayed.

However, at some point the combustion engine speed at a current gear, if the vehicle continues to decelerate, may reach a speed where a change of gear no longer can be avoided, e.g. due to the combustion engine speed reaching the minimum allowed speed n _change _ _slip , e.g. constituting, or being in the vicinity of, engine idling speed. According to the present example, this occurs at time t2, for which reason a change of gear is carried out, although delayed according to the

invention . Furthermore, according to embodiments of the invention, the gear ratio changing operation is only influenced when the driver takes at least one further measure that indicates a reduced grip. Such measures can, for example, be a detection that the driver changes load on wheel shafts, e.g. by reducing load on an auxiliary axle to thereby increase load on drive wheels in an attempt to improve possibilities to obtain a sufficient grip. The drive may also take other actions, such as e.g. lock a differential in order to increase the ability to overcome a section of road where reduced grip is

prevailing. Consequently, according to embodiments of the invention, change of gear is only influenced when at least one such further indication of reduced grip is detected.

Finally, embodiments of the invention have been exemplified above for a particular example of a vehicle, but is applicable for any vehicle in which the vehicle control system controls gear change. The invention also encompasses a vehicle

comprising a system implementing the invention.