DE102007030254B4 - Method and device for supporting a start-up process - Google Patents

Abstract

Method for supporting a starting process of a vehicle on a slope, starting from a vehicle standstill brought about by the driver of the vehicle by actuating the brakes, in which a slope variable (Road_Grad) characterizing the slope angle of the slope and a starting direction desired by a driver are determined, - the determined Starting direction is evaluated in relation to the determined gradient variable (Road_Grad), and- according to a result of the evaluation carried out, a braking force is set in a brake system of the vehicle after a release of the brake on the driver's side has been determined, characterized in that the evaluation determines whether the driver wants to start the vehicle in an uphill direction or in a downhill direction, - in the event that the driver wants to start in a downhill direction, a braking force is set which is dimensioned so that the vehicle due to downhill gradient skraft moved at a low speed and- in the event that the driver wants to start uphill, a braking force is set, which is dimensioned so that the vehicle is held stationary.

Description

technical field

The invention relates to a method for supporting the process of starting a vehicle on a downhill gradient, starting from a driving state brought about by the driver of the vehicle by actuating the brakes, in particular when the vehicle is stationary. The invention also relates to a device suitable for carrying out the method. In particular, the invention includes an addition to a cruise control, for example an HDC control (HDC: Hill Descent Control).

Background of the Invention

Control functions are known, for example, under the designation HDC, which keep a vehicle moving downhill on a downhill gradient at a predetermined set speed by active braking interventions. These control functions are intended in particular for off-road vehicles that are moved on a slope where the engine drag torque provided by the drive motor is no longer sufficient, even in the lowest gear, to decelerate the vehicle. A hydraulic unit, which is also known from driving dynamics control systems, is usually used to carry out the braking interventions, with which the braking force can be adjusted independently of the brake actuation by the driver.

An HDC controller should enable comfortable speed control over a wide range of gradients. Due to the resulting controller structure and due to the limited dynamics of the hydraulic unit, when the vehicle starts to roll, particularly on steep downhill stretches, it can overshoot the target speed and cause high accelerations, which the driver usually perceives as uncomfortable and his confidence in the vehicle the control function is reduced. There is also the problem that the HDC control does not offer the driver any support when starting off in an uphill direction. On steep inclines, even vehicles with an automatic transmission can roll back during the starting process, which the driver can prevent by rapidly changing from actuating the brake to actuating the accelerator pedal or by actuating a parking brake. However, this requires considerable skill on the part of the driver and often cannot completely prevent the vehicle from rolling back unintentionally, particularly on steep inclines.

To increase comfort when rolling downhill, the international patent application WO 01/14 186 A1 or DE 100 39 458 A1 proposed to supplement an HDC controller with a system that builds up a braking force by means of the brake system before or during rolling until the braking force control is taken over by the HDC controller. However, the system described does not offer any support for the driver when starting off in an uphill direction. Furthermore, the known system does not differentiate between starting processes in the forward and reverse direction, in which different starting strategies can be used under certain circumstances in order to make the starting process more comfortable.

From the DE 199 12 878 A1 a method for braking assistance when starting is also known. In this case, when the vehicle is stationary, a braking force is maintained even after a brake pedal has been released. This prevents the vehicle from rolling backwards, for example when starting off in an uphill direction.

the EP 0 822 129 B1 describes a method for starting a vehicle, in which a braking force is built up by external force until there is sufficient engine torque for starting.

From the US 2005 / 0 057 090 A1 a method is known in which it is determined whether a driving power of a motor is only used to keep the vehicle stationary on a slope. In this case, a braking unit is activated to relieve the engine.

Presentation of the invention

It is therefore an object of the present invention to provide support when starting off on a steep gradient, which offers the best possible support for the driver in different starting situations, particularly with regard to driving comfort.

According to the invention, this object is achieved by a method according to patent claim 1 and by a device according to patent claim 14 .

• - eine den Steigungswinkel des Gefälles charakterisierende Steigungsgröße und eine von einem Fahrer gewünschte Anfahrrichtung ermittelt werden,
• - die ermittelte Anfahrrichtung in Bezug auf die ermittelte Steigungsgröße bewertet wird, und
• - nach Maßgabe eines Ergebnisses der vorgenommenen Bewertung eine Bremskraft in einer Bremsanlage des Fahrzeugs eingestellt wird, nachdem ein fahrerseitiges Lösen der Bremse festgestellt worden ist.

Accordingly, a method of the type mentioned is carried out so that

• - an incline variable characterizing the incline angle of the gradient and an approach direction desired by a driver are determined,
• - the determined approach direction is evaluated in relation to the determined gradient size, and
• - According to a result of the assessment made, a braking force is set in a brake system of the vehicle after it has been determined that the brake has been released by the driver.

• - wenigstens eine Ermittlungseinrichtung, mit der eine den Steigungswinkel des Gefälles charakterisierende Steigungsgröße und eine von einem Fahrer gewünschte Anfahrrichtung ermittelbar sind,
• - eine Bewertungseinrichtung, in der die ermittelte Anfahrrichtung in Bezug auf die ermittelte Steigungsgröße bewertbar ist, und
• - eine Steuerungseinrichtung, mit der nach Maßgabe eines Ergebnisses der vorgenommenen Bewertung eine Bremskraft in einer Bremsanlage des Fahrzeugs einstellbar ist, nachdem ein fahrerseitiges Lösen der Bremse festgestellt worden ist.

Furthermore, a device for supporting a starting process of a vehicle based on a driving state brought about by the driver of the vehicle by actuating the brakes, in particular a vehicle standstill, is provided, which comprises the following components:

• - at least one determination device with which a gradient variable characterizing the gradient angle of the downhill gradient and a driving direction desired by a driver can be determined,
• - an evaluation device in which the determined approach direction can be evaluated in relation to the determined gradient variable, and
• - A control device with which a braking force in a brake system of the vehicle can be adjusted according to a result of the assessment made after a release of the brake on the driver's side has been detected.

The invention includes the idea that a starting direction desired by the driver is determined and evaluated in relation to the determined value of a variable characterizing the gradient angle of the present gradient in order to carry out braking force control during a starting process depending on the result of the evaluation. As a result, the brake force control can be adapted to different starting situations, so that driving comfort is increased when starting. The driving off takes place in particular from a standstill of the vehicle, but can in principle also take place from other driving states, in particular from a very slow journey. In addition, it should be pointed out that the term starting in the context of the invention includes, in addition to starting in an uphill direction, in particular starting or rolling in a downhill direction.

The driving direction is understood to mean the driving direction in which the driver probably wants to drive the vehicle. This can be the forward or reverse direction. That is, it is determined whether the driver is likely to want to start the vehicle forward in the vehicle front-rear direction or backward in the vehicle front-rear direction. On the basis of the assessment of the starting direction in relation to the slope variable characterizing the slope angle of the downhill slope, it can be determined in particular whether the driver would like to start the vehicle on a downhill slope in an uphill or downhill direction. The grade variable can be, for example, the longitudinal inclination angle of the vehicle determined by means of a longitudinal acceleration sensor (G-sensor) or a variable derived therefrom.

The braking force control is advantageously carried out after it has been determined that the brake has been released by the driver. This can be the case, for example, if it has been determined that the braking force commanded by the driver has reduced to a predetermined level and/or that an actuating means of the brake is moved by the driver in a predetermined manner, for example with a predetermined release gradient, in particular is resolved. In one embodiment of the method and the device, a release of the brake on the driver's side is determined when a variable characterizing the braking force reaches a threshold value. When using a hydraulic brake system, the variable characterizing the braking force can be, for example, a brake pressure present in the brake system. In such brake systems, the braking force is usually controlled by adjusting a brake pressure.

In one embodiment of the method and the device, it is provided that the starting direction desired by the driver is determined based on a gear selected in a transmission of the vehicle or based on a gear set on a transmission of the vehicle.

This embodiment has the advantage that due to the intended indirect determination of the starting direction desired by the driver, the driver does not have to give any additional signals indicating the starting direction, so that the desired starting direction can be determined without the driver noticing and thus without any loss of comfort . Within the scope of the invention, the term transmission is understood to mean both manual transmissions and automatic transmissions or automated transmissions.

The method and the device according to the invention are characterized in that when evaluating the starting direction in relation to the grade, it is determined whether the driver wishes to start the vehicle in an uphill direction or in a downhill direction.

As previously mentioned, there are different requirements for the launch strategy with regard to whether the vehicle is to be launched in an uphill or downhill direction. Advantageously, the aforementioned configuration allows to differentiate between these starting situations and to adapt the braking force control to the requirements of the respective starting situation.

Furthermore, the method and the device according to the invention provide that in the event that the driver wishes to start off in the downhill direction, a braking force is set which is dimensioned such that the vehicle moves at a low speed due to a downhill force.

Advantageously, in this embodiment, when starting off in the downhill direction, a braking force is set that only allows the vehicle to move at a low speed, so that high accelerations of the vehicle when starting off in the downhill direction can be avoided. In this way, it is possible in particular to prevent vehicle speeds and/or accelerations from occurring before entering into HDC regulation, which the driver finds unpleasant and reduces comfort.

It was also found that a driver generally finds high accelerations in the reverse direction to be particularly unpleasant, but that higher accelerations in the forward direction are often perceived without discomfort.

A development of the method and the device therefore includes that in the event that a reverse direction is determined as the starting direction, a greater braking force is set than in the case that a forward direction is determined as the starting direction.

As a result, the vehicle accelerates less strongly when starting off backwards in a downhill direction than when starting off in a forward direction in a downhill direction.

A further development of the method and the device also includes that in the event that the driver wishes to start off in the downhill direction, a braking force is set which is dimensioned in such a way that the vehicle is held at a standstill.

This development has the advantage that the vehicle does not initially roll downhill when the driver releases the brake. As a result, the driver generally develops a particularly high level of trust in the support function provided, since he will undoubtedly notice the support in this case.

In addition, an embodiment of the method and the device is characterized in that the braking force is reduced with a predetermined braking force gradient.

This takes account of the fact that the initially set braking force cannot be accelerated to a desired speed, in particular to the HDC target speed, for example when the terrain changes. For example, the gradient can change during the start-up process in such a way that the vehicle would come to a standstill again if the braking force were not reduced.

An embodiment of the method and the device also provides that the braking force is reduced only after a predetermined first holding time has elapsed, which begins with the determination that the driver has released the brake.

An advantage of this embodiment is that the vehicle acceleration is kept very low at the start of the starting process, which the driver generally finds comfortable, particularly on very steep inclines, and which further increases the driver's confidence in the HDC function . If, after the driver has released the brake, a braking force is initially set that is dimensioned in such a way that the vehicle is held at a standstill, the vehicle only rolls when the braking force is reduced, i.e. in particular only after the first holding time has elapsed.

As already mentioned, the proposed method and the proposed device with their embodiments are particularly suitable for supporting an HDC function that keeps the vehicle at a predetermined speed when driving downhill. The support provided makes it possible to make the starting process more comfortable for the driver until the braking force is controlled by the HDC controller, i.e. until the HDC target speed is reached.

One embodiment of the method and the device is characterized in that the braking force is reduced until a braking force control is taken over by a speed controller, in particular an HDC controller, which adjusts the speed of the vehicle to a predefined target speed.

Furthermore, the method and the device according to the invention include that in the event that the driver wishes to start off in the uphill direction, a braking force is set which is dimensioned in such a way that the vehicle is held at a standstill.

This prevents the vehicle from rolling backwards in the downhill direction when the driver wants to start the vehicle in the uphill direction, which the driver generally also finds very uncomfortable.

In addition, a development of the method and the device is characterized in that the braking force is reduced after a predetermined second holding time and/or that the braking force is reduced when a drive torque set by the driver on a drive motor of the vehicle exceeds a threshold value.

The advantage of this development is that the driver is given the opportunity, after actuating the brake system, to switch to the accelerator pedal provided for controlling the drive motor in order to start the vehicle without it rolling back downhill. In particular, this allows the HDC function to be expanded to include start-up support, which enables comfortable and safe start-up on a downhill gradient.

In a related embodiment of the method and the device, it is provided that the braking force reduction is accelerated when the drive torque set by the driver exceeds the threshold value.

This reduces the risk of the driver stalling the drive motor of the vehicle due to excessive braking force during the starting process, and the braking force can be reduced sufficiently slowly on the other hand if the driver does not request any drive torque. This can be the case, for example, if he wants the vehicle to start rolling in the opposite direction to the determined starting direction.

Furthermore, one embodiment of the method and the device provides that the threshold value is determined as a function of the gradient variable determined.

In this way, the threshold value can be determined in particular such that the drive torque set when the threshold value is reached is sufficiently large to drive the vehicle uphill or at least to keep it stationary, so that brake assistance is no longer required.

In addition, one embodiment of the method and the device includes the fact that the braking force and/or the braking force gradient is/are determined as a function of the gradient variable determined.

Advantageously, the braking force and/or the braking force gradient can be adapted to the present starting situation in this embodiment, taking into account the determined gradient variable. In particular, the braking force can be determined in such a way that on the one hand it is sufficiently large to compensate for the downhill force acting on the vehicle in the desired manner, i.e. either to keep the vehicle stationary or to let it roll downhill with a low acceleration and on the other hand not is so great that a desired rolling is prevented or a starting process is made more difficult.

In addition, a computer program product is provided that includes software code sections with which a method with individual or all of the features described above can be executed if the software code sections are executed on a processor device.

In order to offer a supplement for an HDC function, one embodiment of the computer program product provides that the software code sections are part of a piece of software with which speed control, in particular an HDC function, is executed when the software is executed on a processor device.

In addition, it is also provided in one embodiment of the device that it is a component of a speed control system, in particular an HDC control system.

The aforementioned and other advantages, special features and expedient configurations of the invention are also clear from the exemplary embodiments, which are described below with reference to the figures.

character list

• 1 eine schematische Darstellung ausgewählter Bestandteile eines Fahrzeug, welches zur Ausführung der Erfindung geeignet ist,
• 2 ein schematisches Hydraulikschaltbild einer Hydraulikeinheit des Fahrzeugs,
• 3 ein schematisches Blockschaltbild einer Einrichtung zur Durchführung eines erfindungsgemäßen Verfahrens,
• 4 ein Ablaufdiagramm zur Veranschaulichung einer Drucksteuerung im Falle eines Anfahrens in Bergabrichtung und
• 5 ein Ablaufdiagramm zur Veranschaulichung einer Drucksteuerung im Falle eines Anfahrens in Bergaufrichtung.

From the figures shows:

• 1 a schematic representation of selected components of a vehicle which is suitable for carrying out the invention,
• 2 a schematic hydraulic circuit diagram of a hydraulic unit of the vehicle,
• 3 a schematic block diagram of a device for carrying out a method according to the invention,
• 4 a flowchart showing a pressure control in the case of a downhill start and
• 5 Fig. 12 is a flowchart showing pressure control in the case of an uphill start.

Detailed description of exemplary embodiments

In 1 selected components of a four-wheel vehicle are shown by way of example, which can be, for example, an all-wheel-drive off-road vehicle. The vehicle has a drive motor 101 in which an engine torque requested by the driver using an accelerator pedal 102 can be generated, which can be transmitted to the four wheels 103 of the vehicle via a drive train. The drive motor 101 is driven by an in 1 controlled engine control device, not shown, which in particular also receives and converts the accelerator pedal signals. The drive train includes a transmission 104, which can be a manual transmission or an automatic transmission, which is operated by the driver using a selector lever 105. The other elements of the drive train are in 1 not shown for reasons of clarity. The transmission 104 provides a plurality of forward gears in which the vehicle is propelled by engine torque in the forward direction and at least one reverse gear in which the vehicle is propelled by engine torque in the reverse direction. If the transmission 104 is a manual transmission, then the selector lever 105 is designed as a shift lever with which the gear engaged in the transmission 104 is selected. If the transmission 104 is an automatic transmission, then several gears are provided which can be selected by the driver using the selector lever 105 and include one or more forward gears and at least one reverse gear. In the automatic transmission, the gears are shifted by an in 1 Not shown transmission control made according to the set gear.

The vehicle also has a hydraulic brake system that can be actuated by the driver using a brake pedal 106 . By moving the brake pedal 106 in a pressure build-up direction, a brake pressure is built up in a master cylinder 108 by means of a brake booster 107, which is also referred to below as the admission pressure. In one embodiment, the master cylinder 108 is designed as a so-called tandem master cylinder, which has two pressure chambers that are each connected to a pressure medium reservoir 115 and supply a brake circuit that includes two wheel brakes 109 . The pressure chambers are connected to the wheel brakes 109 by hydraulic lines 111, via which an admission pressure set in the pressure chambers can be transmitted to the wheel brakes 109. The wheel brakes 109 are embodied as disc brakes, for example, in which a brake piston is actuated when pressure builds up and is pressed against a brake disc which is non-rotatably connected to the corresponding wheel 103 of the vehicle, as a result of which a braking force is generated. The brake system also contains a hydraulic unit (HCU) 110, which allows the brake pressure in the wheel brakes 109 to be controlled independently of the driver's brake actuation.

The hydraulic unit 110 is designed in a manner known per se to a person skilled in the art and includes an arrangement of electronically switchable valves and a pump. A hydraulic circuit diagram of an embodiment of the hydraulic unit 110 is shown in detail in FIG 2 shown in a schematic representation. 2 11 illustrates the section of hydraulic unit 110 that is associated with one of the two brake circuits. Hydraulic unit 110 has an identical valve arrangement for the other brake circuit. The wheel brakes 109 of a brake circuit are connected within the hydraulic unit 110 via an isolating valve 201 and an inlet valve 202 to the corresponding pressure chamber of the master cylinder 108 . The valves 201, 202 have a spring-actuated open position and an electromagnetically switchable closed position. In the case of a braking process controlled exclusively by the driver, the admission pressure built up in the master cylinder 108 is transmitted directly to the wheel brakes 109 . To carry out an automatic brake pressure build-up, the separating valve 201 is actuated (ie closed) and a changeover valve 203, which has a spring-actuated blocking position and an electromagnetically switchable open position, is opened. In this way, a hydraulic connection is created between the master cylinder 108 and the suction side of a pump 204 driven by an electric motor 205 , while the master cylinder 108 is decoupled from the pressure side of the pump 204 . In this valve position, the pump 204 is able to deliver pressure medium from the master cylinder 108 to the wheel brakes 109 and to increase the braking pressure in the wheel brakes 109 . Furthermore, the brake pressure in the wheel brakes 109 can be reduced against an existing form in the master cylinder 108 . For this purpose, each wheel brake 109 is assigned an outlet valve 206, which has a spring-actuated blocking position and an electromagnetically switchable open position. For the pressure reduction in a wheel brake 109, the corresponding inlet valve 202 is closed and the associated outlet valve 206 is opened, so that the pressure medium can escape from the wheel brake 109 into a low-pressure accumulator 207. The pressure medium is then pumped back from the low-pressure accumulator 207 into the master cylinder 108 by means of the pump 204 when the switching valve 203 is closed and the separating valve 201 is open. In addition, the hydraulic unit 110 includes a Pressure sensor 208, with which the form in the master cylinder 108 can be detected.

The hydraulic unit 110 is controlled by a control device 112 which contains an HDC control system which can be activated by the driver using a switch 113, for example. Activation usually takes place when the driver wants assistance from HDC controller 114 when driving off-road. The HDC control system includes, in particular, the actual HDC controller 114, which keeps the vehicle speed at a predetermined target speed when driving downhill by suitable activation of the hydraulic unit 110. The HDC controller 114 is designed in a manner that is fundamentally known to a person skilled in the art and is therefore not explained further here. For an exemplary embodiment of the HDC controller 114, reference is made to the international patent application WO 96/11862 referred. In one implementation, the HDC controller 114 is implemented as software stored in a memory of the controller 112 and executed on a processor of the controller 112 . The processes carried out within the control device 112 are run through cyclically in so-called loops with a specific loop time.

In addition to the HDC controller 114, the HDC control system also includes the control module 115 for executing an additional function that offers support during a starting process on a downhill slope. This support is provided in particular on slopes with an incline greater than 15% or an incline angle greater than 9 degrees. The control module 115 is designed, for example, as a software module that supplements the software containing the HDC controller 114 and is activated at the same time as the HDC controller 114 .

With the additional function, the vehicle acceleration is specifically influenced after the vehicle has been brought to a standstill by the driver in order to make the starting process more comfortable. The additional function is based on the assumption that after bringing the vehicle to a standstill, the driver adjusts an admission pressure in the master cylinder 108 that is large enough to keep the vehicle at a standstill.

A standstill of the vehicle is determined using a speed signal, which is determined from the signals from wheel speed sensors. The speed signal is determined in the control device 112 or in a further control device from which it is transmitted to the control device 112 . In one embodiment, the speed signal corresponds to the wheel speed of the slowest wheel 103 of the vehicle. A standstill of the vehicle is determined when the speed signal is less than a predetermined threshold value of 1 km/h, for example, or when it has assumed the value zero.

If it is determined that the vehicle is stationary, then the additional function provided is executed in the control module 115 . The structure of the control module 115 is in 3 illustrated schematically using a block diagram. It includes a gradient calculation device 301, in which an estimated value Road_Grad is determined for the present gradient, which is defined here as the sine of the present gradient angle. The estimated value Road_Grad is determined from the longitudinal inclination angle of the vehicle, which is detected in a manner known per se to those skilled in the art using a longitudinal acceleration sensor (G-sensor), the signals of which are transmitted to the control device 112 and in particular to the incline calculation device 301.

wobei mit g die Erdbeschleunigung und mit a_Sensor die von dem Längsbeschleunigungssensor gemessene Längsbeschleunigung und mit v die Fahrzeuglängsgeschwindigkeit bezeichnet ist. Die Größe dv/dt bezeichnet die Radbeschleunigung, d.h., die Änderungsrate der Fahrzeuggeschwindigkeit. Das obere (negative) Vorzeichen in Gleichung 1 gilt für Vorwärtsfahrt und das untere (positive) Vorzeichen gilt für Rückwärtsfahrt. Die Vorzeichen wurden in Gleichung 1 so gewählt, dass der Längsneigungswinkel positiv ist, wenn das Fahrzeug in Bergaufrichtung geneigt ist, d.h., wenn die Vorderachse des Fahrzeugs höher liegt als die Hinterachse. Somit ist der Längsneigungswinkel negativ, wenn das Fahrzeug in Bergabrichtung geneigt ist, d.h., wenn die Vorderachse des Fahrzeugs tiefer liegt als die Hinterachse. Im Stillstand ist die Radbeschleunigung gleich null bzw. vernachlässigbar, so dass der Schätzwert Road_Grad direkt aus dem Längsbeschleunigungssignal bestimmt werden kann. Zur Ermittlung des Schätzwerts Road_Grad für die Steigung wird der berechnete Wert für sin(α) vorzugweise gefiltert, um die Effekte eines möglichen Ausschwingens des Längsbeschleunigungssensors zu mindern. Insbesondere kann dabei beispielsweise eine PT1-Filterung vorgenommen werden, so dass Road_Grad = PT1 (a_Sensor) gilt.The following applies to the pitch angle α of the vehicle $$\text{sin}\left(a\right)=\left(a_{SensO\mathrm{right}}\mp \mathrm{i.e}v\text{/}\mathrm{i.e}t\right)\text{/G}$$

where g denotes the gravitational acceleration and a _sensor denotes the longitudinal acceleration measured by the longitudinal acceleration sensor and v denotes the vehicle longitudinal speed. The quantity dv/dt denotes the wheel acceleration, ie the rate of change of the vehicle speed. The upper (negative) sign in Equation 1 is for forward travel and the lower (positive) sign is for reverse travel. The signs in Equation 1 have been chosen such that the pitch angle is positive when the vehicle is inclined in an uphill direction, ie when the front axle of the vehicle is higher than the rear axle. Thus, the pitch angle is negative when the vehicle is inclined in a downhill direction, ie when the front axle of the vehicle is lower than the rear axle. At a standstill, the wheel acceleration is zero or negligible, so that the estimated value Road_Grad can be determined directly from the longitudinal acceleration signal. In order to determine the estimated value Road_Grad for the slope, the calculated value for sin(α) is preferably filtered in order to reduce the effects of a possible ringing out of the longitudinal acceleration sensor. In particular, PT1 filtering can be carried out, for example, so that Road_Grad=PT1(a _Sensor ) applies.

In addition to determining the gradient Road_Grad, the driver's desired direction is also recognized and evaluated in the Block 302. In this block, it is first determined in which driving direction the driver wants to start the vehicle, ie it is determined whether the driver wants to move the vehicle in the forward or reverse direction. This is done by determining the gear engaged in the transmission 104 during the standstill or the gear selected during the standstill, for which purpose the block 302 is supplied with information GI about the gear engaged or the gear selected. If the transmission 104 is a manual transmission, the engaged gear is detected by a sensor system, which transmits the information GI to the control device 112 . The sensors can include, for example, switches that are located on the transmission or in the shift gate of selector lever 105 and are actuated when a specific gear is engaged, or magnetic and/or electrical distance sensors that are located in the shift gate and that monitor the position of selector lever 105 capture, which is characteristic of a particular gear. If the transmission 104 is an automatic transmission, then the set gear is detected by the transmission controller and reported to the control device 112 . For this purpose, for example, information GI indicating the engaged gear can be transmitted to the control device 112 via a data bus, in particular via the CAN bus (CAN: Controller Area Network) commonly used in vehicles.

• - der Fahrer wünscht ein Anfahren in Bergaufrichtung; dies ist der Fall, wenn die Steigung Road_Grad positiv ist und an dem Getriebe 104 ein Vorwärtsgang bzw. eine Vorwärtsfahrstufe eingestellt ist, oder wenn die Steigung Road_Grad negativ ist und an dem Getriebe ein Rückwärtsgang bzw. eine Rückwärtsfahrstufe eingestellt ist;
• - der Fahrer wünscht ein Anfahren in Vorwärtsrichtung bergab; dies ist der Fall, wenn die Steigung Road_Grad negativ ist, und an dem Getriebe 104 ein Vorwärtsgang bzw. eine Vorwärtsfahrstufe eingestellt ist;
• - der Fahrer wünscht ein Anfahren in Rückwärtsrichtung bergab; dies ist der Fall, wenn die Steigung Road_Grad positiv ist und an dem Getriebe 104 ein Rückwärtsgang bzw. eine Rückwärtsfahrstufe eingestellt ist.

Furthermore, in block 302, the driver's desired direction is evaluated in relation to the existing gradient Road_Grad. In particular, the following cases are distinguished:

• - the driver wants to start uphill; this is the case when the gradient Road_Grad is positive and a forward gear or a forward gear is set on the transmission 104, or when the gradient Road_Grad is negative and a reverse gear or a reverse gear is set on the transmission;
• - the driver wants to start in a forward direction downhill; this is the case when the gradient Road_Grad is negative and a forward gear or a forward gear is set on the transmission 104;
• - the driver wants to start in reverse direction downhill; this is the case when the gradient Road_Grad is positive and a reverse gear or reverse gear is set on the transmission 104 .

A brake pressure level is then determined by the additional function as a function of the result E of the evaluation, i.e. depending on which of the three cases mentioned above has been determined. For this purpose, the result E of the evaluation is fed to block 303, in which a pressure threshold P_Thr is calculated on the basis of the result of the evaluation, which pressure threshold is used for brake pressure control during a starting process following standstill. The braking pressure is controlled by block 304 . This checks whether the driver releases the brake pedal 106 and takes over the brake pressure control if the admission pressure reaches the calculated pressure threshold P_Thr during the brake pedal release process. In one specific embodiment, a braking pressure corresponding to the pressure threshold P_Thr is initially maintained in the wheel brakes 106, which is then reduced after a specified pressure holding time has elapsed. In order to carry out the pressure control, the determined gradient Road_Grad is preferably also supplied to block 304 .

In one embodiment, when starting off in the downhill direction, the pressure threshold P_Thr is determined such that the vehicle rolls off with a low acceleration if a brake pressure corresponding to the pressure threshold P_Thr is set in the brake system, in particular in the wheel brakes 109 . In a further specific embodiment, it can be provided that the pressure threshold P_Thr is determined in such a way that the vehicle is kept stationary during the pressure holding time and only starts rolling when the brake pressure is subsequently reduced.

The pressure threshold P_Thr is also determined in block 302 as a function of the existing gradient Road_Grad. Provision can be made here for the existing gradient Road_Grad to be multiplied by a predefined factor in order to determine the pressure threshold P_Thr. In this specific embodiment, the pressure threshold P_Thr is therefore proportional to the gradient Road_Grad. Provision can likewise be made for a characteristic curve to be determined from a polygon stored in control device 112, and for the pressure threshold P_Thr to be determined using this characteristic curve from the gradient Road_Grad. In this case, the characteristic curve can be designed in such a way that with higher gradients Road_Grad, a change in the gradient Road_Grad leads to a greater change in the pressure threshold P_Thr than with lower gradients Road_Grad. In a further specific embodiment, any desired function can also be specified, which is used to determine the pressure threshold P_Thr from the existing gradient Road_Grad, in which case this function can also have the aforementioned property of the polygon. The factor, the traverse or the function can be determined within the framework of driving tests in such a way that the vehicle rolls in the desired manner. In particular, can by specifying the pressure threshold P_Thr appropriately, whether the vehicle is rolling slowly, ie with a low acceleration, or whether the acceleration should be higher when rolling, and whether the vehicle is already rolling during or only after the pressure maintenance phase.

If the downhill direction has been determined as the driver's desired direction, then when determining the pressure threshold P_Thr it is preferably also taken into account whether the driver wants to start off in the forward or reverse direction. Correspondingly, these cases, as described above, are differentiated in block 302 in the evaluation of the driver's desired direction. A higher pressure threshold P_Thr is determined when starting in the reverse direction than when starting in the forward direction, so that the vehicle acceleration is lower when rolling in the reverse direction than when rolling in the forward direction. This takes account of the fact that a driver generally finds rolling in the reverse direction to be particularly unpleasant and therefore has greater confidence in the additional function when rolling backwards sluggishly. Provision can be made here for a separate factor or polygon or a separate function to be specified for determining the pressure threshold P_Thr for the reverse rolling. It can also be provided that the pressure threshold P_Thr determined by means of the factor, the polygonal curve or the function is increased by a predetermined value if it has been determined that the driver wants the vehicle to start rolling in the reverse direction. This value can likewise be determined as a function of the present gradient Road_Grad and, for example, can be greater for larger gradients than for smaller gradients.

In addition, it can be provided that the pressure threshold P_Thr can also be determined as a function of the loading condition of the vehicle, i.e. in particular of the vehicle mass and/or of one or more other variables. Such additional variables can be, for example, the pitch angle of the vehicle, the state of wear of the wheel brakes 109 and the coefficient of friction of the roadway. A non-zero pitch angle can falsify the determination of the gradient Road_Grad, so that a corresponding correction can be made if the pitch angle is known. With high brake wear and a low coefficient of friction on the road, the effectiveness of the vehicle's brake system is limited, so that a higher pressure threshold P_Thr can be determined if increased brake wear or a low coefficient of friction is determined.

The pressure control performed by block 304 is different for a downhill launch than for an uphill launch. In the event that it has been determined that the driver wishes to start downhill, the pressure control is shown schematically in the flowchart in FIG 4 illustrated. In block 304, while the vehicle is stationary, it is first checked whether the driver releases the brake. A release of the brake is detected, for example, by a drop in the initial pressure P_Drv, which is detected by pressure sensor 208, the signals of which are transmitted to control device 112 and, in particular, are fed to block 304. Additionally or alternatively, a release of the brake can also be determined if the position of brake pedal 106 is changed in the pressure reduction direction at a rate of change that exceeds a threshold value. The pedal gradient, ie the rate of change at which the position of brake pedal 106 is changed, can be determined, for example, by evaluating the signals from a pedal travel sensor that is arranged on brake pedal 106 .

During the release process, the block checks in step 401 whether the admission pressure P_Drv falls below the pressure threshold value P_Thr determined in block 303 . For this purpose, the admission pressure P_Thr is preferably monitored using the signals from the pressure sensor 208 . If it has been determined that the preliminary pressure P_Drv has reached or fallen below the pressure threshold value P_Thr, then the brake pressure present in the wheel brakes 109 is maintained or locked in step 402 . This is done, for example, by closing the isolating valves 201 of the two brake circuits and/or by closing the inlet valves 202 of the wheel brakes 109. Furthermore, in one embodiment, a pressure maintenance phase of a predetermined duration is started. This is done, for example, by initializing a counter, the value of which is incremented n loop by loop (step 403) until it has reached a specified threshold value N_Thr1 (see step 404), which is specified in such a way that the N_Thr1 times the loop time is just the intended duration of the pressure maintenance phase is equivalent to. The duration of the pressure maintenance phase, i.e. the threshold value N_Thr1, is determined in driving tests in such a way that a comfortable roll-off is guaranteed. It can be provided that a longer stopping time, i.e. a greater threshold value N_Thr1, is specified for a roll in the reverse direction than for a roll in the forward direction.

After the end of the pressure maintenance phase, ie when the counter has assumed the value n=N_Thr1, the brake pressure in the wheel brakes 109 is increased by a corresponding control of the hydraulic unit 110 with a predetermined pressure gradient in Step 405 dismantled. Here, the brake pressure can be reduced by a predetermined amount, for example per loop. Furthermore, a lower pressure gradient can be set for a reverse roll than for a forward roll in order to avoid the occurrence of high vehicle accelerations when rolling in the reverse direction. The brake pressure reduction can be necessary for various reasons. For example, the determined pressure threshold value P_Thr can be too high, so that the vehicle does not start rolling during the pressure maintenance phase, as desired in one embodiment, or the pressure threshold value P_Thr would be too high due to a change in the terrain to move the vehicle at a low speed. For example, the incline could decrease to such an extent that the vehicle is stopped again after it has started to roll because the brake pressure is too high. If the pressure threshold P_Thr has been determined in such a way that the vehicle is kept at a standstill when a brake pressure corresponding to the pressure threshold P_Thr is present, the brake pressure reduction is required in order for the vehicle to roll.

The additional function ends when the brake pressure has been completely reduced, i.e. when the brake pressure reaches the value zero, or when a control intervention is made by the HDC controller 114 due to the HDC setpoint speed being exceeded (steps 406, 407). Provision is preferably made here for the brake pressure present in the wheel brakes 109 when the HDC target speed is reached to be offset against the first brake pressure request from the HDC controller 114 . After completion of the additional function, the pressure control is carried out in a manner known to those skilled in the art by the HDC controller 114, which controls the hydraulic unit 110 so that the vehicle speed does not exceed the target speed.

If the driver requests an engine torque M _motor during the pressure reduction in order to accelerate the vehicle more, then it is at one in the 4 non-illustrated embodiment also provided that the pressure reduction gradient is increased. This measure in particular prevents the drive motor 101 of the vehicle from stalling due to an excessive braking force. The engine torque request can be recognized by an increase in engine torque M _motor , the value of which can be reported by the engine controller to control device 112, for example via the vehicle's data bus. Provision can also be made for recognizing the engine torque request when accelerator pedal 102 has been depressed by a predetermined pedal travel, which can be determined using signals from a pedal travel sensor arranged on accelerator pedal 102 .

If it has been determined in block 302 that the driver wants to start the vehicle in the uphill direction, then a pressure threshold P_Thr is determined which is so high that the vehicle does not roll in the downhill direction if there is a braking pressure in the wheel brakes 109 that corresponds to the pressure threshold P_Thr is set. This brake pressure is preferably also determined as a function of the gradient Road_Grad using a predetermined factor with which the gradient Road_Grad is converted into a brake pressure. The factor can, for example, be determined in such a way that the vehicle would not roll even with maximum loading. Provision can also be made for the pressure threshold P_Thr to be determined as a function of the vehicle mass and/or one or more other variables already mentioned above.

The pressure control performed by block 304 when it has been determined that the driver desires to start uphill is shown schematically in the flowchart in FIG 5 illustrated. In this case, too, it is checked in the manner described above whether the driver releases the brake. During the release process, the form pressure P_Drv is also checked using the signals from the pressure sensor 208 in order to determine whether the form pressure P_Drv has reached or fallen below the calculated pressure threshold value P_Thr (step 501). If this has been determined, then in step 502 the brake pressure in the wheel brakes 109 is maintained by appropriate activation of the hydraulic unit 110, in particular by closing the isolating valves 201 of the two brake circuits and/or the inlet valves 202 of the wheel brakes 109. During a predetermined pressure maintenance phase the brake pressure is then kept constant. The pressure holding phase is measured in such a way that the driver can easily switch from the brake pedal 106 to the accelerator pedal in order to use the accelerator pedal to set an engine torque M _motor that is sufficient for starting. In particular, the pressure-maintaining phase can be selected to be longer than in the case of starting off in a downhill direction. In order to measure the pressure holding phase, a counter is initialized in step 502, for example, the value n of which is incremented in each loop (step 503). Furthermore, a counter threshold value N_Thr2 is specified, with a multiplication of the loop time by N_Thr2 yielding the intended duration of the pressure maintenance phase.

During the pressure maintenance phase, ie before the counter has assumed the specified value n=N_Thr2, it is checked in block 304 in step 504 whether the drive motor 102 is ready provided engine torque M _engine has exceeded a threshold value M_Thr. As already described above, the value of the engine torque M _motor is reported by the engine controller to the control device 112 . The engine torque threshold value M_Thr is specified in such a way that the vehicle can be started up without braking assistance or be held at a standstill on the downhill gradient if an engine torque corresponding to the threshold value M_Thr is present. The engine torque threshold value M_Thr is preferably determined as a function of the ascertained gradient Road_Grad. In this case, a factor can in turn be determined in driving tests and stored in control device 112, by which factor the determined gradient Road_Grad is multiplied in order to calculate engine torque threshold value M_Thr. Likewise, the engine torque threshold value M_Thr can also be determined using a characteristic curve specified by means of a polygonal curve or using a functional relationship from the ascertained gradient Road_Grad. This makes it possible to specify non-linear relationships between the gradient Road_Grad and the engine torque threshold value M_Thr, so that the engine torque threshold value M_Thr can, for example, be increased disproportionately as the gradient Road_Grad increases. If it is determined in step 504 that engine torque M _Motor has exceeded threshold value M_Thr, then the brake pressure contained in the service brake system is reduced to zero in step 505 with a high brake pressure gradient. If this is done, then in step 506 the additional function is terminated.

If the engine torque has not exceeded the threshold value M_Thr by the end of the pressure maintenance phase, ie until the counter reaches the value n=N_Thr2 (see step 507), then in step 508 the brake pressure begins to be reduced with a predetermined brake pressure gradient, with the brake pressure for example, is reduced by a predetermined value in each loop. The brake pressure gradient thus set is lower than the brake pressure gradient set in step 505, with which the brake pressure is reduced when engine torque M _Motor has exceeded threshold value M_Thr. As during the pressure maintenance phase, it is also checked during the reduction of the brake pressure whether engine torque M _Motor exceeds engine torque threshold value M_Thr (step 509). If this is determined, then the pressure control is continued with step 505, in which the brake pressure that still exists is reduced with a high brake pressure gradient. After the brake pressure has been reduced to the value zero, the additional function is then ended in step 506 .

If the engine torque M _Motor does not exceed the threshold value M_Thr, the brake pressure is reduced further until the vehicle has been accelerated downhill to a speed above the HDC setpoint speed and a control intervention by the HDC controller 114 takes place, or until the brake pressure in the brake system has been completely dismantled (step 510). Then, in step 506, the additional function is ended. In the event of a control intervention by HDC controller 114, the first brake pressure request is offset against the brake pressure still present in the brake system.

The fact that the brake pressure is also released when it has previously been determined using the gear engaged by the driver in transmission 104 or the selected gear that the driver wants to start the vehicle in the uphill direction, it is ensured that the driver The vehicle can also roll in the downhill direction if it has set a gear corresponding to the uphill direction or a gear corresponding to the uphill direction. He can let the vehicle roll backwards downhill even when the forward gear is engaged (and the clutch is open) or when the forward gear is set, and he can let the vehicle roll forwards in the downhill direction when the reverse gear is engaged (and the clutch is open) or when the reverse gear is engaged. The HDC controller 114 ensures that the HDC setpoint speed is not exceeded after the vehicle has started to roll.

Claims (15)

Method for supporting a starting process of a vehicle on a downhill gradient, starting from a vehicle standstill brought about by the driver of the vehicle applying the brakes, in which - a gradient variable (Road_Grad) characterizing the gradient angle of the downhill gradient and a starting direction desired by a driver are determined, - the determined Approach direction is evaluated in relation to the determined gradient variable (Road_Grad), and - according to a result of the evaluation carried out, a braking force is set in a brake system of the vehicle after a release of the brake on the driver's side has been determined, characterized in that determined in the evaluation whether the driver wishes to start the vehicle in an uphill direction or in a downhill direction, - in the event that the driver wishes to start in a downhill direction, a braking force is set which is dimensioned such that driving vehicle is moved at a low speed due to a downhill slope force and - in the event that the driver wishes to start off in the uphill direction, a braking force is set which is dimensioned in such a way that the vehicle is kept stationary. procedure after claim 1 , characterized in that the starting direction desired by the driver is determined on the basis of a gear selected in a transmission (104) of the vehicle or on the basis of a gear set on a transmission (104) of the vehicle Method according to one of the preceding claims, characterized in that in the event that a reverse direction is determined as the starting direction, a greater braking force is set than in the case that a forward direction is determined as the starting direction. Method according to one of the preceding claims, characterized in that in the event that the driver wishes to start off in a downhill direction, a braking force is set which is dimensioned in such a way that the vehicle is held stationary. Method according to one of the preceding claims, characterized in that the braking force is reduced with a predetermined braking force gradient. Method according to one of the preceding claims, characterized in that the braking force is reduced only after a predetermined first holding time has elapsed, which begins with the determination that the driver has released the brake. Method according to one of the preceding claims, characterized in that the braking force is reduced until a braking force control is taken over by a speed controller (114), which regulates the speed of the vehicle to a predetermined desired speed. Method according to one of the preceding claims, characterized in that the braking force is reduced after a predetermined second holding time, and/or that the braking force is reduced when a drive torque (M _Motor ) set by the driver on a drive motor (101) of the vehicle exceeds a threshold value (M_Thr) exceeds. procedure after claim 8 , characterized in that the braking force reduction is accelerated when the drive torque set by the driver (M _Motor ) exceeds the threshold value (M_Thr). procedure after claim 8 or 9 , characterized in that the threshold value (M_Thr) is determined as a function of the ascertained gradient size (Road_Grad). Method according to one of the preceding claims, characterized in that the braking force and/or the braking force gradient is/are determined as a function of the grade variable (Road_Grad) determined. Computer program product, comprising software code sections with which a method according to one of the preceding claims can be executed if the software code sections are executed on a processor device. computer program product claim 12 , characterized in that the software code sections are part of a software with which a speed control is carried out when the software is run on a processor device. Device for supporting a starting process of a vehicle starting from a vehicle standstill brought about by the driver of the vehicle by actuating the brakes, comprising: - at least one determination device (301; 302) with which a gradient variable (Road_Grad) characterizing the gradient angle of the gradient and one from a driver desired starting direction can be determined, - an evaluation device (302), in which the determined starting direction can be evaluated in relation to the determined incline variable (Road_Grad), and - a control device (304), with which, based on a result of the evaluation carried out, a braking force is calculated in a The vehicle's brake system can be adjusted after it has been determined that the brake has been released by the driver, characterized in that the device is set up for a method according to one of Claims 1 until 11 to perform. device after Claim 14 , characterized in that it is part of a cruise control system (112).

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