DE102004027257B4 - A method for preventing a vehicle from rolling away - Google Patents

Abstract

A method of preventing a vehicle from rolling away, wherein a brake pressure (pH) in a brake cylinder is established and / or maintained at at least one wheel of the vehicle during a stoppage of the vehicle after a rate of change (p.D) of a brake pressure (pD) in The braking pressure (pH) is established and / or maintained after several times a rate of change (p D) has been determined with the same sign whose amount exceeds a threshold value ,

Description

The invention relates to a method for preventing a vehicle from rolling away, in which a brake pressure is built up and / or maintained in a brake cylinder on at least one wheel of the vehicle during a standstill of the vehicle.

When starting a vehicle on a slope in a direction opposite to a slope force, the vehicle often unintentionally moves in the direction of the slope force and there is a risk that the vehicle crashes against an arranged in this direction obstacle and road users are harmed.

Substantial skill and skill of an operator of the vehicle is required in releasing the vehicle brake, which is usually to be operated with a foot pedal, and an immediately following operation of an accelerator pedal in order to prevent the vehicle from rolling back on a slope. Also, the use of a usually manually operated mechanical parking brake instead of to be operated by the foot pedal brake system requires skill and is also highly uncomfortable for the driver.

It is known to equip vehicles in support of the vehicle operator with a so-called Hill Start Assist function, which is based on the fact that a hydraulic brake system is acted upon by a mechanical or electronic control means during a standstill of the vehicle with a brake pressure, which is withdrawn, when the vehicle has reached a predetermined speed, or a predetermined drive torque of an engine of the vehicle is detected. The brake pressure usually corresponds to a set by the driver of the vehicle during standstill brake pressure.

Such a method is for example from the German patent DE 196 11 359 C1 out. This relates to a method for preventing a vehicle from rolling away, in which a brake pressure in a wheel brake cylinder is maintained after the driver has increased an actuation force when a brake pedal is actuated during a standstill of the vehicle.

In the known method, there is the problem that activation of the hill start assist function by increasing the braking force for the driver is uncomfortable and cumbersome. In particular, on a slope already a large braking force is required to ensure the stoppage of the vehicle; An increase in the braking force can only be achieved with a considerable effort in this situation.

From the generic DE 199 50 162 A1 a method for preventing the rolling away of a vehicle at start is known in which at least one characteristic actuation quantity of a brake pedal, in particular a pressure gradient of the brake pressure is detected when the vehicle is at rest, wherein a service brake control pressure is activated when the characteristic actuation quantity at least one predetermined criterion in particular, the magnitude of the pressure gradient exceeds a predetermined positive value, and wherein a time-delayed deactivation of the service brake control pressure occurs after the brake pedal has been released.

The invention has for its object to further develop a generic method so that the hill start assist function can be more easily activated by the driver and accidental activation is reliably avoided.

According to the invention this object is achieved by a method according to claim 1.

According to the invention this object is also achieved by a method according to claim 4.

Advantageous developments of the methods are the subject of the dependent claims 2 to 3 and 5 to 6.

The invention provides a method for preventing a rolling away of a vehicle, in which a brake pressure is built up and / or maintained in a brake cylinder on at least one wheel of the vehicle during a stoppage of the vehicle to perform so that the brake pressure is maintained, after a Rate of change of the brake pressure has been determined in the brake cylinder whose amount exceeds a predetermined threshold.

In this case, in order to reliably avoid unintentional activation of the hill start assist function by the driver, according to the invention, the brake pressure is to be built up and / or maintained after a rate of change of the same sign has been determined several times, the magnitude of which exceeds a threshold value. Appropriately, the activation is done by setting twice such a rate of change for the brake pressure.

In contrast to the activation after a one-time setting of a rate of change, the threshold for the amount of the rate of change is Preferably, this embodiment of the invention is smaller, so that the actuation force required for activation is further reduced.

The method enables the driver to activate a hill start assist function by means of an actuation device of a brake system of the vehicle in a particularly simple and comfortable way, by setting a predetermined rate of change for the brake pressure. Due to the apparent common reference of the action to be performed by the driver and the Hill Start Assist function per se to the brake system of the vehicle, the method is quickly and easily adapted by the driver. It has the particular advantage that an activation without great use of force is possible.

In a preferred embodiment of the method, the rate of change is determined during an increase in the brake pressure. The activation takes place by means of a rapid increase of a force applied to operate the actuator of the brake system and force is particularly easy to control for the driver. In particular, in this embodiment only very low demands are placed on a motor coordination ability of the driver.

In a further preferred embodiment of the method, it is provided that the gradient is determined during a reduction of the brake pressure. An increase of the applied force for operating is not required here, so that, especially on slopes comfort in the activation of the Hill Start Assist function is further increased.

The invention also provides a method for preventing a vehicle from rolling away, in which a brake pressure in a brake cylinder is built up and / or maintained at at least one wheel of the vehicle during a stoppage of the vehicle, and which is characterized in that the brake pressure is built up and / is maintained after a predetermined steering wheel movement has been detected.

This allows activation of the Hill Start Assist function, which can be performed with minimal effort. In addition, a steering wheel operation during the stoppage of the vehicle affects the state of motion in any way, so that a particularly secure activation is ensured.

The steering wheel movement is preferably a turning in and a subsequent return to a starting position, so that a steering wheel position is not substantially changed during the activation.

In order to avoid an unintentional activation of the hill start assist function by the driver, it is advantageously provided that the brake pressure is built up and / or maintained only when a steering wheel angle is adjusted during steering, the magnitude of which is a predetermined threshold value exceeds.

A more extensive protection against accidental activation is done in a preferred embodiment of the method in that an activation takes place only when the steering wheel is rotated with a predetermined direction of rotation.

The invention thus provides advantageous methods that allow a very simple and comfortable activation of the Hill Start Assist function. Further advantages and expedient developments emerge from the dependent claims and the following description of preferred embodiments of the invention.

In this case, the starting point is a vehicle with a hydraulic brake system in which the actuating device operated by the driver of the vehicle is connected to a brake booster which builds up a pressure within a hydraulic fluid in a master cylinder of the brake system. This is transmitted via hydraulic lines to the brake cylinders on the braked wheels of the vehicle, which usually consist of two independent brake circuits comprising different wheel brake cylinders. The actuating device is usually a brake pedal to be operated by the driver with one foot.

In addition to mechanical-hydraulic brake systems are particularly suitable electro-hydraulic brake systems for carrying out the method according to the invention, in which the brake booster is replaced by a pressure supply unit, which is electronically connected via a control device to the actuator.

However, the invention is by no means limited to the use of hydraulic brake systems, but can also be carried out with the aid of other types of brake systems, such as pneumatic brake systems.

The starting point for the invention is the possibility of being able to set a braking force of the brake system of the vehicle independently of a position of the foot pedal to be actuated by the driver for braking Vehicles with anti-skid control (ABS), traction control (ASR) and / or braking intervention based yaw rate control (ESP).

Brake cylinders of wheel brakes are typically connected via solenoid valves designed as valves to the master cylinder, which can be operated in three different positions. In a first position, there is an unobstructed passage from the master cylinder to the wheel cylinder, so that the brake pressure in the master cylinder is transmitted to the wheel cylinders. In a second position, the passage from the master cylinder to the wheel cylinder is interrupted, so that the brake pressure in the wheel cylinder is kept constant. In a third position of the wheel cylinder is connected to a return, so that the brake pressure drops in this wheel cylinder.

Further, in many vehicles there is an electronically controlled pressure supply, typically designed as a pump, which makes it possible to set a pressure which is greater than the pressure built up by the driver by means of the brake booster in the master cylinder.

There is also the possibility of adjusting the brake pressure by means of an active brake booster, which in addition to the control by the actuating means allows additional control by a normally inductively generated external force.

For carrying out the method according to the invention and in particular for establishing and / or maintaining the brake pressure, valves and / or a pressure supply are used, which are controlled by a control unit. Control of the active brake booster by the control unit is also possible.

The control unit is either self-contained or integrated in an ABS, ASR and / or ESP controller.

In the proposed hill-start assist function, the control of the valves and / or the pressure supply or the active brake booster is made such that a brake pressure p _{H is} built up and / or maintained in the wheel brake after the function based on an activation signal from the driver has been activated.

The brake pressure p _H ensures the standstill of the vehicle when the driver for starting the vehicle triggers the actuator of the brake system and should therefore be at least as large as required for holding the vehicle minimum brake pressure.

It is determined either on the basis of an evaluation of a progression of a signal representing the brake pressure p _D set by the driver during standstill of the vehicle or in dependence on a mass of the vehicle and / or a present longitudinal inclination angle of the vehicle.

When determining the brake pressure p _H on the basis of the brake pressure p _D set by the driver, the signal representing the brake pressure p _{D is} detected from a time t ₀ , which is marked by the start of the standstill of the vehicle. The standstill can be detected, for example, based on measurement signals ω of wheel speed sensors, which correspond to the angular velocities of wheels of the vehicle.

The signal representing the brake pressure p _D may be a measurement signal of one or more brake pressure sensors which detect the brake pressure p _D in the master brake cylinder or in the wheel brake cylinders.

In addition to such a direct detection of the brake pressure p _D , however, it is also possible to detect by means of other sensors, a signal from which the brake pressure p _D can be derived. For example, force sensors for measuring the force exerted by the driver on the actuating device of the brake system come into question here.

When generated by a brake pressure sensor within the master cylinder or within a wheel brake cylinder, the signal corresponds to the brake pressure p _D itself, while the signals supplied by another sensor typically need to be converted to obtain the value for the brake pressure p _D. Such a conversion can be made already during the detection of the signal or after an evaluation of the signal and will not be considered further below. Instead, no explicit distinction is made between the signal and the brake pressure p _D.

The brake pressure p _H is determined as the mean or minimum value of the detected values of the brake pressure p _D , wherein an additional filtering of the signal representing the brake pressure p _{D is performed} during the detection in order to hide values of the brake pressure p _D from the course of the signal are too small to ensure the vehicle is stationary, but are set by the driver so short that they do not lead to a rolling up of the vehicle.

In particular, in the case of an intended minimum value formation, the filtering is necessary in order to determine a value p _H for the brake pressure, which ensures a safe standstill of the vehicle.

A filter for filtering the signal is a low-pass filter, which is implemented, for example, in the form of a so-called PT ₁ transmission element.

By means of the filter changes the braking pressure p _D are hidden corresponding measurement signal from the course of p to the brake pressure _D which contain frequencies in terms of a Fourier transform of the signal which exceed a reference to the time constant of the PT ₁ -Gliedes predetermined value. In particular, changes in the signal with smaller rates of change are masked out for larger time constants.

It is intended to adapt the time constant of the low-pass filter to a brake pressure p _D0 set at time t ₀ or shortly before by the driver.

For this purpose, the brake pressure p _{D0 is} first evaluated on the basis of a comparison with a reference pressure as to whether the pressure is excessively high or whether the brake pressure p _D0 assumes a value which is close to a minimum value of the brake pressure necessary for holding the vehicle. The reference pressure is either preset statically or determined dynamically.

In a static specification, a maximum required brake pressure is used as the reference pressure, which is determined, for example, as a function of a maximum permissible total weight of the vehicle together with a trailer mass m _max and a maximum slope sin (α _max ) such that a due the reference pressure resulting braking force a slope force m _max · g · sin (α _max ), in which g denotes a gravitational acceleration, just compensated. The maximum permissible total weight and the maximum gradient sin (α _max ) are specified by a manufacturer of the vehicle.

In contrast, a dynamic determination of the reference pressure takes into account a current gradient sin (α) and / or a currently present mass m of the vehicle. In particular, the reference pressure is determined such that the braking force resulting from the reference pressure just compensates for the downward force m · g · sin (α).

The gradient sin (α) can be determined taking into account a rotational behavior of the wheels of the vehicle by means of the measured values of a longitudinal acceleration sensor. In this case, a measurement signal of the longitudinal acceleration sensor corresponds to a sum of a change in a speed of the vehicle to be determined on the basis of the rotational behavior of the wheels and of a slope acceleration. The slope sin (α) or the longitudinal inclination angle α of the vehicle can be determined on the basis of a downhill acceleration g · sin (α) which can thus be determined from the measurement signal.

In addition, it is possible to ascertain the gradient sin (α) during the deceleration process preceding the standstill of the vehicle on the basis of the braking force and a deceleration of the vehicle.

The mass m of the vehicle can be detected on the basis of a detected by corresponding sensors deflection of spring-mounted body parts of the vehicle or determined at a known propulsion or braking force based on a vehicle acceleration.

If the comparison of the brake pressure p _D0 with the reference pressure _reveals that the difference between the two pressures is less than a predetermined value, the low-pass filter is set so that even very slow changes in the brake pressure are filtered. This is achieved by setting a high value for the time constant of the low-pass filter.

It is thereby ensured that even such brake pressures p _D hidden and not taken into account in the determination of the brake pressure p _H , resulting in a reduction of the brake pressure by the driver, which is made very slowly.

When the difference between the brake pressure exceeds p _D0 and the predetermined value, the driver has set a brake pressure p _D0 which is far above the brake pressure required to hold the vehicle.

In this case, it is generally not to be assumed that the driver temporarily sets a brake pressure that is too low for holding the vehicle by slowing down the brake pressure p _D very slowly.

Therefore, a small time constant can be set, so that a more accurate detection of the actual course of the measurement signal takes place. In particular, even lower pressure reductions are detected, which enable an effective limitation of the brake pressure to be established or maintained on the basis of the mean value or minimum value formation.

The detection of the filtered brake pressure p _D beginning at the time t ₀ is continued until a predetermined event occurs.

As one of the events that cause an end of the detection, a rolling of the vehicle is provided, which is detected by a change in the angular velocity ω of the wheel speed sensors. A consideration of present during a roll of the vehicle brake pressures p _D for the determination of the brake pressure p _H does not seem appropriate, since these brake pressures p _{D are} usually too low to ensure a stoppage of the vehicle.

If the amounts of the angular velocity ω of the wheels of the vehicle do not exceed a threshold within a predetermined period Δt _A and again assume the value ω = 0 within the period Δt _A , the detection of the brake pressure p _D continues.

It can be assumed in this case that the driver has reduced the brake pressure without intending to start.

Another event leading to the termination of the detection is a lapse of a time period Δt _B , which begins at the time t ₀ .

A duration of the time interval .DELTA.t _B should be selected so that a brake pressure p _H can be reliably determined, which ensures the stoppage of the vehicle.

In addition, the experience has been made that a driver often brings the vehicle to a standstill with a very high brake pressure and reduces the brake pressure at or just before the standstill.

Particularly in the case of an intended average value for determining the brake pressure p _{H is} the duration of the period of time .DELTA.t _B is therefore sufficient to select long, so that the average p _H is as low as possible influenced by a very high braking pressure p _D at the beginning of the standstill of the vehicle. The duration of the period is, for example, 0.5 s to 5 s.

Furthermore, it can be provided that the detection of the signal for activating the hill-start assist function is an event whose occurrence results in termination of the detection when the brake pressure p _{H is set} by the control unit immediately after activation.

The control unit recognizes the occurrence of the events which cause an interruption or the interruption of the detection of the brake pressure p _D. After its occurrence, it makes an evaluation of the detected values of the brake pressure p _D , if this has not already been carried out online, as may be the case, for example, with a minimum value formation. From this evaluation, the brake pressure p _H results.

In addition to determining the brake pressure p _H on the basis of the mean or minimum value formation, it may also be provided to determine the brake pressure p _H as a function of the mass m of the vehicle and the present pitch angle α so that a resulting due to the brake pressure p _H braking force downhill force m · g · sin (α) acting on the vehicle is just compensated.

The control unit also monitors whether an activation signal is given by the driver of the vehicle during the stoppage of the vehicle.

In this case, a change of the brake pressure p _D set by the driver is provided as an activation signal, in which an amount of a rate of change p, designated as a pressure gradient, is provided. _{D of} the brake pressure p _D exceeds a predetermined threshold.

To detect this signal, in addition to the brake pressure p _D also the gradient p. _{D of} the brake pressure monitored by the control unit and compared continuously or quasi-continuously with the threshold. The pressure gradient p. _D can be determined, for example, by means of a differentiator from the signal p _D.

It can be adjusted by the driver during an increase and / or a reduction of the brake pressure p _D , ie with a positive and / or a negative sign.

If a variant is implemented, in which the activation of both a positive and a negative pressure gradient p. _D , the controller compares the magnitude of the pressure gradient with a threshold and detects the activation signal when the magnitude exceeds the threshold.

A comparison of the value of the pressure gradient p. _D even with the threshold occurs when the driver to set a positive or negative pressure gradient, the activation signal is detected in these cases, if the value of p. _D exceeds or falls below the threshold.

In alternative embodiments, the signal for activating the Hill Start Assist function in a single or multiple, for example, a two-time setting of an amount above a predetermined threshold pressure gradient p. _D with the same sign.

If a repeated setting is provided, a lower threshold value can be used than with the activation by a single setting of a gradient p. _D , as in this case, even with a low threshold, there is sufficient protection against inadvertent activation.

If it is not the brake pressure p _D itself but a signal from which the brake pressure p _{D can be} derived, the monitoring of the activation signal takes place in an analogous manner by monitoring the rates of change of this signal and comparing its rates of change with a predetermined threshold value.

Alternatively, the Hill Start Assist function can be activated by a defined steering wheel movement. This consists, for example, the steering wheel, starting from a present during standstill of the vehicle starting position δ ₀ to turn a steering wheel angle Δδ, the amount exceeds a predetermined threshold.

In addition, it can be provided that the steering wheel then has to be returned to the starting position.

If this type of activation is provided, there is a monitoring of a steering wheel angle δ by the control unit, for which signals of a steering wheel angle sensor are transmitted.

In order to detect the activation signal, thereby a at the time t ₀ the present steering angle δ ₀ is stored by the control unit, and the difference Δδ = δ - δ ₀ δ between the current steering wheel angle and the steering wheel angle δ ₀ is continuously monitored.

Depending on whether only a turning in with a given amount or additionally a turning in a predetermined direction is required for activation, the amount of the difference Δδ or the value Δδ itself is compared with a corresponding threshold value. If the threshold value is positive, the activation signal is recognized in particular by the fact that the value of Δδ or the amount of Δδ exceeds the threshold value. Accordingly, detection at a negative threshold occurs when the value of Δδ falls below the threshold.

The steering wheel angle δ is usually defined as having a steering wheel angle δ = 0 when rim planes of steerable wheels of the vehicle are aligned parallel to a vehicle longitudinal direction and causes an increase / decrease by turning the steering wheel counterclockwise / clockwise becomes.

If, for example, a turning in by an angle δ _thr <0 in the clockwise direction is provided for activating the hill-start assist function, then the control unit recognizes the activation if Δδ <δ _thr holds.

If in addition to the steering also a return of the steering wheel to the starting position δ ₀ for activating the Hill Start Assist function is required, the detection of the signal takes place in that the difference .DELTA.δ after the detected exceeding or falling below the threshold within again assumes the value Δδ = 0 for a given period of time.

Between the steering wheel angle δ and the steering angle at steerable wheels of the vehicle there is a gear ratio i. The aforementioned steps for detecting a steering wheel movement for activating the hill start assist function can therefore be carried out in an analogous manner on the basis of an evaluation of the steering angle on the steerable wheels of the vehicle.

The brake pressure p _H is set either immediately after the activation of the Hill Start Assist function, after the expiration of the time period Δt _B , after a period beginning with the detection of activation or then when the brake pressure p _D after activation the value p _{H falls} below.

For setting the brake pressure p _H , the currently present brake pressure p _D is first compared with the brake pressure p _H.

When both brake pressures are equal, the valves of the wheel brake cylinders are closed so that the pressure in the wheel brake cylinders is maintained.

If the comparison shows that the brake pressure p _{D is} greater than the brake pressure p _H , the position of the valves, at which a pressure reduction takes place, so long activated until the brake pressure in the brake cylinders reaches the value p _H to the valves then close.

A pressure increase by means of the pressure supply is made in the case in which the brake pressure p _{D is} smaller than the value p _H. After reaching the value p _H in this case, the valves are closed.

In particular, when determining the pressure p _H by the minimum value formation is usually a pressure reduction may be required. A pressure build-up by means of the pressure supply is required in a minimal value formation in any case, so that no pressure supply is required in this embodiment.

If an adjustment of the brake pressure p _{H is} not provided immediately after the detection of the activation signal, information is stored in the area of the control unit that the Hill Start Assist function has been activated by the driver or the control unit is turned into a offset here as active mode designated operating mode.

In the active mode, the above-mentioned steps for setting the brake pressure p _{H are performed} after the elapse of the time period Δt _B or after the lapse of the period beginning with the detection of the activation signal.

Alternatively, it may be provided that the detection of the brake pressure p _D in the active mode is continued beyond the time period Δt _B , in order to maintain the brake pressure by closing the valves when the brake pressure set by the driver is just below the value p _H sinks. This embodiment is particularly suitable when a minimum value formation for the determination of p _{H is} provided.

In order to bring about a timely closing of the valves, while the gradient p. _D monitors. The pressure to be expected after a certain short time T ₁ results in the form p _D + p. _D · T ₁ from the current brake pressure p _D and the pressure gradient p. _D. By considering the pressure gradient p. _D can thus be precisely determined the time for closing the valves, and it is ensured that is set by the control unit not too low brake pressure.

The advantage of this embodiment is that the wheel brake cylinders are only separated from the master cylinder when it is actually required, making the hill start assist function more comfortable for the driver.

When the valves between the master cylinder and the wheel brake cylinders are closed, the hydraulic fluid can not escape into wheel brake circuits when the brake device is actuated, so that the actuating device becomes very stiff. Likewise, a release of the actuating device is made more difficult by the fact that no hydraulic fluid can flow from the wheel brake circuits into the master brake cylinder. The problems associated with this implementation do not occur.

LIST OF REFERENCE NUMBERS

• α

  Longitudinal angle of the vehicle

  α _max

  maximum permissible longitudinal inclination angle of the vehicle steering wheel angle

  δ ₀

  Steering wheel angle at time t ₀

  Δδ

  Steering angle difference

  Δt _A

  Period of time

  Δt _B

  Period of time

  G

  acceleration of gravity

  m

  vehicle mass

  m _max

  maximum permissible vehicle mass

  ω

  angular velocity

  p _D

  Brake pressure set by a driver

  p. _D

  Rate of change of the brake pressure set by the driver

  p _D0

  initially set by the driver brake pressure

  p _H

  to be built up and / or maintained brake pressure to hold the vehicle

  T

  time

  T ₁

  time

  t

  Time

  t ₀

  Time at which a standstill of the vehicle or a detection of the brake pressure set by the driver begins

Claims (6)

A method for preventing a rolling of a vehicle in which a brake pressure (p _H) is built in a brake cylinder on at least one wheel of the vehicle during a standstill of the vehicle and / or maintained after a change rate (p. _D) a brake pressure (p _D ) has been determined in the brake cylinder whose amount exceeds a predetermined threshold, characterized in that the brake pressure (p _H ) is built up and / or maintained after several times a rate of change (p _D ) has been determined with the same sign whose amount exceeds a threshold. A method according to claim 1, characterized in that the rate of change (p. _D) during an increase of the brake pressure (p _D) is determined. A method according to claim 1, characterized in that the rate of change (p. _D) during a reduction of the brake pressure (p _D) is determined. Method for preventing a vehicle from rolling away, in which a brake pressure (p _H ) is built up and / or maintained in a brake cylinder on at least one wheel of the vehicle during a standstill of the vehicle, characterized in that the brake pressure (p _H ) is built and / or maintained after a predetermined steering wheel movement has been detected. A method according to claim 4, characterized in that the predetermined steering movement consists in starting the steering wheel, starting from a present during standstill of the vehicle starting position to a steering wheel angle, the amount exceeds a predetermined threshold. A method according to claim 4 or 5, characterized in that it is at the predetermined steering movement to a steering and a subsequent return to the starting position.