DE10057279A1 - Regulation method for directional stability of motor vehicles with modification of ESP longitudinal velocity regulation for rapid traveling speed reduction - Google Patents

Abstract

The method determines pressures for individual brakes to increase stability by yaw momentum regulation (ESP). During mainly stationary cornering and with understeered driving, measurements are made to detect whether during an ESP operation an actual longitudinal velocity deviates from a nominal longitudinal velocity. After the size of the deviation has been measured, the actual longitudinal velocity is brought towards the level of the nominal velocity within a set time period.

Description

The invention relates to a method for regulating the Driving stability, based on several input variables Pressures for the individual brakes are determined so that by ESP intervention increases the driving stability of the vehicle becomes.

In order to counteract vehicle instabilities automatically a variety of driving stability regulations have become known. Four come together under the term driving stability regulation Principles for influencing the driving behavior of a vehicle by means of presettable pressures or braking forces in or on individual wheel brakes and by intervention in the Motor management of the drive motor. It refers to Brake slip control (ABS), which occurs during a braking process to prevent individual wheels from locking in order Traction control system (ASR), which prevents the vehicle from spinning driven wheels prevented to electronic Brake force distribution (EBV), which is the ratio of the Braking forces between the front and rear axles of the vehicle regulates and a yaw moment control (ESP), which for stable driving conditions when yawing the vehicle around the vertical axis provides.

In this context, a vehicle is a motor vehicle meant with four wheels, which with a hydraulic, electro-hydraulic or electro-mechanical brake system is equipped. In the hydraulic brake system can be of a pedal-operated master cylinder by the driver applying brake pressure be built up while the electro-hydraulic and  electro-mechanical brake systems one of the sensed Build driver braking request dependent braking force. Hereinafter is referred to a hydraulic brake system. each Wheel has a brake, which each have an inlet valve and an exhaust valve are assigned. Stand over the inlet valves the wheel brakes in connection with the master cylinder while the Outlet valves to a pressureless container or Lead low pressure accumulator. Finally, there is one Auxiliary pressure source available, which is also independent of the Position of the brake pedal a pressure in the wheel brakes is able to build up. The inlet and outlet valves are for Pressure control in the wheel brakes can be actuated electromagnetically.

There are four for recording dynamic driving conditions Speed sensors, one for each wheel Yaw rate sensor, a lateral accelerometer and at least one pressure sensor for that generated by the brake pedal Brake pressure available. The pressure sensor can also be replaced be by a pedal travel or pedal dynamometer if the Auxiliary pressure source is arranged such that a driver built up brake pressure from that of the auxiliary pressure source not is distinguishable.

With a driving stability control, the driving behavior of a Vehicle so influenced that it is in for the driver critical situations become more manageable. A critical one The situation here is an unstable driving state in which In extreme cases, the vehicle does not follow the driver's instructions. The function of the driving stability control is therefore within the physical limits in such Situations the vehicle desired by the driver To lend vehicle behavior. An unstable driving behavior of a vehicle can occur in the event of understeer at which the measured yaw rate in the manner of  The difference to be achieved is that the vehicle is not so turns strongly into the curve, as expected.

An ESP understeer intervention taking place in this case braking of the inside rear wheel especially in stationary with strong tendency to understeer and high level of lateral acceleration through the curves Normal force reduction on the inside of the vehicle only one limited effect. Understeering driving situations of this kind arise when the driver has a high friction coefficient Prescribes or specifies the steering angle based on the course of the curve the vehicle must not at the current speed can follow. The real reason for that in such Situations recognized by the ESP regulation understeering So there is no instability in the course of the curve adjusted vehicle speed and that caused by it high level of lateral acceleration. Through the ESP Understeer intervention is caused by vehicle instability the initiation of a yaw moment about the vehicle vertical axis degraded, however, the vehicle speed can be reduced by the Braking forces built up during the intervention are limited be reduced. A reduction in the understeer tendency will only achieved when the low braking effect of the Understeer intervention and due to the delay due to high Cross slip the vehicle speed was reduced. This The time period can extend over several seconds.

The ESP understeer intervention is particularly useful for vehicles with high-lying assembly focus (e.g. off-road vehicles) Importance. These vehicles are due to the high Lateral accelerations under the boundary conditions mentioned Normal force of the wheels on the inside of the curve is reduced so much that the braking forces that can be built up are even lower and furthermore, not even a stabilizing one Building a yaw moment is possible. Because the vehicle speed  is reduced only to a small extent, increases due to the unlimited lateral acceleration level too the danger of these vehicles tipping over. In addition, can a reduction in the normal force level that affects the effect of the ESP Understeer intervention further reduced, also by a Shifting the center of gravity towards the front axle (e.g. Front-wheel drive vehicles with less at the same time Total mass) can be brought about.

The invention has for its object a modification of To create ESP driving stability control, which in addition to the Generation of an additional torque about the vertical axis allows faster lowering of the vehicle speed.

One way of reducing the speed of the Vehicle in the event of understeer is in EP 0945320 A1 described in the from the deviation of the measured Steering angle from a calculated steering angle a setting value is calculated for the longitudinal deceleration. When calculating the Steering angle is one of other sizes in real time estimated maximum value of the lateral acceleration (Saturation value) is taken into account.

According to the invention this object is achieved in that a generic method is carried out so that at a essentially stationary cornering at a understeering driving behavior is determined whether the ESP Intervention a deviation of an actual longitudinal speed of a target longitudinal speed of the vehicle is present, wherein according to the deviation within a predetermined Period the actual longitudinal speed to the target Longitudinal speed is led.

The ESP understeer intervention is modified such that in addition to braking by braking all wheels with a  suitable distribution through the lateral acceleration level Lowering the vehicle speed while maintaining Vehicle stability criteria the unstable driving behavior in a predetermined short period of time is reduced. The engagement is used here for simple naming with USLAR (Under Steering Lateral Acceleration Reduction). The procedure enables an improvement of the vehicle stabilization in the Understeer case, especially for vehicles with high Center of gravity and / or center of gravity near the Front axle. Especially for these vehicles, the special calculation of the target longitudinal speed directly too the lateral acceleration level can be set. Still is by coupling the USLAR intervention with regard to its Entry to the actual ESP entry or ESP intervention also ensured that only when vehicle instability Speed reduction is made, d. H. the driver only then does the system select the curve speed limited if there is a driving need. If however, if the intervention occurs, the level of delay can be chosen higher. Furthermore, by coupling the USLAR regulation entry to the ESP entry through which Friction value estimation of the ESP regulation, the currently critical Lateral acceleration level is known, so the dimensioning of the nominal value of the lateral acceleration and subsequently the Calculation of the nominal longitudinal speed only in the proposed form is possible. By the special choice of Brake pressure distribution to implement the target longitudinal deceleration is an effective compromise between Vehicle stabilization and braking ensured.

In order to increase driving stability, it is expedient that the desired longitudinal speed is determined directly from quantities that are present in an ESP-controlled vehicle. The target longitudinal speed directly after the relationship is advantageous

determined with
δ - steering angle on the wheel
l - distance of vehicle axles (wheelbase)
v _to - target vehicle speed
EG - self-steering gradient of the vehicle
a _q - vehicle _{lateral acceleration} .

It is expedient that the lateral vehicle acceleration a _q according to the relationship a _q = ka _q at ESP control entry with k = constant, preferably k = 0.8, and a _q ≦ 7 to 8 m / s ² , in particular a _q ≦ 5 m / s ² , is determined.

A deceleration request according to the relationship is advantageous for leading the actual longitudinal speed to the desired longitudinal speed

determined directly from the actual and target longitudinal speed with
v _is - speed of the vehicle when ESP control occurs
v _to - target vehicle speed
_should - delay request
Δt - predetermined time.

Over the specified period of time, the Excess speed can be reduced in a targeted manner, the time period depending on vehicle condition variables and / or information about driving behavior or the Driving situation determined or depending on the sizes and / or information can be changed online.  

The driving stability control is only modified in the ESP intervention in such a way that the deviation between the actual and target longitudinal speed is determined if at least one, preferably all, of the following conditions is met:

• 1. a.) The coefficient of friction is greater than 0.5
• 2. b.) Driver does not brake
• 3. c.) Vehicle speed is less than 100 km / h, preferably less than 80 km / h.

To further improve the modification of the ESP understeer intervention, the deviation between the actual and target longitudinal speed is only determined if at least one of the conditions that essentially represent a steady cornering is fulfilled:

• 1. d.) The vehicle yaw acceleration is less than 35 degrees / s ²
• 2. e.) The steering angle speed is less than 150 degrees / s
• 3. f.) The actual yaw rate and the target Yaw rate have the same direction of movement.

The pressures for the brakes are advantageously formed from the wheel-specific requirement of the ESP control and the requirement p _uslar in accordance with the longitudinal speed difference. The wheel-specific requirements are overlaid by the requirements from the longitudinal speed difference.

It is appropriate that the requirement on the inside of the curve Rear wheel to a slip value between 30 and 80%, preferably 50%. It will be advantageous addition, the requirement on the inside front wheel and the outer wheels limited to a slip value that the ABS blocking pressure level corresponds.  

In the event of an ESP regulation exit, the requirement on the The rear wheel on the inside of the bend is also advantageous on one Slip value limits the ABS blocking pressure level equivalent.

To further reduce the longitudinal speed, in the case of understeering driving behavior during the ESP control or outside the ESP control, the pressures for the brakes from the Puslar requirement are developed in accordance with the difference in longitudinal speed and controlled in the wheel brakes until at least one of the conditions

• 1. g.) Detection of oversteering driving behavior from the ESP control and / or
  Performing an ESP override,
• 2. h.) V _is ≦ v _should

is satisfied.

Furthermore, it is advantageous that the brake pressures on the front and rear wheels on the outside of the curve are each limited to an upper limit pressure p _{gr, va} or p _{gr, ha} .

For the implementation of the specified longitudinal _{deceleration,} the pressure requirement p _{uslar is formed} on all wheels until the limit pressure p _{gr, especially} on the front wheel on the outside of the curve, is reached in accordance with the following relationship:

is with
η - Efficiency wheel piston
r _effective - effective radius
r _dyn - dynamic radius
C * - brake constant
A - Brake piston area
m - vehicle mass and
v and h are indices for the front and rear axles.

It is expedient for a suitable distribution of the lateral acceleration level that the pressure requirement p _uslar on the wheels of the rear axle after reaching the limit pressure p _{gr, especially} on the front wheel on the outside of the curve until reaching the limit pressure p _{gr, ha} on the outside wheel on the corner according to the following relationship is formed:

With
m - vehicle mass,
_should - delay request,
p _{gr, va} - limit pressure of the front wheel on the outside of the curve,
K _{v / h} constant.

The invention first determines the actual target speed for the desired one Lateral acceleration based on the current driving condition and determines the speed profile for reaching this Target speed. The second element is the determination of the Conditions of entry into the regulation necessary. The third Element includes the determination of the brake pressure distribution in Dependence on the driving condition.

1.1 Calculation of target speed and deceleration profile

Using the stationary transfer function of a linear single-track model

in which
- yaw rate of the vehicle
δ - steering angle on the wheel
l - distance of vehicle axles (wheelbase)
v - vehicle speed
EG - self-steering gradient of the vehicle
and the force balance in the vehicle transverse direction for stationary vehicle behavior

v = a _q (2)

where a _{q is} the lateral vehicle acceleration, the following equation for the target longitudinal speed is derived:

The equation specifies the nominal longitudinal speed at which the vehicle should steadily follow a curve at a given steering angle and lateral acceleration. With the help of equation (3) at ESP control entry for the steering angle specified by the driver for the desired lateral acceleration (0.8 * lateral acceleration at ESP control entry, with a maximum limitation of 7 to 8 m / s ² , preferably approx. 5 m / s ² ) calculates the target speed. This is adapted to current changes in the steering angle and thus the driver's request during the ESP control. According to Eq. (3) calculated target longitudinal speed is only accepted if it is less than the current vehicle speed. Furthermore, the speed is limited downwards.

The time period Δt until the target speed is reached is set to a constant value, preferably 1 second, so that with constant deceleration from the equation

With
v _is - speed of the vehicle at ESP control entry
v _soll - longitudinal velocity according to Eq. (3)
the desired delay request _{is to be} determined.

1.2 Conditions of entry and termination

The USLAR regulation begins, ie the vehicle is decelerated to the nominal longitudinal speed derived under 1.1 if the following conditions are fulfilled individually or in combination:

• 1. a.) Coefficient of friction is greater than 0.5
• 2. b.) Driver does not brake
• 3. c.) Vehicle speed is less than 100 km / h, preferably less than 80 km / h
• 4. i.) ESP understeer intervention is active, i. H. understeering The ESP controller detects a situation requiring control
• 5. d.) Vehicle yaw acceleration is less than 35 degrees / s ²
• 6. e.) Steering angle speed is less than 150 degrees / s
• 7. f.) The actual and the target yaw rate have the same direction of movement (sign of the measured Yaw rate of the vehicle and the Reference model calculated are the same).

The conditions d.) To f.) Prevent the USLAR regulation in the case of highly unsteady and dynamic change maneuvers. The USLAR intervention is prevented. The USLAR brake intervention on all wheels is canceled if

• 1. g.) The ESP controller performs an oversteer intervention or
• 2. h.) The actual longitudinal speed of the vehicle the calculated Falls below target longitudinal speed.

The brake intervention is not canceled when the actual one ESP understeer intervention has ended. This is necessary because on the largely stationary, understeering reached Stability limit under high coefficient of friction a consequence of short ESP understeer interventions and the Target speed not reached within an ESP control becomes.

1.3 Brake pressure distribution

When calculating the wheel-specific wheel pressure requirement p _req , the requirement of the ESP control p _esp and the pressure requirement p _uslar are additively superimposed according to the following relationship:

p _req = p _esp + p _uslar (5)

In the understeer case under consideration, there is a pressure request from the ESP control only for the inside rear wheel. The slip of the wheel on this wheel is limited to approx. 50% by the ESP slip controller (pressure requirement of the slip controller: p _slip ). On all other wheels, the ABS pressure requirement is limited to the blocking pressure level, as can be seen in the following table.

p _esp = pressure request from the ESP control, p _uslar = pressure request from the USLAR control, p _slip = pressure request from the traction control system.

If the ESP is no longer active, the pressure requirement on this wheel is also reduced to the minimum of P _uslar and p _abs . Here, a modification of the ABS control strategy is necessary so that no select low control is carried out. With Select Low control, the wheel on the inside of the curve also determines the pressure modulation of the wheel on the outside of the curve in this ABS control mode. This is the only way to build up pressure on the outside rear wheel, possibly up to the ABS control level, regardless of the ABS activity on the inside wheel. Then a value for p _uslar is determined based on the delay _request from equation (4). The requirement for the brake pressure distribution is to generate braking forces which, if possible, ensure the deceleration determined according to equation (4) and which simultaneously stabilize the vehicle or at least do not have a destabilizing effect in the sense of the current vehicle state.

In the considered situations with high lateral acceleration, the inside of the vehicle is relieved relatively strongly, so that due to the low normal force level, the braking forces that can be applied are not very high. If significant braking forces are nevertheless to be built up, this can only be done on the outside of the curve. However, if one braked both wheels on the outside of the curve equally, a relatively high deceleration would be built up, but the already pronounced unstable understeer tendency would be further intensified. The effect on the transferable lateral forces must also be taken into account. If the pressure build-up on the front wheel on the outside of the curve is too great, the ability to transfer lateral forces is also impaired. This increases the understeer tendency of the vehicle. In order to find the contradicting requirements between braking and stability, the pressure build-up on the front wheel on the outside of the curve is limited to an upper level p _{gr, especially} . A pressure range is between 10 and 30 bar, preferably between 12 and 18 bar, preferably about 15 bar. The tendency to reduce the lateral forces on the outside rear wheel as a result of the pressure build-up creates a yaw moment which counteracts the understeer tendency. Since the pressure requirement of the ESP control (p _esp) the request _uslar p according to Eq. (5) is also added, the lateral force reduction and thus the ESP intervention is increased on the inside rear wheel. This means that the ESP intervention on the rear axle tends to reduce the lateral force in terms of quality, so to a certain extent the rather understeering torque can be compensated for by the braking forces on the outside of the curve. If the brake pressure on the rear wheel on the outside of the curve is increased above the blocking pressure level, the ABS control limits the further pressure build-up. Due to the slippage during the first ABS control cycle, the lateral force reduction can be too great, so that the understeer tendency abruptly changes into an unstable oversteering vehicle behavior. To prevent this, an upper limit pressure p _{gr, ha} is additionally introduced for the pressure request on the rear wheel on the outside of the curve, which is below the current blocking pressure level of the wheel. The upper limit pressure p _{gr, ha} is determined using an estimate based on the current lateral and longitudinal deceleration with the vehicle mass without a load.

The following table summarizes the printing requirements and qualitative effects together.

The limitation of the pressure level on the front wheel on the outside of the curve canceled as soon as the USPAR Regulation a transition from an under tax to an Oversteer situation is recognized, but not yet Oversteer intervention takes place. In the same turn, this will Limitation on the outer rear wheel to the previous one Front axle level lowered (preferably to one Pressure range of approx. 15 bar). This allows one Oversteer tendency to be counteracted more effectively.

The calculation of the pressure requirement p _uslar for a predetermined _{target deceleration} according to equation (4) is now carried out in the following way. In order to make the pressure request and thus the target deceleration tend to be higher, the already small braking force components on the inside of the curve are not taken into account. This results in the pressure requirement p _uslar (the same for all wheels), taking into account the force balance in the vehicle's longitudinal direction before the limit pressure p _{gr, va}

and
η - Efficiency wheel piston
r _effective - effective radius
r _dyn - dynamic radius
C * - brake constant
A - Brake piston area
m - vehicle mass
and the indices v for the front axle and h for the rear axle of the vehicle. If the limit pressure p _{gr, va is} exceeded, no further braking force is built up on the front axle and the relationship for calculating the pressure request is now until the limit pressure p _{gr, ha of} the rear wheel on the outside of the curve is reached:

The calculated pressure requirement according to equations (6) and (8) could also be a subordinate Determine the delay controller.

For convenience reasons, the setpoint pressure requirement is implemented with a limitation of the maximum change per loop in the following manner:

p _{req, i} = p _{req, i-1} + sign (p _{uslar, i} - p _{req, i-1} ) min (Δp _{req, max} ; abs (p _{uslar, i} - p _{req, i - 1} )) (9 )

Here is:
i - current loop
i-1 - last loop
p _req - pressure request for pressure regulator
Δp _{req, max} - absolute value of the maximum permissible change in the pressure requirement per loop.

A particularly advantageous application of the USLAR intervention exists with an electro-hydraulic brake (EMS), because the autonomous brake intervention on all wheels very comfortable (Noise level) can be carried out and also the Possibilities of hydraulic representability simply given are (volume requirement, necessary gradient for pressure build-up). Furthermore, it is also very easy and without any Effects on the brake pedal active driver braking overlay by taking the maximum from the driver pressure request and the autonomous brake intervention is used as the target pressure. This would enable the driver to conveniently Exceed the pressure request and the intervention must be done at one active driver braking cannot be canceled.

Claims (14)

1. A method for controlling the driving stability, in which pressures for the individual brakes are determined on the basis of a plurality of input variables, so that the driving stability of the vehicle is increased by an ESP intervention, characterized in that in the case of an essentially stationary cornering with an understeering driving behavior it is determined whether there is a deviation of an actual longitudinal speed from a desired longitudinal speed of the vehicle during the ESP intervention, the actual longitudinal speed being guided to the desired longitudinal speed within a predetermined period of time in accordance with the deviation. 2. The method according to claim 1, characterized in that the target longitudinal speed according to the relationship
is determined with
δ - steering angle on the wheel
l - distance of vehicle axles (wheelbase)
v _to - target vehicle speed
EG - self-steering gradient of the vehicle
a _q - vehicle _{lateral acceleration} . 3. The method according to claim 2, characterized in that the vehicle lateral acceleration a _q according to the relationship a _q = ka _q at ESP control entry with preferably k = 0.8 and a _q ≦ 7 to 8 m / s ² , in particular a _q ≦ 5 m / s ² , is determined. 4. The method according to any one of claims 1 to 3, characterized in that a delay request according to the relationship for leading the actual longitudinal speed to the desired longitudinal speed
is determined with
v _is - speed of the vehicle when ESP control occurs
v _to - target vehicle speed
_should - delay request. 5. The method according to any one of claims 1 to 4, characterized in that the deviation in the ESP intervention is determined if at least one of the conditions is met:

• 1. a.) The coefficient of friction is greater than 0.5
• 2. b.) Driver does not brake
• 3. c.) Vehicle speed is less than 100 km / h, preferably less than 80 km / h.

6. The method according to any one of claims 1 to 5, characterized in that the substantially stationary cornering is determined when at least one of the conditions is met:

• 1. d.) The vehicle yaw acceleration is less than 35 degrees / s ²
• 2. e.) The steering angle speed is less than 150 degrees / s
• 3. f.) The actual yaw rate and the target yaw rate have the same direction of movement.

7. The method according to any one of claims 1 to 6, characterized in that the pressures for the brakes from the wheel-specific requirement of the ESP control and the requirement p _{uslar are formed} in accordance with the longitudinal speed difference and the wheel-specific requirements from the requirements from the longitudinal speed difference be overlaid. 8. The method according to any one of claims 1 to 7, characterized characterized that the requirement on the inside of the curve Rear wheel to a slip value between 30 and 80%, preferably 50%. 9. The method according to any one of claims 1 to 8, characterized characterized that the requirement on the inside of the curve Front wheel and the outer wheels on a slip value is limited, which corresponds to the ABS blocking pressure level. 10. The method according to any one of claims 1 to 9, characterized characterized in that if the ESP rules exit the Requirement on the inside rear wheel on you Slip value is limited, the ABS blocking pressure level equivalent. 11. The method according to any one of claims 1 to 10, characterized in that in the _{understeering} driving behavior, the pressures for the brakes from the request p _{uslar are formed} in accordance with the longitudinal speed difference until at least one of the conditions

• 1. g.) Detecting an oversteering driving behavior from the ESP control and / or
  Performing an ESP override,
• 2. h.) V _is ≦ v _should

is satisfied. 12. The method according to any one of claims 1 to 11, characterized in that the braking pressure on the outer front and rear wheel is each limited to an upper limit pressure p _{gr, va} or p _{gr, ha} . 13. The method according to any one of claims 1 to 12, characterized in that the pressure _request p _{uslar is formed} on all wheels until the limit pressure p _{gr, especially} on the front wheel on the outside of the curve, is formed according to the following relationship:
is with
η - Efficiency wheel piston
r _effective - effective radius
r _dyn - dynamic radius
C * - brake constant
A - Brake piston area
m - vehicle mass and
v and h are indices for the front and rear axles. 14. The method according to any one of claims 1 to 13, characterized in that the pressure _request p _uslar on the wheels of the rear axle after reaching the limit pressure p _{gr, especially} on the front wheel on the outside of the curve until reaching the limit pressure p _{gr, ha} on the outside wheel on the curve is formed according to the following relationship:
m - mass of vehicle, _will - delay requirement, p _{gr, va} - limit pressure curve-outward front wheel, K _{v, h} - constant.