DE19830189A1 - Method of improving tilt stability of motor vehicle - Google Patents

Abstract

The method involves reducing tilting if a parameter which correlates with vehicle (1) transverse acceleration exceeds a first threshold value. When the vehicle starts a journey, the threshold value is at a lowest value at which the danger of tilting occurs for permitted vehicle loading. The correlating parameter is at a point approximately corresponding to the unladen centre of gravity (G) and is a function of the transverse acceleration and its time derivative

Description

The present invention relates to a method for Increase the lateral tilt stability of at least one two-axle and two-lane vehicle, i.e. one at least three-wheeled vehicle.

In T.D. Gillespie's book "Fundamentals of Vehicle Dynamics", Society of Automotive Engineers, Inc., Warrendale 1992, Chapter 9, pp. 309-333, are different models for Rollover accidents described. Starting with a quasi stationary model for a rigid vehicle over a quasi-stationary model for a sprung vehicle up to to dynamic models taking into account Rolling natural frequencies become conditions for existing ones Risk of tipping calculated.

While it is for trucks, trucks, buses, minibuses and off-road vehicles due to their high center of gravity and / or narrow track width already when the o.g. Book was known that when cornering with large Roll motion there is a risk of tipping, it has only turned in recently shown that passenger cars - especially with sinusoidal steering movements - sideways to can rock up to tip over. Such a risk of tipping over is caused by improper loading, for example extreme  one-sided or on the vehicle roof, significantly increased because the position of the center of gravity of the vehicle upwards or is shifted to one side.

DE-A 197 46 889 describes a system for increasing the Side stability described when cornering, which with an inclination detection device. This Tilt detector either measures the height below differentiated between the right and left side of the vehicle or the Lateral acceleration of the vehicle to the roll angle between the vehicle horizontal and the road horizontal to capture. If the inclination detection device If the danger of tipping is recognized, a counteracting yaw moment is created generated by braking the front wheel on the outside of the curve.

However, as already described above, the allowable Lateral acceleration and the permissible roll angle depend on the location, in particular the height of the vehicle's weight point.

The present invention is therefore based on the object to create a process that is also taking into account unfavorable loading of a risk of tipping as required counteracts.

This object is achieved in that Starting the journey is not the center of gravity of the empty vehicle, but the most unfavorable vehicle center under Taking into account the permissible vehicle loading for Calculation of a stability threshold is used. This stability threshold can be a lateral acceleration or a correlating variable, for example a yaw rate be. However, the choice of lateral acceleration  compared to the yaw rate the advantage that a vehicle-fixed built-in lateral accelerometer automatically lateral Road inclinations, for example in excessive bends, considered.

In the further course of the journey will be at the start of the journey given mass distribution in the vehicle not too much change: other seating positions can be adopted, and the level of the vehicle tank drops. In relation to Vehicle dimensions are only small and / or slow Changes. While driving you can from the Vehicle sensors further observations of the acceleration and Delay behavior are made up of more precisely the actual location of the vehicle's center of gravity can be calculated.

For a more detailed description of details of the invention in the following a drawing is used.

Fig. 1 shows a schematic representation of a four-wheeled vehicle in rear view when driving through a left curve.

The following sizes are shown in the drawing:
G Center of gravity of the vehicle 1
h _G height of the center of mass M
R Center of rotation of a vehicle roll
h _R Height of the center of rotation R
s Vehicle track width 1
Φ roll angle
F _G Gravity of the vehicle 1
F _yi lateral force on the inside tire 2
F _zi high power on the inside tire 2
F _yo lateral force on the outside tires 3
F _zo high power on the outside tires 3

The following considerations are made without restricting the general validity for the vehicle 1 traveling through a left curve. In order to be equally applicable to a right-hand curve, the terms of the following equations would have to be provided with dashes, which was omitted for the sake of clarity.

Lifting of the inside wheel 2 from the ground begins when the lever torques in the vehicle-fixed system cancel the center of gravity around the wheel contact area of the outside wheel 3 , which is regarded as a point here. Counterclockwise for small Φ with m as vehicle mass:

mg [s / 2-Φ. (h _M -h _R )]

with g = 9.81 m / s ² .

The virtual centrifugal force -Fy = -a _y .m acts clockwise and causes the lever torque:

-ma _ykrit .h _M.

Equation of the lever moments, substitution of Φ with the help of R _Φ , the dimensionless roll rate Φ.g / a _y , which gives the change of the roll angle Φ (in rad) with the lateral acceleration (in multiples of g) and results in resolution according to a _ykrit for the critical lateral acceleration at which the inner wheel lifts off, the following condition:

The critical lateral acceleration is the smaller, ever further the center of mass above the roll rotation center lies. The location of the center of gravity is therefore for Statements about the lateral stability are important.

This simple calculation of the critical lateral acceleration only applies in the quasi-stationary case. The vehicle can rock up during dynamic steering maneuvers with deflections similar to vibrations. Therefore, a stabilizing system should have its entry _threshold a _yon even with smaller lateral accelerations:

a _yon = da _ykrit

with 0 <d <1.

The exit threshold from a stabilizing control is determined analogously:

a _yoff = ea _ykrit

with 0 <e <d.

The change, that is to say the time derivative _{y of} the lateral acceleration a _{y, can be} used as an additional variable for recognizing a risk of tipping, as a result of which a rocking can be detected, in particular in the case of alternating-dynamic maneuvers. Exceeding a threshold value _yon , which is dependent on the current and critical lateral acceleration, can be introduced as an AND condition to initiate a stabilizing measure:

a _y <a _yon

and _y < _yon .

Or an overall view can be made with a stability condition according to:

ia _y + yes _y ≦ a _ykrit

with i, j as empirically obtained vehicle-specific parameters, where i is dimensionless and j has the dimension of a time. As soon as it applies

ia _y + jy <a _ykrit ,

a stabilizing measure is initiated. Yourself can also be understood instead of a linear relationship square or other shape can be chosen. The best suitable relationship will show through if necessary Have test series determined.

According to the selected entry condition, the exit condition is then:

a _y <a _yoff
and _y < _yoff
with _yoff = f. _yon and 0 <f <1; or:
ia _y + j. _y ≦ ka _ykrit
with 0 <k <1;

or a correspondingly more complex criterion.

A high lateral acceleration occurs when large Lateral forces are transmitted between the tire and the road. The friction in the tire contact patch, i.e. the contact area between The road surface and tires determine how high the transferable Horizontal force is. The vector sum of longitudinal force and Lateral force can be determined from the coefficient of friction Do not exceed maximum force. To be a dangerous one So reducing lateral acceleration can be a  Control intervention is carried out using the longitudinal force Brake intervention increases and therefore the maximum transferable lateral force reduced. The vehicle will then assume an understeering behavior, i.e. a larger one Follow curve radius.

A stabilizing control intervention that is active, that is automatic brake actuation will definitely brake the front wheel on the outside of the curve. For one, are at when cornering, the outside of the vehicle and when braking the front axle is heavier. The front wheel on the outside of the curve can therefore significantly higher forces between tires and Roadway transmitted than the other wheels, so that here the greatest effect is expected. On the other hand, the Force vector of the front wheel built on the outside of the curve Longitudinal force on the outside of the curve on Center of gravity over and thus contributes supportively Stabilization at. A similar supportive effect is to be observed on the outside rear wheel. However here, if there is no anti-lock braking system, Caution should be exercised as blocking the more heavily loaded outer rear wheel in front of the front wheels, as is well known Can break out of the rear of the vehicle. Of the Longitudinal force vector of the front wheel on the inside of the curve shows only when the steering wheel is turned extremely hard outer side of the center of gravity, so that the longitudinal force usually works against stabilization, but what in Not difficult considering the reduced side force weight. To brake the inside rear wheel brings no advantages for the side stabilization, because this bike as a lightly loaded bike plays only a minimal role plays an additional role when transferring lateral forces  unfavorable longitudinal force vector and Blocking would favor a skid tendency.

Basically, there should be no total loss of cornering can be brought about, otherwise the vehicle will simply be from the road slips, which of course not desirable alternative to overturning the vehicle represents.

If during a braking initiated by the driver critical lateral acceleration is recognized, the existing braking forces taking into account the desired lateral force reduction can be redistributed, whereby the total braking torque must not decrease. As long as the wheels on the rising section of the known µ (λ) curve are, is an increase in braking force not critical. When the maximum is reached, however Be considered that a further increase in Brake pressure a - albeit slight - loss Brings braking torque with it.

For the reasons mentioned above, it is advisable to use one Brake intervention to reduce lateral force only on the linearly increasing branch of the µ (λ) curve below the Saturation, so in the so-called partial braking range.

In addition, an active vehicle suspension can be provided that at least partially compensates for the roll angle, by lifting the outside of the vehicle. Such systems are e.g. B. developed for trucks and buses been.  

At the start of the journey, the stability threshold for the Lateral acceleration assumed a value that ensures that with any - legally permissible - loading a tipping over through a rule intervention with the greatest probability can be avoided if physically possible. in the further course of a journey can be observed by observing the Wheel sensor signals further conclusions on the location of the Center of gravity are drawn, which u. U. an increase allow this threshold:

Provided that the drive motor at Acceleration applied drive torque is known can the driving force, i.e. the longitudinal force between the tire and Lane can be calculated. The wheel sensors preferably Non-driven wheels deliver the vehicle achieved speed from which the vehicle acceleration can be determined by time derivation. From the Acceleration divided by the driving force results the vehicle mass. To consider the at a roll or The pitching movement of the moving mass m becomes the well-known vehicle specific mass of all unsprung parts subtracted.

Similar considerations can of course also be done during start braking with the vehicle deceleration, whereby a defined braking force is to be assigned to a braking pressure.

The braking of a vehicle can also be used to infer the given mass shift, that is to say the pitch angle of the vehicle, for given decelerations on the basis of slip differences occurring on the front and rear axles. With known elasticities in the vehicle suspension and known sprung mass m, the height h _{m of} the center of gravity can be calculated from the pitch lever moment. Slip differences on the front and rear axles can also be determined from the acceleration in all-wheel drive vehicles.

The determined height of the center of gravity can then be used in the equation for calculating the critical lateral acceleration a _ykrit in order to use it to modify the entry _{threshold value} a _yon or a more complex entry condition.

This type of center of gravity can be determined once immediately at the start of the journey during the first acceleration cleaning and braking maneuvers are carried out after repeated at certain time intervals or at each suitable braking and / or acceleration (preferably without turning the steering angle).

If the vehicle in question has a system for Yaw moment control should be equipped, this should be System undergo certain modifications. A Yaw moment control regulates the yaw rate of a vehicle a setpoint. The setpoint is usually set to one limited physically meaningful value. The physical However, considerations generally only take into account the Coefficient of friction of the road surface: With high friction the maximum applicable target yaw rate is higher than at Low friction. However, there is especially high friction increased risk of tipping due to large transferable side forces. The limitation of the target yaw rate should therefore also below Consideration of the critical lateral acceleration or the Entry threshold for the lateral stability control respectively. This is particularly critical in systems that  not just a tendency to oversteer, but also one counteract excessive understeer tendency. While the Fighting oversteer is basically one too Reduced risk of tipping, can combat a Understeer create or promote a risk of tipping. Therefore, the increase in lateral tilt stability in the When in doubt take precedence over avoiding understeer to have.

The following requirements are placed on a brake system for carrying out the method for lateral stabilization according to the invention:
It must be possible to carry out active braking on a wheel-specific basis. This means that at least on the individual front wheels it must be possible to apply the brakes without the driver having to do anything. This condition applies, for example, to vehicles with front axle drive and traction control. Small modifications in the valve arrangement then enable active braking into each of the wheels. But vehicles that have a system for yawing torque control by means of brake intervention are also equipped for active braking, usually individually for each wheel. Anti-lock braking systems, however, are normally dependent on the brake pedal in order to be able to build up brake pressure. These systems can be equipped with an active brake booster, for example, or with a self-priming pump with a connection to the brake fluid reservoir and check valve in the brake line. It may also be advisable to ensure wheel-specific control of the rear axle brakes by means of additional valves if necessary.

Claims (10)

1. Procedure for increasing the lateral tilt stability one at least biaxial and at least two-lane vehicle, when crossing a first threshold one with the Vehicle lateral acceleration correlating size Measures to prevent tipping are initiated, the threshold value being at least at the start of the journey represents the lowest value, at which at permissible Vehicle loading of any kind is a danger of tipping over occurs. 2. The method according to claim 1, characterized in that the correlating quantity the lateral acceleration at a is about the location of the vehicle point with the vehicle unladen. 3. The method according to claim 1, characterized in that the correlating size is a function of the Vehicle lateral acceleration and its temporal Derivative is. 4. The method according to any one of the preceding claims, characterized in that when falling below the first threshold or a second, lower Threshold of the same physical quantity stabilizing measures are canceled. 5. The method according to any one of the preceding claims, characterized in that while driving through Observing the vehicle's response to Speed changes are estimated with  the stipulation, the first threshold value and / or the second threshold of the center of gravity of the Adapt vehicle accordingly. 6. The method according to any one of the preceding claims, characterized in that the vehicle cross acceleration only from the wheel sensor signals is determined. 7. The method according to any one of the preceding claims, characterized in that the vehicle cross acceleration from a steering angle signal and Wheel sensor signals is determined. 8. The method according to any one of the preceding claims, characterized in that the measure for Tilt prevention a brake application on the front Axis includes. 9. The method according to any one of the preceding claims for a vehicle with an active vehicle suspension, characterized in that the measure for Prevention of tipping into the vehicle hanging includes.   10. The method according to any one of the preceding claims for a vehicle with an avoidance device excessive understeering, characterized in that the measure to prevent tipping takes precedence over one Has measures to avoid excessive understeering.