Process to prevent the overturning of a vehicle around its longitudinal axis

[0002] The present invention relates to the control of a vehicle. More specifically, the present invention relates to a process for preventing a vehicle from overturning around its longitudinal axis.

[0003] A process of this type is known from the German patent DE 196 02 879 C1, which is incorporated herein by reference.

[0004] This prior art process relates to recognizing the likelihood of a vehicle overturning, and in particular, a vehicle consisting of a tractor and a trailer. The danger of overturning is recognized through a minimal actuation of the trailer brakes, combined with the observation of the reaction of an ABS anti-lock braking system incorporated in the trailer. If the ABS starts regulating and carries out a locking prevention action in conjunction with this relatively weak braking, there is an imminent danger of the tractor-trailer overturning. Upon this situation occuring, either a warning signal is emitted, or a stronger brake intervention is carried out in order to reduce the transverse acceleration of the tractor-trailer.

[0005] In addition to recognizing the right time to start such a stabilizing braking intervention, it is also important to recognize the right time to terminate the braking intervention. That is, at the time when the danger of overturning is no longer acute, the braking intervention should be reduced to avoid braking the vehicle unnecessarily.

[0006] It is therefore an object of the present invention to disclose a simple and reliable process for preventing a vehicle from overturning around its longitudinal axis, in which a stabilizing braking intervention is terminated in a timely manner.

[0007] A process to prevent the overturning of a vehicle around its longitudinal axis, as e.g., when rounding a curve, automatically evaluates the potential danger of overturning, and then proceeds to apply braking as required, in the following manner:

[0008] a) applying a first braking force to at least one of the vehicle wheels on the outside of the curve, to prevent the vehicle from overturning;

[0009] b) applying a second braking force to at least one of the vehicle wheels on the inside of the curve, where the second braking force is weaker than the first braking force;

[0010] c) terminating the first and second braking forces when a rotational speed of a wheel on the inside of the curve accelerates in a predetermined characteristic manner.

[0011] The inventive process determines the potential danger of overturning by determining the transverse acceleration level of the vehicle, based on the rotational speeds of the wheels, and comparing this transverse acceleration level to a predetermined threshold level. When the transverse acceleration level of the vehicle exceeds the predetermined threshold level, a potential danger of overturning is indicated.

[0012] The process further evaluates the danger of overturning by checking a wheel on the inside of the curve to sense a characteristic reduction of the wheel's rotational speed as a result of the second braking force. At the same time, the process ascertains whether a rotational speed of a wheel on the outside of the curve remains essentially unchanged.

[0013] Where applicable, the process also disables the anti-lock braking system slippage signals for a wheel on the inside of the curve when the second braking force is applied.

[0014] One advantage of the present invention is that the time when there is no longer a danger of overturning can be ascertained indirectly from the wheel load; i.e., by evaluating the behavior of the wheel with the lesser applied braking force. Using this technique, no further information need be obtained, such as the level of the center of gravity, or the actual transverse acceleration, which can only be ascertained by a sensor. Therefore, no additional sensors are needed, and the inventive process can be implemented very economically through a simple expansion of the control program of an electronic control device.

[0015] In the prior art processes to prevent overturning, in which the wheels on both sides of the vehicle are subjected to approximately the same, relatively high braking force, the wheels on the inside of a curve don't show that slippage has decreased until the wheel load is relatively great. This results in a calculated value of transverse acceleration that is relatively high, which in turn results in a continuation of the high braking force. Such a relatively later termination of the ABS-regulation process is regarded as uncomfortable. If the vehicle is braked in this manner so that it almost comes to a stop, the further consequence may be endangering the surrounding traffic. In the present invention, however, a considerably lower braking force is applied to the wheels on the inside of the curve, which results in an earlier start-up of those wheels, and thereby to a lower calculated transverse acceleration level, which enables the braking intervention to be terminated earlier and more safely.

[0016] The present invention is applicable to conventional compressed-air-controlled braking systems for utility vehicles, as well as to braking systems using any other type of actuating energy, such as hydraulic compression or electrical servomotors.

[0017] Another advantage of the present invention is that the braking intervention to prevent overturning is terminated when the rotational speed of at least one wheel on the inside of the curve accelerates in a characteristic manner. This eliminates the need for a separate load sensor to recognize the load increase on the wheel on the inside of the curve. Instead, the rotational-speed sensor already included in an anti-locking system can also be used for this purpose. It is a further advantage of the present invention that the calculation for transverse acceleration requires no special programming to recognize that there is no longer a danger of overturning. Instead, the increasing rotational speed of the wheel on the inside of the curve is included in this calculation, which then shows a decrease of the transverse acceleration level as shall be explained in further detail below.

[0018] In the present invention, test braking with a relatively lower braking force is applied to at least one wheel on the inside of the curve, in order to recognize a potential danger of overturning. Also, the ABS slippage signals of at least one wheel subjected to the test braking are disabled. As a result, the wheel subjected to test braking is not influenced by the ABS. It is therefore not possible that the brake of the wheel subjected to the test braking is bled by the ABS due to excessive wheel slip, and it is not able to start up again. In contrast to a complete elimination of the ABS function, this suppression of the ABS slippage signals enables the anti-locking function to be maintained on the basis of acceleration signals, for as long as the wheel has contact with the ground. This helps to prevent damage to the tires, such as flat spots, for example.

[0019] The invention is described in greater detail below through the example of an embodiment shown in the drawings, wherein

[0020] FIG. 1 shows a vehicle in a left curve, as seen from above;

[0021] FIGS. 2, 3 and 4 show a preferred embodiment of the invention in flow-chart format, and

[0022] FIG. 5 shows the embodiment of FIGS. 2, 3, and 4 in the form of a timing diagram.

[0023] FIG. 1 shows a vehicle, consisting of a tractor vehicle 2 and a semi-trailer 3, traveling in a left curve on a road 1 as seen from above. While the present invention is applicable to this type of tractor-trailer configuration, it is not limited to vehicles of this type. The trailer 3 has a pneumatic braking system that receives braking pressure from the tractor vehicle 2 in response to a brake pedal actuation by the driver. The braking system can also receive braking pressure by certain control and regulating functions of the tractor vehicle 2. To supply these functions, the tractor vehicle 2 is connected to the semi-trailer 3 via electrical and pneumatic lines 11.

[0024] The tractor vehicle 2 and the semi-trailer 3 are rotatably connected to each other at a pivot point 10.

[0025] The braking system of the semi-trailer 3 is preferably provided with electrically actuated components, such as anti-lock braking system (ABS) braking pressure modulators, or with purely electrically powered brake actuators. The brake modulators, or brake actuators, are controlled by an electronic control system 13. The control system 13 and the brake modulators/brake actuators are supplied with electrical energy and with the braking energy pressure medium via electrical and pneumatic lines 12. In addition, the rotational speed signals of the wheels 4, 5, 7, 8 (designated as v₄, v₅, v₇, v₈) are inputted to the electronic control system 13, as is known in the art of anti-lock braking systems.

[0026] In the illustrated embodiment, the semi trailer wheels 4, 5, 6 are on the outside of the curve, and the wheels 7, 8, 9 are on the inside of the curve.

[0027] The electronic control system 13 executes a number of control and regulating tasks in the semi-trailer 3. Importantly, one of these tasks is to recognize a likely possibility of the vehicle 2, 3 overturning around its longitudinal axis, and to prevent such an overturning action by means of a controlled braking intervention. This preventive action is illustrated by an example flow chart in FIG. 2.

[0028] The process starts at step 20. At step 21, the rotational speeds v₄, v₅, v₇, v₈of the wheels 4, 5, 7, 8 are read in. In step 22, a first transverse acceleration signal a_q,1is calculated from the rotational speeds v₄, v₇, and a second transverse acceleration signal a_q,2is calculated from the rotational speeds v₅, v₈in accordance with the following formulas:

[0029] a_q,1=½S×(v₄−v₇)×(v₄+v₇)  [1]

[0030] a_q,2=½S×(v₅−v₈)×(v₅+v₈)  [2]

[0031] The value S represents the wheel gauge of the vehicle. The transverse acceleration signals a_q,1, a_q,2are used jointly in this illustrative embodiment in order to make the process less prone to malfunctions, such as signal discrepancies, different tire diameters, etc. In this manner, erroneous triggering of the inventive process can be avoided.

[0032] In step 23, it is determined whether or not a braking force F₂had already been applied during a previous programmed execution of the process to avoid overturning. If so, the inventive process bypasses the sub-program block 26, to be described in detail in conjunction with FIGS. 3 and 4, and branches off directly to step 24, where it is determined if there is still a danger of overturning.

[0033] Referring again to step 23, if F₂has not already been applied, the process continues with the sub-program block 26, which is described in FIG. 3. This sub-program begins with a step 30. In step 31, the process determines whether the first transverse acceleration signal a_q,1, and the second transverse acceleration signal a_q,2, exceed a predetermined transverse acceleration threshold a_q,krit. If so, the wheels 4, 5, 6, 7, 8, 9 of the semi-trailer 3 are subjected to a relatively low braking force F₁, as indicated in step 32. The braking force F₁is set so that only a relatively slight braking effect occurs, which is barely perceptible to the driver. Also, braking force F₁is set so that no locking of wheels occurs, even on road surfaces with a relatively low frictional value, if there is no danger of overturning at this point. In a typical compressed-air braking system, a braking force of approximately 1 to 2 bar is applied in order to deliver the braking force F₁.

[0034] Also in step 32, the ABS slippage signals for the wheels 7, 8 are disabled, in order to avoid activation of the anti-locking function due to high slippage. However, activation of the anti-locking function based on acceleration signals continues to be enabled, so that possible damage to the tires can be avoided.

[0035] In step 33, at the end of a sufficiently long build-up time for the braking force F₁, two evaluations are made. First, it is ascertained whether the rotational speeds v₇, v₈of the wheels 7, 8 on the inside of the curve are characteristically lower than the rotational speeds v₄, v₅of the wheels 4, 5 on the outside of the curve. This is done by comparing the sum of the rotational speeds v₇, v₈with the sum of the rotational speeds v₄, v₅times a factor K₁. Then, the rotational speeds v₄, v₅are checked to see that they remain essentially unchanged. This is accomplished by checking the sum of the retardations of the wheels 4, 5; i.e., of the first time derivation of the appertaining rotational speeds v₄, v₅. Checking the wheels 4, 5 on the outside of the curve for continued relatively high rotational speeds serves to avoid triggering the inventive process by mistake, in the case of relatively low frictional values; e.g., on an icy surface. In that type of situation, it can happen that not only the rotational speeds of the inner wheels (7, 8), are reduced intentionally by the banking force F₁which is applied in the form of test braking, but also the rotational speeds v₄, v₅of the outer wheels are reduced intentionally by the braking force F₁, which is applied in the form of test braking. In this case, the speed reduction of the inner wheels 7, 8 does not presage an imminent overturning of the vehicle 2, 3.

[0036] If both conditions of step 33 are met, it is assumed that there is an imminent danger of overturning. At this point (step 34), the wheels 4, 5, 6 on the outside of the curve, which have the better frictional contact between road surface and tire, are subjected to a braking force F₂, that is greater than the braking force F₁. The level of braking force F₂is selected to immediately reduce the vehicle speed, so that the transverse acceleration, and thereby the danger of overturning, is also reduced immediately. The physical relationship between the vehicle's transverse acceleration a_qand the vehicle speed v is determined according to the following equation, where the value R indicates the curve radius:

[0037] a_q=v²/R  [3]

[0038] The anti-locking system prevents the wheels to which the braking force F₂is applied from locking up. The weaker braking force F₁continues to be applied to the wheels 7, 8, 9 on the inside of the curve. In order to produce the braking force F₂, a pressure of 4 to 8 bar is preferably applied in a conventional compressed air braking system.

[0039] In step 35, the transverse acceleration threshold a_q,krit, which is used to recognize a danger of overturning and to trigger the test braking, is set to a starting value a_q,start. This will be described in further detail below, in an embodiment of the invention wherein the transverse acceleration threshold a_q,kritcan be varied.

[0040] The sub-program 26 then ends with a step 38.

[0041] Referring back to FIG. 2, if the decision in step 23 is yes, the process continues at step 24. Here, a determination is made as to whether or not the first transverse acceleration signal a_q,1,and the second transverse acceleration signal a_q,2,fall below the transverse acceleration threshold a_q,krit. If they do fall below, there is no longer any danger of overturning, and the braking forces F₁, F₂can be terminated in step 25. In addition, the ABS slipping signals, which were disabled in step 32 of FIG. 3, can be released. If the first and second transverse acceleration signals a_q,1and a_q,2do not fall below the transverse acceleration threshold a_q,krit(step 24, FIG. 2), the process bypasses block 25, and branches directly to step 27, where the process ends.

[0042] The above described inventive process, and especially the utilization of the transverse acceleration signals a_q,1, a_q,2, can be summarized as follows. When a danger of overturning threatens, the wheels 7, 8, 9, having a lighter load on the inside of the curve, tend towards a decreased rotational speed due to the test braking force F₁. This causes a relatively large difference in rotational speed between the wheels on the inside and those on the outside of the curve. By applying the equations [1] and [2], this difference results in a rapid increase of the first and second transverse acceleration signals a_q,1, a_q,2. On the other hand, the return of the wheels 7, 8, 9 to the road surface; e.g., as a result of the larger braking force F₂in step 34 of FIG. 3, causes the calculated transverse acceleration signals a_q,1, a_q,2to decrease rapidly. Due to this rapid change in the transverse acceleration signal levels, the test braking force F₁, which stops the wheels bearing the lesser load when there is a danger of overturning, can be used to recognize the return of the wheels to the road surface, indicating the end of the danger of overturning. Under these conditions, the wheels on the inside of the curve start up again due to the increasing wheel load, in spite of the effect of braking force F₁, resulting in a characteristic increase in rotational speeds v₇, v₈.

[0043] Referring again to FIG. 3, if one or both conditions checked in step 33 are not met, the transverse acceleration threshold a_q,kritis increased by a value K₃(step 36), and the process branches off to step 38, where it ends. Steps 36, 35, and sub-program step 37 are part of an embodiment of the invention to be explained in further detail below.

[0044] If one or both conditions checked in step 31 are not met, the process branches off to the sub-program block 37, which is shown in further detail in FIG. 4.

[0045] The sub-program 37 starts at step 40. In step 41, the test braking force F₁is terminated. Also, the ABS slippage signals that had been disabled in step 32 are released. The process then continues with step 42, where the condition previously checked in step 24 (FIG. 2) is evaluated. This evaluation determines whether the termination of the brake intervention has fallen short by a given value K₄. That is, it is ascertained whether every transverse acceleration signal a_q,1, a_q,2falls short of the transverse acceleration threshold a_q,kritby a value K₄. If this is true, then the vehicle 2, 3 is relatively safe from the danger of overturning, and the transverse acceleration threshold a_q,krit, which was raised by a value K3 in step 36 (FIG. 3), can be lowered with little risk of erroneously triggering the process to prevent overturning. However, the lowered level of the transverse acceleration threshold a_q,kritmust not fall below a certain minimum value K₅, which is predetermined as a function of the vehicle characteristics.

[0046] This condition is checked in step 43. If it has been met, the transverse acceleration threshold a_q,kritis decreased in step 44 by a value K₆, and the process ends at step 45. However, in the event of a negative result of the check in step 42 or 43, the process moves directly to step 45.

[0047] A starting value a_q,startof the transverse acceleration threshold a_q,kritcan be predetermined and stored as a parameter in a non-volatile memory, based on the characteristics of the vehicle. In an advantageous embodiment of the present invention, this starting value may be determined in dependance of the vehicle loading; e.g., by measuring the pressure in the air suspension bellows when the vehicle is so equipped.

[0048] Two typical applications of the inventive process are illustrated in the timing diagrams of FIG. 5. One application depicts a vehicle traveling on a curve with no danger of overturning, and the other application depicts a vehicle traveling on a curve with imminent danger of overturning. The above described signal magnitudes are represented as velocity values in FIG. 5a, as transverse acceleration values in FIG. 5b, and as braking forces F (or braking pressures p) for both left and right sides of the vehicle in FIG. 5c. All the figures (5a, 5b, and 5c) have a common x-axis time base. In order to simplify the representation, only the rotational speed signals v₄, v₇(of the wheels 4, 7) of the semi trailer 3 are considered, in addition to the signals a_q,1, a_q,kritderived from them. The inventive process can also apply to vehicles with only one axle, or with only one axle equipped with rotational speed sensors.

[0049] Starting at time t₀, the vehicle 2, 3 travels in a straight line at a normal speed. At this time, the rotational speeds v₄, v₇have identical values, and the transverse acceleration signal a_q.1has a value=0. At time t₁, the vehicle 2, 3 begins to travel into a left curve as shown in FIG. 1. At the same time, the rotational speed v₇(on the inside of the curve) decreases relative to the rotational speed v₄(on the outside of the curve), where a relatively small decrease in rotational speed v₄is caused by the semi-trailer 3. This small decrease in rotational speed v₄is considered negligible, however, and is disregarded in FIG. 5a.

[0050] Due to the difference between the rotational speeds v₄and v₇, the transverse acceleration signal a_q,1increases (Equation 1), and at time t₂, reaches the transverse acceleration threshold a_q,krit. As a result, the test braking action is triggered at all the wheels (4, 5, 6, 7, 8, 9) of the semi-trailer 3 in FIG. 1. This is illustrated in FIG. 5cby an increase in braking force to the value F₁(signals 50, 51). Furthermore, the transverse acceleration threshold a_q,kritis increased, in accordance with every embodiment of the inventive process depicted in FIGS. 2 to 4. Illustratively, this increase can be made in fine-tuned steps (K₃in step 36 of FIG. 3) over intervals of 10 ms, as indicated by the ramp-shaped rise in FIG. 5b. At time t₃, the transverse acceleration signal a_q,1falls below the transverse acceleration threshold a_q,krit, and this results in a termination of the test braking action (FIG. 5c). The transverse acceleration threshold a_q,krit, which had been increased between times t₂and t₃, is initially maintained, since no serious danger of overturning was recognized in this time period. As such, the higher transverse acceleration threshold a_q,kritis compatible with safety, since it can be assumed that the critical value of transverse acceleration is higher when there is a danger of overturning than the transverse acceleration threshold a_q,start, which was assumed at the start of vehicle travel.

[0051] At time t₄, the condition in step 42 (FIG. 4) is met; i.e., the transverse acceleration signal a_q,1(FIG. 5b) falls below the transverse acceleration threshold a_q,kritby a value K₄. Therefore, the transverse acceleration threshold a_q,kritis decremented in fine-tuned steps (K₆in step 44, FIG. 4) for as long as the transverse acceleration threshold a_q,kritis larger than the minimum value K₅(step 43 in FIG. 4). This is illustrated in FIG. 5bby the ramp-shaped waveform between times t₄and t₅. At time t₅, the transverse acceleration threshold a_q,krithas reached the minimum value K₅, so that the condition verified in step 43 is no longer met, and the transverse acceleration threshold a_q,kritremains at the minimum value K₅. In this manner, the transverse acceleration threshold a_q,kritis adjusted as a self-learning function, in order to match the actual conditions of travel, in accordance with the embodiments of the process shown in steps 35, 36, and 42, 43, 44 of FIGS. 3 and 4, respectively.

[0052] Referring again to FIG. 5, the vehicle 2, 3 has ended its curve travel (time t₅), which leads to an adjustment of the rotational speeds v₄, v₇to match each other, and to a return of the transverse acceleration signal a_q,1to a value=0. At time t₆, the vehicle 2, 3 again travels into a left curve, but this time with a danger of overturning, so that the inventive process of recognizing and preventing such a mishap can be explained. Starting at time t₆, the rotational speed v₇(on the inside of the curve) decreases, while the rotational speed v₄(on the outside of the curve) remains essentially constant for the reasons previously discussed. At time t₇, the transverse acceleration signal a_q,1exceeds the transverse acceleration threshold a_q,krit, causing the test braking action to be initiated (signals 52, 53 in FIG. 5c), and the transverse acceleration threshold a_q,kritis again increased in a ramp-like manner. At approximately time t₈, the load on wheel 7 is reduced to such an extent that the test braking force F₁on the wheel 7 rapidly reduces the rotational speed v₇, as can be seen in FIG. 5a. This rapid reduction of the rotational speed v₇causes a brief decrease (54) of the braking force on the wheel 7 shortly after time t₉, due to the action of the anti-locking function.

[0053] At time t₉, the criteria listed in step 33 (FIG. 3) are met, with constant K₁assumed to have an illustrative value of 0.5 (see dashed line 56 in FIG. 5a). Therefore, an imminent danger of overturning is present. Starting at time t₉, a braking force F₂(signal 55 in FIG. 5c) is applied to the wheel 4 (on the outside of the curve), as well as to the other wheels 5, 6 on the outside of the curve. Due to this braking force F₂, the vehicle 2, 3 decelerates, as can be seen in FIG. 5afrom the downward sloping of the rotational speed v₄.

[0054] Also at time t₉, the transverse acceleration threshold a_q,kritis set back to the starting value a_q,start(step 35 in FIG. 3).

[0055] Due to the rapid reduction of the rotational speed v₇, the transverse acceleration signal a_q,1increases to a maximum value when the rotational speed v₇is reduced to 0. Due to the reduction of the vehicle's traveling speed, and the accompanying reduction of the rotational speed v₄, the transverse acceleration signal a_q,1also decreases.

[0056] At time t₁₀, the wheel 7 resumes its rotational movement, thus increasing the rotational speed v₇. At time t₁₁, the transverse acceleration signal a_q,1falls below the transverse acceleration threshold a_q,krit, which had been set back to the level a_q,startat time t₉. This results in the immediate termination of the braking intervention. Finally, at time t₁₂, the vehicle 2, 3 is again represented as traveling on a straight course.

[0057] Illustratively, the following values are preferred for the magnitudes K₁, K₂, K₃, K₄, K₅, K₆, a_q,start:

[0058] K₁=0.5

[0059] K₂=5 m/s²

[0060] K₃=0.01 m/s²

[0061] K₄=1 m/s²

[0062] K₅=5 m/s²

[0063] K₆=0.01 m/s²per second

[0064] a_q,start=3 m/s²

[0065] While the invention has been described by reference to specific embodiments, this was for purposes of illustration only and should not be construed to limit the scope of the invention. Numerous alternative embodiments will be apparent to those skilled in the art.