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Influences of Braking System Faults on the Vehicle Dynamics
Copyright @ lFAC Advances in Automotive Control. Karlsruhe. Gennany. 2001
INFLUENCES OF BRAKING SYSTEM FAULTS ON THE VEHICLE DYNAMICS
Harald Straky, Michael Kochem, Jiirgen Schmitt, Rainer Hild, Rolf Isermann
Institute of Automatic ControL Laboratory of ControL Engineering and Process Automation Darmstadt University of Technology Landgraf-Georg-Str. 4. D-64283 Darmstadt. Germany Phone: +496151 16-7412. FAX: +49615116-7421 { hstraky. mkochem. jschmitt. risermann}@iat.tu-darmstadt.de
Abstract: From a safety point of view the braking system is, besides the driver, one of the key subsystems in a car. The driver, as an adaptive control system, might not notice small faults in the hydraulic part of the braking system and sooner or later critical braking situations, e.g. due to a brake-circuit failure, may occur. Most of the drivers are not capable to deal with such critical situations. Therefore this paper investigates the influence of faults in the braking system on the dynamic vehicle behavior and the steering inputs of the driver to keep the vehicle on the desired course. Copyright fS>20011FAC
Keywords: vehicle dynamics, hydraulics, brakes, failure
Each of the above mentioned control systems has a multiple supervision of their electronic devices (hardand software) due to their active influence on the vehicle dynamics. A fault occurrence in these control systems generally causes a shut down of the respective subsystem. In the worst case, the hydraulic part of the braking system is the last back-up level for the driver. Although this subsystem of the braking system which is used in every braking situation is the only unsupervised part of the brake system. For safety reasons usually two hydraulic brake circuits are used (see Fig. I), therefore, in case of a brakecircuit failure half of the braking force can still be applied.
1. INTRODUCTION The developments in the field of braking systems over the past years enhanced the active driving safety constantly (Fennel, 1998; Kiesewetter, et aI., 1997, van Zanten et aI., 1998). Systems such as the anti lock braking system (ABS), the traction control system (TCS), the electronic stability program (ESP), or the brake assist (BAS) support the driver in critical driving situations by influencing the vehicle dynamics through the hydraulic subsystem of the braking system (master brake cylinder, solenoidvalve unit, brake lines, brake hoses and the wheel brake cylinders). As the anti lock braking system (ABS) reduces the wheel slip while braking the traction control system (TCS) causes the same effect while accelerating keeping the car steerable in both situations. The electronic stability program (ESP) supports the driver in critical lateral dynamic driving situations (over- and understeering), causing a yawing moment by a selective braking of the wheels and keeps the vehicle on the desired trajectory. The BAS supports the driver in panic situations, where the driver realizes the criticality of the situation but he does not react in an appropriate way, i. e. does not perform the necessary full braking. In such a case an integrated magnetic valve in the vacuum-operated brake booster causes the maximum pneumatic amplification of the brake booster which leads to a full braking.
Material wear or poor maintenance cause small, drifting faults, such as brake-fluid loss (leakage) or water / air inclusions in the brake system. Critical driving situations may not only occur in a brakecircuit failure, but also due to reduced friction between brake disc and brake shoe induced by brakefluid loss on a wheel brake cylinder, or due to a jammed or plugged ABS-solenoid valve. These faults result in an increased yawing moment while braking, which usually can not be corrected by the ESP while a fault in the hydraulic part of the braking system occurs. As will be shown later, it is nearly impossible for the average driver to control these situations.
113
•
I
P wbclront lelt
Hydraulic Brake-System with x-distribution pattern
P wbc Iront right
Pwbc_"lght
Fig. 1: Hydraulic brake test stand: 1. Control laptop, 2.Controller for the brake actuation actuator 3. Brake actuation actuator (DC-motor) 4. DSPsystem 5. Power supply for the DSP-system 6. Real-time simulation of the vehicle dynamics 7. Arrangement for leakage and air-blister installation 8. Vacuum-operated brake booster 9. Hydraulic modulator with solenoid-valve unit and integrated ECU 10. Wheel brake front left.
VII, VII, V I I,
= 0,00 em 3
= 0,45 em 3 = 0,90 em'
VoI ' = 1,35 ems
Apparently an early detection of small faults in the hydraulic part of the braking system is of major importance. At the Institute of Automatic Control, at Darmstadt University of Technology, a model based supervision- and diagnosis-system of a hydraulic braking system is developed. Figure I shows the hydraulic brake test stand setting up the development environment. The system consists of a standard braking system with integrated ABS, TCS, ESP and BAS and a digital signal processing system (DSPsystem) for the real-time simulation of the vehicle dynamics (lateral, longitudinal and vertical dynamics). For the vehicle model see (Halfmann et aI., 1999). An lateral MISO-PI-controller was also developed, which can keep the simulated vehicle on the desired trajectory by an automated correction of the steering angle. The inputs of the MISO-PIcontroller are: the yaw rate, the lateral acceleration and the lateral displacement of the vehicle. The inputs of the model are: the initial velocity of the vehicle VStarl at the beginning of the brake actuation, the throttle position (identical to zero while braking), the steering angle (arbitrary or from the lateral controller), the measured brake pressure in the four wheel brake cylinders and the external influences such as road condition (dry, wet, ice, undulation) or head- and cross-wind.
I c
., o·: ~~~~~ ..:l 7Rt--t--t--t----j--t--r--t:::;;;;;Iiiiil+-1
~}lt~-+-~-+-~~~-1-<;,:~.9 20r--+-~.Fr--t
:1 O~-+-~;"'-F--t-
Fig. 2: Influences of faults on the hydraulic pressure: a. Arrangement for leakage and air-blister installation, b. Pressure characteristic in the master (P mbe) an the wheel brake (P wbe) cylinders in a fault free case, c. Pressure characteristic in the wheel brake cylinder rear left with and without air-blister, d. Pressure characteristic in the wheel brake cylinder rear left with and without brake-fluid loss (leakage),
114
2. INSTALLED FAULTS AT THE HYDRAULIC BRAKE TEST STAND
in the force-controlled application of the braking force by the BAS, which results in a final constant pressure of 70 bar for the whole braking system. This kind of braking actuation is maintained for all of the experiments in the following sections.
To investigate the influence of faults in the braking system on the vehicle dynamics, several leakages (up to a volume of 6.5 cm 3 ) and air-blister (up to 1.6 cm3 ) were applied to the front right, the rear right and the rear left wheel brake cylinder, respectively. Figure 2a shows the arrangement for both rear wheel brake cylinders. The overall braking fluid volume is approx. 100 cm 3 , but there is only 13 cm 3 available per brake actuation, which is the total volume of the main brake cylinder (6.5 cm 3 per brake circuit). In case of brake-fluid loss or an air-blister of this size (13 cm 3) the main brake piston stops at the end and cannot generate pressure anymore. Figure 2b shows the dynamic pressure differences in the wheel brake cylinder in a fault free case for a BAS supported full braking. These pressure differences are a result of the hydraulic resistances in the ABS-solenoid valves and depend strongly on the dynamics of the braking actuation. An air-blister inclusion causes large differences (see Fig. 2c, rear left wheel brake cylinder) compared to the fault free case, whereas the leakage results (see Fig. 2d, rear left wheel brake cylinder) does not show any differences (for both experiments the same braking actuation has been applied). The reason for that behavior can be found
3. INFLUENCES OF SMALL FAULTS ON THE DYNAMIC VEHICLE BEHA VIOR In the following the vehicle dynamics are simulated by a real-time vehicle model described in (Halfmann et aI., 1999).
3.1 Influences o/the Dynamic o/the Braking System
The dynamic pressure differences between the wheel brake cylinders (see Fig. 2b) influence the vehicle dynamics in a braking situation even for the fault free case slightly (see Fig. 3). The influence increases slightly due to a different initial velocity. Figure 3a shows the influence of different initial vehicle velocities for a locked steering wheel (steering angle 8 =0°) on a plain, dry road. Due to a slower pressure increase at the rear left wheel brake cylinder the vehicle deviates slightly to the right (negative yaw 5""
:.:..~
'::
Votort = 140 kmlh v ...rt = 120 kmlh v.tort = 100 kmlh
1
;-+!J, ~
1\
0
5
--
steering angle
= 0 deg
-
";
.--..,....-. ,~
"'_ .
I--
IV
r--
V. tort
-0.1 5
_..
I
=100 kmlh ~
steering angle
=
0 deg
t
;~~,,;,;:-
.'
-~ -
::::0:25 ... -
,,1:~:
.,
-.
A
I v..."
1
0 B;':':';::O,5
~~~q
v.tort =100 kmlh
.5
~=~~:~~~
-
-1
~~t~~;)~~t,:3~~:",,~=2.". ,.~~.~
.5
Fig. 3: Influences of the dynamic of the braking system on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle Ijf - 0 deg.
=
100 kmlh
.
. ,
I
I controlled yaw angle - 0 deg 1
0
.5
....-. ,
VIookage= 1.83 cm'
..•..v::O.2 .'~
~
VIookage= 0.28 cm'
..r-
~-
..... ,-
I-- VIookag.= 0.98 cm'
~
;~ .
.-...
.:.~-
,O!"
11
Jp...
~ !-r--...
Vr-----= !-!--
V,..kage= 0.28 cm' VIookago= 0.98 cm' VIookago= 1.83 cm' _
I -- I
-~ -.~
.-.
Fig. 4: Influences of brake-fluid loss (leakage) of the front right wheel brake cylinder on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle Ijf - 0 deg.
115
Voir = 1.35 cm' V. =0.90 ems V....= 0.45 ems
V. W= 1.35 cm' Volf =0.90 em 3 V..Ir= 0.45 cm'
VoWt
= 100 kmIh
controlled yaw angle - 0 deg
Fig. 5: Influences of air-blister in the front right wheel brake cylinder on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle ljI- 0 deg .
Fig. 6: Influences of air-blisters in the rear right wheel brake cylinder on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle ljI- 0 deg.
rate), which results in a leftward displacement of the center of gravity. Now the left tires can bring a higher braking force to the ground, which results in the vehicle to be drawn to the left (positive yaw rate at approx. 0.4 seconds). The whole deviation for an initial speed of 140 kmlh is about 2 cm. The necessary steering actions to keep the vehicle on track can be seen in figure 3b.
compared to the pressures in the other wheel brake cylinders. First the vehicle deviates to the left (positive yaw rate) and then due to a center-ofgravity-shift to the right. The maximum deviation, induced by the biggest air-blister (1.35 cm 3), is 10 cm. The necessary steering angle required to keep the vehicle on track is plotted in Fig. 5. The steering angle still is comparatively small.
3.2 Influences of Brake-fluid Loss (Leakage) of the Front Right Wheel Brake Cylinder
3.4 Influences of Air-Blisters in the Rear Right Wheel Brake Cylinder
Figure 4a shows the influences of brake-fluid loss (leakage) of the front right wheel brake cylinder on the lateral vehicle behavior. A force-controlled brake actuation reveals almost no difference between the faulty and the fault-free case. The braking system shows the same behavior for brake-fluid loss (leakage) in other parts of the hydraulic subsystem.
Air-blisters in the rear right wheel brake cylinder cause similar effects (fig. 6a) as in the front right wheel brake cylinder (fig. 5a), which leads to a yaw rate turning the vehicle leftward. But due to the static and dynamic brake-power distribution between front and rear axle this yaw rate is smaller. Therefore, the lateral controller needs a smaller range of steering angel to keep the vehicle on track (compare fig . 6b with fig . 5b).
3.3 Influences of Air-Blisters in the Front Right Wheel Brake Cylinder 4. INFLUENCES OF BRAKE-CIRCUIT FAILURE In case of an air-blister in the front right wheel brake cylinder, the pressure increases slower (the air is to be compressed adiabetically and it takes additional brake-fluid volume to build up the pressure)
Small faults like brake-fluid loss or air-blister (see sections 2 and 3) installed in the hydraulic brake system can lead to a brake-circuit failure.
116
v.tart : 100 kmlh
v ...,,: 100 kmlh controlled yaw angle - 0 deg
-., .-
~
;,;
":.' -
-~
;--:
Fig. 7: Influence of a main brake-circuit failure due to brake-fluid loss (leakage) of the front right wheel brake cylinder: a. Yaw rate changes for a steering angle () = 0 deg., b. Required steering angle for an yaw angle lI' - 0 deg.
Fig. 8: Influence of a secondary brake-circuit failure due to brake-fluid loss (leakage) of the rear right wheel brake cylinder: a. Yaw rate changes for a steering angle () = 0 deg. b. Required steering angle for an yaw angle lI' - 0 deg. wards it deviates to the right (now also the front left wheel brake cylinder is without pressure). Figure 8b shows the necessary steering angle to keep the vehicle on track. Figure 9 shows the course deviations for the above mentioned brake-circuit failures in comparison with the fault-free case on a plain, dry road.
4.1 Main Brake-Circuitfailure due to Brake-Fluid Loss of the Front Right Wheel Brake Cylinder A leakage was installed in to the front right brake cylinder. Because of this, the brake pressure at this wheel cannot build up and also leads to insufficient brake pressure at the left rear wheel. The resulting yaw moment causes the vehicle to drift to the left if no steering is applied (Fig. 7a). Figure 7b shows the necessary correcting steer-angle. Only skilled drivers will be capable of executing such a maneuver. Fluctuations in the yaw rate and in the correcting steer-angle are due to the dynamic axle-load transfer.
Notice the increased braking distance of 46 m in case of a main brake-circuit failure front right and 37 m in case of a secondary brake-circuit failure rear right for a full braking.
5. CONCLUSIONS
4.1 Secondary Brake-Circuit failure due to BrakeFluid Loss of the Rear Right Wheel Brake Cylinder
The simulation of the vehicle dynamics together with the real hydraulic brake system have shown that small faults like brake-fluid loss (leakage) or airblisters in the hydraulic subsystem of the brake system have a negligible effect on the vehicle dynamics, which can be compared with the effects of changes in road surfaces or a small shifting of the center of gravity of the vehicle.
For this case the leakage was introduced at the rear right wheel brake cylinder. The pressure at the rear axle increases slower than the pressure at the front axle. Consequently the pressure at the front left wheel brake cylinder increases for a short period of time before it decays to 0 bar. The vehicle dynamics reflect this dynamic braking behavior. At the beginning the vehicle deviates to the left (the rear right wheel brake cylinder is without pressure), after-
A driver subconsciously corrects these influences on the vehicle path without attributing this to brake system faults.
117
'.~
REFERENCES Bbrner M., H. Straky, T. Weispfenning and R. Isermann (2000). Model based fault detection of vehicle suspension and hydraulic brake systems, 1st IFAC-Conference on Mechatronic Systems, 18.-20. September 2000, Darmstadt. Fennel, H. (1998). Ein Konzept zur Beherrschung der Fahrdynamik, Automobiltechnische Zeitschrift ATZ 100, S302 ff. 1998. Halfmann c., H. Holzmann, R. Isermann, C.-D. Hamman and N. Simm (1999). Adaptive Echtzeitmodelle fUr die Kraftfahrzeugdynamik, Automobiltechnische Zeitschrift ATZ 101, S 1006 ff., 1999. Kiesewetter, W., W. Klinkner, W. Reichelt and M. Steiner (1997). Der neue Brake-Assist von Mercedes-Benz, Automobiltechnische Zeitschrift ATZ 99, S. 330 ff., 1997. Moseler 0., H. Straky (2000). Fault Detection of a Solenoid Valve for Hydraulic Systems in Vehicles, 4th IFAC Symposium on Fault Detection, Supervision and Safety for Technical Processes (SA FEPROCESS) , 1416. June 2000, Budapest. Straky, H., Weispfenning, T. (1999a). ModeIl based data processing in a mechatronic system. European Automotive Congress, EAEC 1999, Barcelona, Spain, 30th June - 2 nd July 1999. Straky, H., Weispfenning, T., Isermann R. (1 999b). ModelI based fault detection of hydraulic brake system components. European Control Conference, ECC 1999, Karlsruhe, Germany, 31 Aug. - 3 th Sept. 1999. Straky H., M. Bbrner and R. Isermann (2000) Steigerung der Fahrsicherheit von Kraftfahrzeugen durch modellgestlitzte on board Diagnose, 2-te Tagung Mechatronik im Automobil, 15-16. November 2000, Mlinchen. van Zanten A., R. Erhardt, K. Landesfeind and G. Pfaff (1998). VDC System Development, International Congress & Exposition SAE, Feb. 23-26, 1998, Detroit.
E E
~-+--~~~~~ ~ ~r.~--r-~~-r--;
Fig. 9: Deviation of the vehicle from its desired course and the increased brake distance caused by brake-circuit failures . If on the other hand an entire brake-circuit fails due to undetected faults only skilled drivers are capable of controlling the vehicle. Most of the time though the vehicle will crash. Therefore these small faults should be detected early to warn the driver. Such methods are presently developed at the Institute of Automatic Control, at Darmstadt University of Technology. First results are published in (Bbrner, 2000; Moseler, 2000; Straky, 1999a; Straky, 1999b; Straky, 2000).
Acknowledgment: This work is supported by Deutsche Forschungsgemeinschaft (DFG) through the special research program 241, 'Integrated Mechatronic Systems, (IMES).
118
INFLUENCES OF BRAKING SYSTEM FAULTS ON THE VEHICLE DYNAMICS
Harald Straky, Michael Kochem, Jiirgen Schmitt, Rainer Hild, Rolf Isermann
Institute of Automatic ControL Laboratory of ControL Engineering and Process Automation Darmstadt University of Technology Landgraf-Georg-Str. 4. D-64283 Darmstadt. Germany Phone: +496151 16-7412. FAX: +49615116-7421 { hstraky. mkochem. jschmitt. risermann}@iat.tu-darmstadt.de
Abstract: From a safety point of view the braking system is, besides the driver, one of the key subsystems in a car. The driver, as an adaptive control system, might not notice small faults in the hydraulic part of the braking system and sooner or later critical braking situations, e.g. due to a brake-circuit failure, may occur. Most of the drivers are not capable to deal with such critical situations. Therefore this paper investigates the influence of faults in the braking system on the dynamic vehicle behavior and the steering inputs of the driver to keep the vehicle on the desired course. Copyright fS>20011FAC
Keywords: vehicle dynamics, hydraulics, brakes, failure
Each of the above mentioned control systems has a multiple supervision of their electronic devices (hardand software) due to their active influence on the vehicle dynamics. A fault occurrence in these control systems generally causes a shut down of the respective subsystem. In the worst case, the hydraulic part of the braking system is the last back-up level for the driver. Although this subsystem of the braking system which is used in every braking situation is the only unsupervised part of the brake system. For safety reasons usually two hydraulic brake circuits are used (see Fig. I), therefore, in case of a brakecircuit failure half of the braking force can still be applied.
1. INTRODUCTION The developments in the field of braking systems over the past years enhanced the active driving safety constantly (Fennel, 1998; Kiesewetter, et aI., 1997, van Zanten et aI., 1998). Systems such as the anti lock braking system (ABS), the traction control system (TCS), the electronic stability program (ESP), or the brake assist (BAS) support the driver in critical driving situations by influencing the vehicle dynamics through the hydraulic subsystem of the braking system (master brake cylinder, solenoidvalve unit, brake lines, brake hoses and the wheel brake cylinders). As the anti lock braking system (ABS) reduces the wheel slip while braking the traction control system (TCS) causes the same effect while accelerating keeping the car steerable in both situations. The electronic stability program (ESP) supports the driver in critical lateral dynamic driving situations (over- and understeering), causing a yawing moment by a selective braking of the wheels and keeps the vehicle on the desired trajectory. The BAS supports the driver in panic situations, where the driver realizes the criticality of the situation but he does not react in an appropriate way, i. e. does not perform the necessary full braking. In such a case an integrated magnetic valve in the vacuum-operated brake booster causes the maximum pneumatic amplification of the brake booster which leads to a full braking.
Material wear or poor maintenance cause small, drifting faults, such as brake-fluid loss (leakage) or water / air inclusions in the brake system. Critical driving situations may not only occur in a brakecircuit failure, but also due to reduced friction between brake disc and brake shoe induced by brakefluid loss on a wheel brake cylinder, or due to a jammed or plugged ABS-solenoid valve. These faults result in an increased yawing moment while braking, which usually can not be corrected by the ESP while a fault in the hydraulic part of the braking system occurs. As will be shown later, it is nearly impossible for the average driver to control these situations.
113
•
I
P wbclront lelt
Hydraulic Brake-System with x-distribution pattern
P wbc Iront right
Pwbc_"lght
Fig. 1: Hydraulic brake test stand: 1. Control laptop, 2.Controller for the brake actuation actuator 3. Brake actuation actuator (DC-motor) 4. DSPsystem 5. Power supply for the DSP-system 6. Real-time simulation of the vehicle dynamics 7. Arrangement for leakage and air-blister installation 8. Vacuum-operated brake booster 9. Hydraulic modulator with solenoid-valve unit and integrated ECU 10. Wheel brake front left.
VII, VII, V I I,
= 0,00 em 3
= 0,45 em 3 = 0,90 em'
VoI ' = 1,35 ems
Apparently an early detection of small faults in the hydraulic part of the braking system is of major importance. At the Institute of Automatic Control, at Darmstadt University of Technology, a model based supervision- and diagnosis-system of a hydraulic braking system is developed. Figure I shows the hydraulic brake test stand setting up the development environment. The system consists of a standard braking system with integrated ABS, TCS, ESP and BAS and a digital signal processing system (DSPsystem) for the real-time simulation of the vehicle dynamics (lateral, longitudinal and vertical dynamics). For the vehicle model see (Halfmann et aI., 1999). An lateral MISO-PI-controller was also developed, which can keep the simulated vehicle on the desired trajectory by an automated correction of the steering angle. The inputs of the MISO-PIcontroller are: the yaw rate, the lateral acceleration and the lateral displacement of the vehicle. The inputs of the model are: the initial velocity of the vehicle VStarl at the beginning of the brake actuation, the throttle position (identical to zero while braking), the steering angle (arbitrary or from the lateral controller), the measured brake pressure in the four wheel brake cylinders and the external influences such as road condition (dry, wet, ice, undulation) or head- and cross-wind.
I c
., o·: ~~~~~ ..:l 7Rt--t--t--t----j--t--r--t:::;;;;;Iiiiil+-1
~}lt~-+-~-+-~~~-1-<;,:~.9 20r--+-~.Fr--t
:1 O~-+-~;"'-F--t-
Fig. 2: Influences of faults on the hydraulic pressure: a. Arrangement for leakage and air-blister installation, b. Pressure characteristic in the master (P mbe) an the wheel brake (P wbe) cylinders in a fault free case, c. Pressure characteristic in the wheel brake cylinder rear left with and without air-blister, d. Pressure characteristic in the wheel brake cylinder rear left with and without brake-fluid loss (leakage),
114
2. INSTALLED FAULTS AT THE HYDRAULIC BRAKE TEST STAND
in the force-controlled application of the braking force by the BAS, which results in a final constant pressure of 70 bar for the whole braking system. This kind of braking actuation is maintained for all of the experiments in the following sections.
To investigate the influence of faults in the braking system on the vehicle dynamics, several leakages (up to a volume of 6.5 cm 3 ) and air-blister (up to 1.6 cm3 ) were applied to the front right, the rear right and the rear left wheel brake cylinder, respectively. Figure 2a shows the arrangement for both rear wheel brake cylinders. The overall braking fluid volume is approx. 100 cm 3 , but there is only 13 cm 3 available per brake actuation, which is the total volume of the main brake cylinder (6.5 cm 3 per brake circuit). In case of brake-fluid loss or an air-blister of this size (13 cm 3) the main brake piston stops at the end and cannot generate pressure anymore. Figure 2b shows the dynamic pressure differences in the wheel brake cylinder in a fault free case for a BAS supported full braking. These pressure differences are a result of the hydraulic resistances in the ABS-solenoid valves and depend strongly on the dynamics of the braking actuation. An air-blister inclusion causes large differences (see Fig. 2c, rear left wheel brake cylinder) compared to the fault free case, whereas the leakage results (see Fig. 2d, rear left wheel brake cylinder) does not show any differences (for both experiments the same braking actuation has been applied). The reason for that behavior can be found
3. INFLUENCES OF SMALL FAULTS ON THE DYNAMIC VEHICLE BEHA VIOR In the following the vehicle dynamics are simulated by a real-time vehicle model described in (Halfmann et aI., 1999).
3.1 Influences o/the Dynamic o/the Braking System
The dynamic pressure differences between the wheel brake cylinders (see Fig. 2b) influence the vehicle dynamics in a braking situation even for the fault free case slightly (see Fig. 3). The influence increases slightly due to a different initial velocity. Figure 3a shows the influence of different initial vehicle velocities for a locked steering wheel (steering angle 8 =0°) on a plain, dry road. Due to a slower pressure increase at the rear left wheel brake cylinder the vehicle deviates slightly to the right (negative yaw 5""
:.:..~
'::
Votort = 140 kmlh v ...rt = 120 kmlh v.tort = 100 kmlh
1
;-+!J, ~
1\
0
5
--
steering angle
= 0 deg
-
";
.--..,....-. ,~
"'_ .
I--
IV
r--
V. tort
-0.1 5
_..
I
=100 kmlh ~
steering angle
=
0 deg
t
;~~,,;,;:-
.'
-~ -
::::0:25 ... -
,,1:~:
.,
-.
A
I v..."
1
0 B;':':';::O,5
~~~q
v.tort =100 kmlh
.5
~=~~:~~~
-
-1
~~t~~;)~~t,:3~~:",,~=2.". ,.~~.~
.5
Fig. 3: Influences of the dynamic of the braking system on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle Ijf - 0 deg.
=
100 kmlh
.
. ,
I
I controlled yaw angle - 0 deg 1
0
.5
....-. ,
VIookage= 1.83 cm'
..•..v::O.2 .'~
~
VIookage= 0.28 cm'
..r-
~-
..... ,-
I-- VIookag.= 0.98 cm'
~
;~ .
.-...
.:.~-
,O!"
11
Jp...
~ !-r--...
Vr-----= !-!--
V,..kage= 0.28 cm' VIookago= 0.98 cm' VIookago= 1.83 cm' _
I -- I
-~ -.~
.-.
Fig. 4: Influences of brake-fluid loss (leakage) of the front right wheel brake cylinder on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle Ijf - 0 deg.
115
Voir = 1.35 cm' V. =0.90 ems V....= 0.45 ems
V. W= 1.35 cm' Volf =0.90 em 3 V..Ir= 0.45 cm'
VoWt
= 100 kmIh
controlled yaw angle - 0 deg
Fig. 5: Influences of air-blister in the front right wheel brake cylinder on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle ljI- 0 deg .
Fig. 6: Influences of air-blisters in the rear right wheel brake cylinder on the lateral vehicle behavior: a. Yaw rate changes for a steering angle 8 = 0 deg. h. Required steering angle for an yaw angle ljI- 0 deg.
rate), which results in a leftward displacement of the center of gravity. Now the left tires can bring a higher braking force to the ground, which results in the vehicle to be drawn to the left (positive yaw rate at approx. 0.4 seconds). The whole deviation for an initial speed of 140 kmlh is about 2 cm. The necessary steering actions to keep the vehicle on track can be seen in figure 3b.
compared to the pressures in the other wheel brake cylinders. First the vehicle deviates to the left (positive yaw rate) and then due to a center-ofgravity-shift to the right. The maximum deviation, induced by the biggest air-blister (1.35 cm 3), is 10 cm. The necessary steering angle required to keep the vehicle on track is plotted in Fig. 5. The steering angle still is comparatively small.
3.2 Influences of Brake-fluid Loss (Leakage) of the Front Right Wheel Brake Cylinder
3.4 Influences of Air-Blisters in the Rear Right Wheel Brake Cylinder
Figure 4a shows the influences of brake-fluid loss (leakage) of the front right wheel brake cylinder on the lateral vehicle behavior. A force-controlled brake actuation reveals almost no difference between the faulty and the fault-free case. The braking system shows the same behavior for brake-fluid loss (leakage) in other parts of the hydraulic subsystem.
Air-blisters in the rear right wheel brake cylinder cause similar effects (fig. 6a) as in the front right wheel brake cylinder (fig. 5a), which leads to a yaw rate turning the vehicle leftward. But due to the static and dynamic brake-power distribution between front and rear axle this yaw rate is smaller. Therefore, the lateral controller needs a smaller range of steering angel to keep the vehicle on track (compare fig . 6b with fig . 5b).
3.3 Influences of Air-Blisters in the Front Right Wheel Brake Cylinder 4. INFLUENCES OF BRAKE-CIRCUIT FAILURE In case of an air-blister in the front right wheel brake cylinder, the pressure increases slower (the air is to be compressed adiabetically and it takes additional brake-fluid volume to build up the pressure)
Small faults like brake-fluid loss or air-blister (see sections 2 and 3) installed in the hydraulic brake system can lead to a brake-circuit failure.
116
v.tart : 100 kmlh
v ...,,: 100 kmlh controlled yaw angle - 0 deg
-., .-
~
;,;
":.' -
-~
;--:
Fig. 7: Influence of a main brake-circuit failure due to brake-fluid loss (leakage) of the front right wheel brake cylinder: a. Yaw rate changes for a steering angle () = 0 deg., b. Required steering angle for an yaw angle lI' - 0 deg.
Fig. 8: Influence of a secondary brake-circuit failure due to brake-fluid loss (leakage) of the rear right wheel brake cylinder: a. Yaw rate changes for a steering angle () = 0 deg. b. Required steering angle for an yaw angle lI' - 0 deg. wards it deviates to the right (now also the front left wheel brake cylinder is without pressure). Figure 8b shows the necessary steering angle to keep the vehicle on track. Figure 9 shows the course deviations for the above mentioned brake-circuit failures in comparison with the fault-free case on a plain, dry road.
4.1 Main Brake-Circuitfailure due to Brake-Fluid Loss of the Front Right Wheel Brake Cylinder A leakage was installed in to the front right brake cylinder. Because of this, the brake pressure at this wheel cannot build up and also leads to insufficient brake pressure at the left rear wheel. The resulting yaw moment causes the vehicle to drift to the left if no steering is applied (Fig. 7a). Figure 7b shows the necessary correcting steer-angle. Only skilled drivers will be capable of executing such a maneuver. Fluctuations in the yaw rate and in the correcting steer-angle are due to the dynamic axle-load transfer.
Notice the increased braking distance of 46 m in case of a main brake-circuit failure front right and 37 m in case of a secondary brake-circuit failure rear right for a full braking.
5. CONCLUSIONS
4.1 Secondary Brake-Circuit failure due to BrakeFluid Loss of the Rear Right Wheel Brake Cylinder
The simulation of the vehicle dynamics together with the real hydraulic brake system have shown that small faults like brake-fluid loss (leakage) or airblisters in the hydraulic subsystem of the brake system have a negligible effect on the vehicle dynamics, which can be compared with the effects of changes in road surfaces or a small shifting of the center of gravity of the vehicle.
For this case the leakage was introduced at the rear right wheel brake cylinder. The pressure at the rear axle increases slower than the pressure at the front axle. Consequently the pressure at the front left wheel brake cylinder increases for a short period of time before it decays to 0 bar. The vehicle dynamics reflect this dynamic braking behavior. At the beginning the vehicle deviates to the left (the rear right wheel brake cylinder is without pressure), after-
A driver subconsciously corrects these influences on the vehicle path without attributing this to brake system faults.
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REFERENCES Bbrner M., H. Straky, T. Weispfenning and R. Isermann (2000). Model based fault detection of vehicle suspension and hydraulic brake systems, 1st IFAC-Conference on Mechatronic Systems, 18.-20. September 2000, Darmstadt. Fennel, H. (1998). Ein Konzept zur Beherrschung der Fahrdynamik, Automobiltechnische Zeitschrift ATZ 100, S302 ff. 1998. Halfmann c., H. Holzmann, R. Isermann, C.-D. Hamman and N. Simm (1999). Adaptive Echtzeitmodelle fUr die Kraftfahrzeugdynamik, Automobiltechnische Zeitschrift ATZ 101, S 1006 ff., 1999. Kiesewetter, W., W. Klinkner, W. Reichelt and M. Steiner (1997). Der neue Brake-Assist von Mercedes-Benz, Automobiltechnische Zeitschrift ATZ 99, S. 330 ff., 1997. Moseler 0., H. Straky (2000). Fault Detection of a Solenoid Valve for Hydraulic Systems in Vehicles, 4th IFAC Symposium on Fault Detection, Supervision and Safety for Technical Processes (SA FEPROCESS) , 1416. June 2000, Budapest. Straky, H., Weispfenning, T. (1999a). ModeIl based data processing in a mechatronic system. European Automotive Congress, EAEC 1999, Barcelona, Spain, 30th June - 2 nd July 1999. Straky, H., Weispfenning, T., Isermann R. (1 999b). ModelI based fault detection of hydraulic brake system components. European Control Conference, ECC 1999, Karlsruhe, Germany, 31 Aug. - 3 th Sept. 1999. Straky H., M. Bbrner and R. Isermann (2000) Steigerung der Fahrsicherheit von Kraftfahrzeugen durch modellgestlitzte on board Diagnose, 2-te Tagung Mechatronik im Automobil, 15-16. November 2000, Mlinchen. van Zanten A., R. Erhardt, K. Landesfeind and G. Pfaff (1998). VDC System Development, International Congress & Exposition SAE, Feb. 23-26, 1998, Detroit.
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~-+--~~~~~ ~ ~r.~--r-~~-r--;
Fig. 9: Deviation of the vehicle from its desired course and the increased brake distance caused by brake-circuit failures . If on the other hand an entire brake-circuit fails due to undetected faults only skilled drivers are capable of controlling the vehicle. Most of the time though the vehicle will crash. Therefore these small faults should be detected early to warn the driver. Such methods are presently developed at the Institute of Automatic Control, at Darmstadt University of Technology. First results are published in (Bbrner, 2000; Moseler, 2000; Straky, 1999a; Straky, 1999b; Straky, 2000).
Acknowledgment: This work is supported by Deutsche Forschungsgemeinschaft (DFG) through the special research program 241, 'Integrated Mechatronic Systems, (IMES).
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