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Testing of ESP/ESC in vehicles. A wholistic approach to the complex

Wissenschaftlicher Aufsatz 2018 9 Seiten

Ingenieurwissenschaften - Fahrzeugtechnik

Leseprobe

ABSTRACT:

Electronic stability control (=ESC) as active safety system is getting more and more common in todays vehicles. First established in 1995 by Bosch and Daimler under the well known brand ESP it has obviously prevented many accidents all around the world. This evidence led to the fact that many states passed law which makes an ESC system mandatory in every vehicle sold in their territory. ESC is based on fundamentals of control engineering and vehicle dynamics and is using the vehicle brake as well as reducing engine power to stabilize vehicle behaviour in critical driving situations like over- and understeering. Therefor it is making us of the hardware of the Antilock-System (=ABS) and the Anti Slip Control (=ASC). Since ESC is always in it’s general component structure the same but in its specific reaction (=control) and modeling differing due to the desired driving behaviour of a particular manufacturer, tests need to be performed to prove the stabilizing effectiveness of its output signals which are represented as brake torques and reduction of engine power. These verifications can be virtually and/or physically conducted. In case of th EU there’s even a defined mandatory test to be completed in order to receive the sales permission by local authorities.

KEYWORDS: Electronic Stability Control, Active Safety Systems, Vehicle Safety, Vehicle Dynamics, System Testing

1. ESP® - A spreading brand

ESP ® is a brand name protected by the Daimler AG since 1995 [1]. The brand has established in common language for the general function of an electronic stability control known as ESC which is just a word by word translation of the protected notation. Thus many other car manufacturers circumvent the brand protection by introducing their own labelling for their system interpretation. BMW e.g. uses DSC (=Dynamic stability control). Since there are various descriptions for an in its essence and overall functional point of view identical system this paper will stick to the ESC term.

ESC as part of the active safety systems of a vehicle preventis a vehicle from skidding or side slip which occur in different extreme driving situations (2.2). It is generally based on complex control engineering (2.3). In the beginning of the millennium when ESC system got more common a study from the United States pointed out the huge effectiveness of ESC [2]. Fig. 1 shows the reduction in different crash classes of identical vehicles equipped with an ESC system compared to their identical models without ESC system.

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Fig. 1: Crash reduction vehicles equipped ESC [3]

The question that arises is whether the ESC itself prevented from these in a sense “not occurred” accidents or whether other factors like improved driver skills and other additional vehicle systems provided a bit of this reduction as well. Since the study was done in between two years the technological and driving skill aspects can be neglected. Therefore there is a great evidence that ESC is really enhancing the probability to escape safely special situations in which most drivers wouldn’t know how to react since they occur seldomly but on the other hand most of the time rapidly with a dynamic that’s hard to control when human reaction is not intuitively adequate (2.2).

Such studies led to the fact that ESC got mandatory vehicle equipment in many regions [3], e.g. the U.S. and the EU. The EU directive 2007/46/EC article 11 defines the need to prove the overall vehicle safety during an European Conformity (EC) approval process. Concrete action concerning making ESC a standard have been taken with directive 661/2009/EC that introduced the prescript of ESC as standard equipment in the EU for new cars from November 2014 on.

But ESC developed by Bosch and Daimler in the beginning is not a plug and play solution, it needs to be applied to every different vehicle depending on it’s characteristics. Therefore not only the existence of the necessary components for a proper ESC control loop should be proved but also it’s actual performance in combination with the particular vehicle in highly demanding driving manoveurs has to be considered in a defined way.

In 2016 the UNECE (United Nations Economic Comission f. Europe) then managed to agree on a standard ESC test procedure that every vehicle has to pass in order to receive the ECE approval. Besides this defined procedure exist many other driving situations that can be tested in order to assess the performance of an ESC system. Both will be illustrated (3) after explaining the main components and functioning of an ESC system (2).

2. ESC System operation

2.1 Mechanical Background

Understanding an ESC control algorithm is based on basic knowledge about vehicle dynamics. Especially lateral dynamics Especially lateral dynamics play an important role for the calculation of the systems behaviour.

Most crucial factor for secure driving either lateral or transversal is the proper interference of the car body with the road through the tires. The transmission of side forces depends highly on the attributes of both. Fig. 2 shows the side forces while cornering caused by the centrifugal force split up on the tires up to their maximum capability to carry the load.

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Fig. 2: Forces on vehicle while cornering [4]

These specific capabilities at each individual wheel are illustrated in the Kammsche Circle shown in Fig. 3.

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Fig. 3: The Kammsche circle for tire forces [8]

The image shows the dependency between overall transmittable tire force (black circle = Kammsche circle), the actual acting horicontal force at the wheel and the road (pink) and it’s components consisting of side force (red) and tangential force (yellow). As long as a tire stays inside the black circle during operation adhesive friction is guaranteed. The final 3D wheel force (green) consisting of the derived horicontal force (pink) and the normal force (blue) is not directly of interest of the lateral dynamics.

The circle diameter is continuously defined by the normal force acting on each wheel:

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and the combined actual friction coefficient

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The circular shape pretends that the lateral and longitudinal friction coefficient are always equal which then define the diameter of the circle in combination with the normal force. In reality the shape is more an ellipse due to the fact that a tire doesn’t have identical coefficients in both directions. The lateral friction coefficient is lower than longitudinal which will squeeze the Kammsche circle to an elliptic form. The mathematic calculation of the different u’s is difficult since they are the result of a summation of different prevailing µ at the road. In general in Fig. 4 is shown that, neglecting the specific conditions between road and tire, the maximum of the friction coefficients in both force direction curve is mainly dependend on the recent longitudinal slip. On top the lateral coefficient is dependend on the slip angle of the particular axle and compared to the longitudinal coefficient also highly on the normal force acting on the wheel [4]. These friction coefficients change to much lower values once the horicontal force exceeds the Kammsche circle. From this follows the conclusion that in different driving situations certain combinations of slip and axle slip angle are advantageous for the shape of the Kammsche circle. The calculation of that optimum is task of the ESC system together with ABS (Anti Block System) and ASC (Anti Slip Control).

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Fig. 4: Friction coefficients depending on slip [4]

A vehicle operating under it’s maximum possible tangential force either breaking or accelerating can’t be steered anymore which also means that just a small side force will cause a break- out of the vehicle. Vice versa operating in cornering on the edge of the Kammsche circle for lateral forces will end in an unstable manoveur once the driver starts breaking or tries to accelerate. Both riskful situations should be prevented by a proper ESC together with its subsystems.

2.2. Operation conditions

Before defining test procedures for any technical system it is necessary to evaluate the operating conditions. Although the ESC is observing the recent behaviour of a car all the time while running it is not always active by means of constant outputs. In order to bring the system into a state of action special driving situations are necessary.

Whereas known system like ABS (Anti block system) and ASC (Anti slip control) aim to stabilize mainly the longitudinal behaviour of a vehicle, ESC is in charge of keeping lateral dynamics under control. An abstract description is the check whether the vehicle is still following the drivers cornering intention and if yes whether it is doing it in a way that is still controllable by the average driver. If the system recognizes an unstable situation, these stati are called under- and oversteering. This doesn’t mean that those non-linear driving modes aren’t controllable at all by a human. An experienced driver is able to stabilize the car with certain measures. But an ESC system has just more opportunities and quicker reactions than a human being.

In understeering conditions the vehicle is not sufficiently following the indicated direction of the driver. This happens due to a reduced or just an exceeding of the applicable force on the tires of the front axle (2.1). Root causes are too high velocity in combination with a too big steering angle. Since the rear wheels still have grip the intuitive human reaction of braking will reduce the speed and thus have the possibility to shift the front tire load back into the Kammsche circle so that the steering wish will be transmitted again. A less intuitive reaction would be opening the steering angle just a bit which also gives the possibility to move the overall horicontal tire force back into the Kammsche circle. In comparison inside the boundaries of physics ESC has the capability to apply break torque individually to each wheel in order to approximate the desired and actual vehicle direction. In terms of understeering that leads mainly to a breaking of the inner (curve) rear wheel. This generates a drag torque on the vehicle around its yaw axis. It pulls the vehicle back into the corner. The break torque is applied to the rear wheel because front wheels are operating in sliding friction area (2.1) which reduces applicable break forces.

Oversteering occurs when the vehicle is following the driver’s steering wish but finally not able to stop the inertia of the rear axle load at the desired yaw angle. The vehicle’s rear axle tends forwarding the front axle and it starts rotating around his own yaw axis. This situation is much more difficult to control by the driver himself than understeering. Once the rear axle starts leaving the Kammsche circle the appropriate counter measure would be quick anti steering. Releasing the throttle pedal at the same time which is a typical human reaction in such a situation might lead to a load shift from the rear to the front axle which could end up in a so called “counter-swing”. Professional drivers are able to maintain this “unstable” driving condition by modulating the throttle pedal and the steering wheel angle in an appropriate way in a so called “drift”. Since car manufacturers can not expect this ability from every driver ESC applies brake torque especially to the front outer wheel in order to create a drag torque out of the curve again.

Considering these facts in general most of the cars are designed with a tendency to understeer when approaching a cornering situation too fast. Hence it easier to control the car even with turned off or malfunction of the ESC. Still oversteering might to be provoked by load shifts or swinging of the vehicle body.

Figure 5 shows the different conditions indicating one of the previous mentioned driving manoveurs and the consecutive reaction of the ESC system.

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Fig. 5: Under- and oversteering illustration [4]

In general an unintended high yawing angle or rate normally occurs if either different longitudinal forces act at the right and left side wheels (e.g. u-split surface) or the difference of lateral forces at front and rear axle exceeds a certain limit.

2.3. Control loop

The necessity of the evaluation of the test data from real physical ESC systems requires knowledge about the control system acting in the background while executing a driving action at the physical limits (2.2). Fig. 6 shows the overall structure of the control loop. The ESC Control unit has the task to permanently compare desired driving behaviour with the actual driving behaviour via the equipped sensors and adjust the actors output accordingly to move it closer to the desired one.

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Fig. 6: ESC Topology overview [4]

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Details

Seiten
9
Jahr
2018
ISBN (eBook)
9783668924369
Sprache
Englisch
Katalognummer
v461412
Institution / Hochschule
Technische Hochschule Ingolstadt
Note
1,3
Schlagworte
ADAS Driver assistance system vehicle safety ESP ESC Vehicle testing Vehicle dynamics Control Engineering

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Titel: Testing of ESP/ESC in vehicles. A wholistic approach to the complex