Designing an integrated control system to improve the lateral stability and rolling dynamics of the vehicle

Presented for master's degree

Presented for master's degree

Abstract

In addition to lateral instability, one of the major threats to passenger vehicles, especially chassis vehicles, is the risk of overturning. In this thesis, a coordination strategy based on fuzzy logic is designed for the integrated operation of active steering, differential, brake and anti-roll bar subsystems. Separate analyzes have been done on each of the subsystems independently as well as their synergistic effect. This coordination strategy tries to eliminate the interference between the subsystems and their control objectives, which include: following the rate of rotation, lateral acceleration and rolling motion of the vehicle, while maintaining the desired longitudinal acceleration of the driver, and establish the balance between them. Investigating the performance of this strategy in the absence of active differential has also brought successful results. Lateral slip angle and rotation rate are considered as indicators of lateral stability and roll angle, roll rate and lateral weight transfer are considered as indicators of roll stability. The simulation results on a ten-degree-of-freedom model built in Simulink software show that the performance of the integrated system has improved compared to the independent performance of individual subsystems, and that rolling stability has been maintained along with lateral stability. Also, the simulation results for the "worst case" maneuver indicate the satisfactory performance of this integrated system. The results have been validated by modeling in the Carsim software environment.

Key words:

integrated control, stability control, roll control, active steering, active differential, active braking

necessity Research

In recent years, public and private institutions have carried out extensive research on active safety technologies [1] of cars. It has been estimated that in the member states of the European Union, the direct and indirect costs caused by road accidents in 2009 were 130 billion euros [1]. One of the most effective ways to reduce these accidents is the use of integrated stability control systems [2]. The American NHTSA [2] has estimated that the use of electronic stability control systems [3] (ESC) has reduced the occurrence of accidents for a passenger car by 34% and the same accidents for SUVs [4] (SUV) by 59%. The amount of this reduction has been much higher in accidents leading to overturning[2].

In the field of car safety, extensive efforts have been made, which in a division, they are divided into two parts, passive[6] and active[7]. All measures that are used to save the lives of passengers after an accident are among the passive methods of car safety, including airbags, seat belts, head protectors, and shock absorbers (Figure 1-1). These methods are not the subject of this thesis. On the other hand, there are active methods that include lane departure warning systems, collision warning systems, and controllers that are used to maintain vehicle stability and prevent accidents (Figure 2-1). In the mentioned active systems, the first two are merely warning systems, while stability control systems directly affect vehicle dynamics. These methods are widely developed today, and the most important of them are anti-lock braking systems, wheel slip adjustment, active steering, active braking, active differential, and semi-active and active suspension. These systems are designed with the purpose of regulating the behavior of sets of vehicle dynamic variables such as rotation rate, lateral slip, longitudinal slip and rolling variables. In the following sections, there is a brief description of the mentioned methods.

The background of vehicle stability control

1-2-1 Rotation rate control

One ??of the important criteria of lateral and rotational stability of the vehicle is its rotation rate[8]. In fast and sudden maneuvers, the car may experience one of two acute situations of understeer [9] or oversteer [10] in which, respectively, the rotation rate of the car is much lower and higher than the desired value (which depends on the driver's speed and steering wheel angle).Therefore, this criterion is one of the important issues in car behavior.

In order to control the rotation rate of the car, various operators have been used, some of which are steering wheel (angle of the wheels), differential (differential thrust force under the wheels), brake (differential braking between the right and left wheels), suspension system (vertical load distribution between the wheels and as a result of changing the longitudinal and lateral forces).

Manning [11] and colleagues [4] have reviewed this research. In this review, it is mentioned that Kramer [12] and colleagues used the active command [13] and feed forward algorithm [14] (Figure 1-3). In this method, the controller reduces the response time by increasing the driver's steering angle. The more common method is feedback algorithms [15] in which the controller tries to bring the rotation rate to the desired rotation rate of a reference model by modifying the angle of the wheels (Figure 4-1). In the continuation of the review, it is pointed out that Ackerman [16] and colleagues (1992, 1996 and 1997) have done separate research in the field of separation [17] of turning dynamics from lateral dynamics with the aim that the driver follows the desired path and the controller removes disturbances caused by side winds and different friction levels on the road.

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Matsumoto[18] et al. (1992) describes the basic "brake force distribution" (BFD[19]) system used in Nissan. In this system, as well as similar systems of "direct stability control [20]" (used in BMW), the feed-back control method is used (Figure 5-1). The important point here is that if the goal of the controller is to ensure the stability of the car and the comfort of the passengers at the same time, except when the driver intends to brake, the use of this system is somewhat undesirable due to the unwanted decrease in speed. But, if the only goal is stability, braking is the most powerful tool for this purpose. However, it should be noted that braking is used more to reduce lateral slip than to control the rotation rate.

-2 Lateral slip control

One ??of the oldest methods of minimizing the side slip angle is the linear feed control of active rear steering systems [21] [5]. Figure 6-1 shows the general scheme of this method. In this algorithm, the control law is calculated from the solution of the two-degree-of-freedom model of the car to make the side slip zero.

The initial methods were based on extracting the control law based on stable conditions. In 1994, Inagaki [22] [6] proposed to design the controller based on the behavior of the vehicle in the phase plane to better analyze the response dynamics, including its decay and natural frequency. In this method, the design of the control law is based on the lateral slip values ??and its rate. In 1996, Yasui [23] et al. [7] presented experimental results of this approach on an Aisin Seiki model that used active braking. Several studies have also been done on the effect of integrated control systems on side slip. Among them, Smackman [24] [8] in 2000 compared the performance of the active braking system with the integrated system of active braking and wheel load control [25] (active suspension) and concluded that differential braking has the greatest effect on lateral dynamics, but it interferes with the desired longitudinal speed of the driver. While the wheel load control, although it has a small effect on the longitudinal dynamics, it is not capable of generating the large required turning moments. In the coordination strategy presented in that research, the suspension is active until the wheels reach saturation, and after that, the brakes come into action. rollover during cornering. A coordination strategy based on fuzzy logic has been devised to coordinate the sub-controls, namely active steering, active differential, active brake and active roll control. Independent analysis of each sub-control, as well as an analysis on their inter-relationship has been carried out.