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Intelligent Control and Automation, 2011, 2, 57-68 doi:10.4236/ica.2011.***** Published Online August 2011 ( An Antilock-Braking Systems (ABS) Control: A Technical Review Ayman A. Aly 1,2, El-Shafei B. Zeidan1,3, Ahmed M. Hamed1,3, Farhan A. Salem1 1 Department of Mechanical Engineering, Faculty of Engineering, Taif University, Al-Haweiah, Saudi Arabia 2 Department of Mech
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   Intelligent Control and Automation ,   2011, 2, 57-68 doi:10.4236/ica.2011.***** Published Online August 2011 ( Copyright © 2011 SciRes.  ICA   An Antilock-Braking Systems (ABS) Control: A Technical Review Ayman A. Aly 1,2 , El-Shafei B. Zeidan 1,3 , Ahmed M. Hamed 1,3 , Farhan A. Salem 1   1  Department of Mechanical Engineering  ,  Faculty of Engineering  , Taif University ,  Al-Haweiah ,  Saudi Arabia 2  Department of Mechanical Engineering  ,  Faculty of Engineering  ,  Assiut University ,  Assiut  ,  Egypt 3  Department of Mechanical Power Engineering  ,  Faculty of Engineering  ,  Mansoura University ,  Mansoura ,  Egypt  E-mail  :  Received ************************** Abstract Many different control methods for ABS systems have been developed. These methods differ in their theo-retical basis and performance under the changes of road conditions. The present review is a part of research  project entitled “  Intelligent Antilock Brake System Design for Road-Surfaces of Saudi Arabia ”   In the present  paper we review the methods used in the design of ABS systems. We highlight the main difficulties and summarize the more recent developments in their control techniques. Intelligent control systems like fuzzy control can be used in ABS control to emulate the qualitative aspects of human knowledge with several ad-vantages such as robustness, universal approximation theorem and rule-based algorithms. Keywords:   ABS, Intelligent Control, Fuzzy Control   1. Introduction Since the development of the first motor driven vehicle in 1769 and the occurrence of first driving accident in 1770, engineers were determined to reduce driving acci-dents and improve the safety of vehicles [1]. It is obvious that efficient design of braking systems is to reduce ac-cidents. Vehicle experts have developed this field through the invention of the first mechanical anti-lock-braking system (ABS) system which have been de-signed and produced in aerospace industry in 1930 [2,3]. In 1945, the first set of ABS brakes were put on a Boeing B-47 to prevent spin outs and tires from blowing and later in the 1950s, ABS brakes were commonly installed in airplanes [4,5]. Soon after, in the 1960s, high end automobiles were fitted with rear-only ABS, and with the rapid progress of microcomputers and electronics tech-nologies, the trend exploded in the 1980s. Today, all-wheel ABS can be found on the majority of late model vehicles and even on select motorcycles [6-10]. ABS is recognized as an important contribution to road safety as it is designed to keep a vehicle steerable and stable during heavy braking moments by preventing wheel lock. It is well known that wheels will slip and lockup during severe braking or when braking on a slip- pery (wet, icy, etc.) road surface. This usually causes a long stopping distance and sometimes the vehicle will lose steering stability [11-13]. The objective of ABS is to manipulate the wheel slip so that a maximum friction is obtained and the steering stability (also known as the lateral stability) is maintained. That is, to make the vehi-cle stop in the shortest distance possible while maintain-ing the directional control. The ideal goal for the control design is to regulate the wheel velocity. The technologies of ABS are also applied in traction control system (TCS) and vehicle dynamic stability control (VDSC) [14]. Typical ABS components include: vehicle’s physical  brakes, wheel speed sensors (up to 4), an electronic con-trol unit (ECU), brake master cylinder, a hydraulic modulator unit with pump and valves as shown in Figure 1 . Some of the advanced ABS systems include acceler-ometer to determine the deceleration of the vehicle. This  paper is intended to present a literature review of re-search works done by many researchers concerning various aspects of ABS technology in an effort to im- prove the performance of its applications.   2. Principles of Antilock-Brake System The reason for the development of antilock brakes is in essence very simple. Under braking, if one or more of a vehicle’s wheels lock (begins to skid) then this has a  A. A. ALY  ET AL  58   Figure 1. Typical ABS components [4].   number of consequences: a) braking distance increases, b) steering control is lost, and c) tire wear will be abnormal. The obvious consequence is that an accident is far more likely to occur. The application of brakes generates a force that impedes a vehicles motion by applying a force in the opposite direction. During severe braking scenarios, a point is obtained in which the tangential velocity of the tire surface and the velocity on road surface are not the same such that an optimal slip which corresponds to the maximum friction is obtained. The ABS controller must deal with the brake dynamics and the wheel dynamics as a whole plant [15]. The wheel slip, S   is defined as: VR sV      (1) where ω  , R, and V denote the wheel angular velocity, the wheel rolling radius, and the vehicle forward velocity, respectively. In normal driving conditions, V = ω .R, therefore S = 0 . In severe braking, it is common to have ω  = 0   while S = 1  , which is called wheel lockup. Wheel lockup is undesirable since it prolongs the stopping dis-tance and causes the loss of direction control [16,17]. Figure 2 . shows the relationship between braking co-efficient and wheel slip. It is shown that the slide values for stopping/traction force are proportionately higher than the slide values for cornering/steering force. A locked-up wheel provides low road handling force and minimal steering force. Consequently the main benefit from ABS operation is to maintain directional control of the vehicle during heavy braking. In rare circumstances the stopping distance may be increased however, the directional control of the vehicle is substantially greater than if the wheels are locked up. The main difficulty in the design of ABS control arises from the strong nonlinearity and uncertainty of the prob-lem. It is difficult and in many cases impossible to solve this problem by using classical linear, frequency domain methods [17]. ABS systems are designed around system hydraulics, sensors and control electronics. These sys-tems are dependent on each other and the different sys-tem components are interchangeable with minor changes in the controller software. [18] The wheel sensor feeds the wheel spin velocity to the electronic control unit, which based on some underlying control approach would give an output signal to the  brake actuator control unit. The brake actuator control unit then controls the brake actuator based on the output from the electronic control unit. The control logic is  based on the objective to keep the wheels from getting locked up and to maintain the traction between the tire and road surface at an optimal maximum. The task of keeping the wheels operating at maximum traction is complicated given that the friction-slip curve changes with vehicle, tire and road changes. The block diagram in Figure 3 . shows the block representation of an antilock  brake system. It shows the basic functionality of the various components in ABS systems and also shows the data/information flow. The ABS (shown in Figure 4 .) consists of a conven-tional hydraulic brake system plus antilock components. Copyright © 2011 SciRes.  ICA    A. A. ALY  ET AL 59   Figure 2. Illustration of the relationship between braking coefficient and wheel slip [14]. Wheel Velocity Sensor Vehicle Velocity Sensor Tire Road Interaction Control AlgorithmBrake Actuator Valve Brake Actuator Figure 3. Block representation of an ABS. The conventional brake system includes a vacuum  booster, master cylinder, front disc brakes, rear drum  brakes, interconnecting hydraulic brake pipes and hoses,  brake fluid level sensor and the brake indicator. The ABS components include a hydraulic unit, an electronic  brake control module (EBCM), two system fuses, four wheel speed sensors (one at each wheel), interconnecting wiring, the ABS indicator, and the rear drum brake. Most ABS systems employ hydraulic valve control to regulate the brake pressure during the anti-lock operation. Brake pressure is increased, decreased or held. The amount of time required to open, close or hold the hy-draulic valve is the key point affecting the brake effi-iency and steering controllability. c  Copyright © 2011 SciRes.  ICA    A. A. ALY  ET AL  Copyright © 2011 SciRes.  ICA  60   Figure 4. Anti-lock braking system [14]. 3. ABS Control ABS brake controllers pose unique challenges to the de-signer: a) For optimal performance, the controller must operate at an unstable equilibrium point, b) Depending on road conditions, the maximum braking torque may vary over a wide range, c) The tire slippage measurement signal, crucial for controller performance, is both highly uncertain and noisy, d) On rough roads, the tire slip ratio varies widely and rapidly due to tire bouncing, e) brake  pad coefficient of friction changes, and f) The braking system contains transportation delays which limit the control system bandwidth [19]. As stated in the previous section of this paper, the ABS consists of a conventional hydraulic brake system  plus antilock components which affect the control char-acteristics of the ABS. ABS control is a highly a nonlin-ear control problem due to the complicated relationship  between friction and slip. Another impediment in this control problem is that the linear velocity of the wheel is not directly measurable and it has to be estimated. Fric-tion between the road and tire is also not readily meas-urable or might need complicated sensors. Researchers have employed various control approaches to tackle this  problem. A sampling of the research done for different control approaches is shown in Figure 5.  One of the technologies that has been applied in the various aspects of ABS control is soft computing. Brief review of ideas of soft computing and how they are employed in ABS control are given below. 3.1. Classical Control Methods Based on PID Control Out of all control types, the well known PID has been  Antilock Brake control  systems ResearchClassical Control     Intelligent control  Robust control    Otimal Control  Adaptive control     Nonlinear Control    Figure 5. Sampling of ABS control. Signal WireWd heel SpeeSensor Sensor RingBrake Cylinder PistonsDrumCableTyBrer o Emergencake LevEmergencyBrake MechanismAdjuster Mechanism Brake Shoes
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