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Anti-lock Braking System

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My research module report on Anti-lock Braking system for motorcycles.
  Anti-Lock Braking System for Motorcycles Karthikeyan Manga Loganathan Summer Semester 2013, Matriculation #: 334707 Dept. of Electrical Engineering & Computer Science (Micro & Nano Systems) Technische Universität Chemnitz Chemnitz, Germany   Abstract   —  This paper elucidates the employment of Anti-lock Braking System (ABS) in motorcycles as an important safety utility from both technology and application points of view. It also briefly explains the challenges faced when designing such a specific system. The current state of the art models from BMW and Honda are compared and their features are also briefly studied. Keywords  —   ABS; motorcycle dynamics; RLP; rear lift-off; Honda CBS; slip ratio; lean angle I.   M OTIVATION Although motorcycles are regarded as machines for recreational transport and circuit racing in most countries of  North America, Europe and Oceania, they largely contribute to one of the primary and practical modes of daily transport in several countries of Asia, Africa and South America. In 2005, Asia accounted for 80% of the world’s motorcycle fleet of 290 million units and 90% of the world motorcycle sales [8]. With such widespread use of motorcycles in these countries, the figures associated with motorcycle accidents are also  proportionally high. Incorporating the technology of Anti-lock Braking System (ABS) in motorcycles has been sought to save the accidents arising due to: (1) panic or hard braking, (2) wet riding conditions and (3) human incompetence in efficient  braking. II.   I  NTRODUCTION :   ABS  FOR MOTORCYCLES  The principle and deployment of ABS in motorcycles, though identical in its core working with the technology used in cars, should consider a few unique factors owing to the inherent dynamics of a motorcycle and the riding style of a common motorcyclist. These factors are described as follows:  A.    Lean Angle One of the primary differences between a car and a motorcycle is the existence of lean angle which is typically the angle between the longitudinal axis of the motorcycle and the normal through the contact patch on the road. The effect of lean angle is more pronounced as the lateral acceleration and the radius of the corner increases. As illustrated in Fig. 1, λ is the lean angle and it is mathematically described as: λ = arctan( v 2  / R · g  ) (1) Fig. 1.   Definition of lean angle .   with the velocity v , cornering radius  R  and acceleration due to gravity  g   [2]. It is evident that the grip available to the tyre is inversely proportional to the development of lean angle which  becomes critical at high speeds and due to sudden lateral acceleration. Any rapid disturbance, both external and internal, during the equilibrium driving state with lean angle would result in a crash.  B.   Gyroscopic Effect Motorcycles are highly sensitive to the Gyroscopic effect. Like a gyroscope, the tyres of the motorcycle also experience a gyroscopic moment or couple along the axis perpendicular to the plane of rotation. Fig. 2 describes the stabilizing effect on the motorcycle from the gyroscopic moment as the tyre rolls along the road.  Fig. 2.   Stabilizing effect due to gyroscopic moment. Under hard braking this stabilizing effect is lost as the angular momentum responsible for setting up the gyroscopic moment is suddenly reduced. C.    Rear Lift-off Braking under panic could make the rider apply excessive  braking force to the front tyre when compared to the rear tyre. Exceeding a critical limit of this uneven braking force would eventually result in the shifting of centre of gravity towards the front suspension and hence cause the rear wheel to lift off the ground. Extreme cases can cause the rider to be thrown off from the motorcycle that could either prove injurious or fatal. Thus most ABS mechanism in motorcycle should be designed to mitigate such an adverse scenario. Fig. 3 shows an instance of such a risky situation [5]. Fig. 3.   Biker experiencing rear lift-off. III.   T YPICAL ABS   M ODEL  A typical scheme of ABS in motorcycles is shown in Fig. 4. Its parts are briefly described as follows: 1.    Brake handle . A press lever mechanism that serves as the human input for mechanical braking of the motorcycle. Fig. 4. Typical scheme of ABS. 2.    Brake pump.  Essentially an reservoir of brake fluid and generates amplifies brake fluid pressure from the lever pressure. 3.    Anti-lock modulator.  An electro-hydraulic control unit consisting of relays, servo motor and valves that acts as governor of fluid pressure at calipers during both manual and assisted braking. 4.    Brake calipers.  Abrasive liners attached to the rotor on the wheel, causing actual braking by friction. 5.   Controller.  Electronic control unit that senses wheel speeds using wheel speed sensors and brake fluid  pressure at calipers using pressure sensors, and sends modulation signals to the anti-lock modulator. It can  be powered by the storage battery in the vehicle [1] [3]. IV.   S ENSORS IN ABS Various sensors act as peripheral components in ABS aiding in detecting various parameters that are essential for its operation. These sensors are listed as follows:  A.   Wheel Speed Sensor: These sensors are either based on Induction or Hall Effect. They are harnessed to a toner or toothed ring that is freely allowed to rotate along with the disc rotor attached to the wheel. The frequency of the pulses is used to gather data on the speed of the wheels [3].  B.    Pressure Sensor: They are used to sense pressure of brake fluid and rider input pressure at the brake lever. The commonly used type is a strain gauge type with a bending stainless steel diaphragm connected in a Wheatstone bridge.  C.    Lean Angle Sensor: They are a cluster of inertial sensors and accelerometers measuring the various dynamic attributes of the vehicle like [5]:    roll rate (ΩX)    yaw rate (ΩZ)    longitudinal acceleration (aX)    transverse acceleration (aY)    vertical acceleration (aZ)     pitch angle     bank/lean angle (ψ)  V.   P RINCIPLE  Threshold braking or limit braking is the prime objective of any ABS. Threshold braking refers to the slowing down of the vehicle at an optimum rate without locking up the wheels. Earlier, this technique was left completely to the skill of the rider by applying precise and pulsating pressure on the brake levers. With the advent of ABS, the motorcycle is always maintained in the safe zone of braking while slowing down with help of electronically controlled hydraulic actuating units. This safe zone of braking is quantized in the form of a Braking Force Chart for various road surfaces as depicted in Fig, 5 [3]. Another way of achieving threshold braking is by minimizing wheel hysteresis. The dimensionless quantity Slip Ratio is used to quantify wheel hysteresis. Slip ratio is defined as: σ   s = ((v v    –   v w  ) / v v  ) * 100  (2) where σ   s refers to the slip ratio , v v and v w are vehicle speed and wheel speed respectively. 100% slip ratio indicates a completely wheel-locked condition resulting in ‘ skidding ’ . It has been experimentally observed that a slip ratio of 10  –   30% is regarded as safe braking zone [3]. Fig. 5. Braking Force Chart (X axis  –   Slip Ratio). VI.   OPERATION The crude operation of ABS in a motorcycle in effecting safe braking without wheel lock up is sequentially listed as follows:    ABS is powered upon turning on the ignition key switch and remains in calibration stage till the motorcycle achieves a certain threshold speed.    The electronic control unit (ECU) constantly senses the wheel speeds by monitoring the electric signals from the wheel speed sensors.    This data is processed by the ECU to get data about Slip ratio.    During braking, the condition of Slip ratio to be within the safe zone is checked by the ECU for criticality. o   If the slip ratio exceeds the safe limit, ECU commands the modulator unit to take over control from rider input and subsequently regulates the brake fluid pressure at the caliper by sending corresponding step signals to it so that the slip ratio is brought within the safe limit o   If the slip ratio is maintained within the safe limit, the ECU maintains the parameters of the step signal to the modulator. This maintains the braking force at the calipers. o   If the slip ratio is well below the safe limit, the control is restored to the rider input,  bypassing regulation from the ECU [9]. Fig. 6. Sequential operation of ABS  This operation is summarised as a flowchart depicted in Fig. 6. VII.   STATE   OF   THE   ART Of all the commercial ABS models available for motorcycles as a standard option from the manufacturer,  popular variants include those from the pioneers BMW and Honda.  A.    BMW ABS I & II In 1988, BMW became the first motorcycle manufacturer to equip their K-100s with ABS. BMW's system employs a plunger to regulate hydraulic pressure on the brake calipers and a ball valve to provide isolation  between the brake lever and the system, thereby nullifying the transfer of vibration from the brake to the lever. The first generation system weighed about 11 kg. However the second generation of BMW's ABS, introduced in 1993, weighed just over half the weight of that of the first generation system. It enhanced the reliability of the ABS, and an upgraded control system more efficiently regulated the travel of the hydraulic  plunger. This new approach allowed the system to apply appropriate braking force sooner upon the activation of the  brake, resulting in smoother stops. Fig. 7. ABS Setup in BWM R1200 Both ABS I & II controlled the braking of the front and rear tyres independently. A layout of all the ABS components in BMW R1200 is shown in Fig. 7 [4].  B.    Honda Combined ABS Honda ’ s C-ABS offered linked control over braking  between the front and rear tyres which provided the rider the freedom to apply excessive braking force on any one tyre. The excess braking force would be automatically distributed to the other tyre to ensure smooth and jerk-free  braking [6]. The schematic of Honda CBS is illustrated in Fig. 8. Fig. 8. Honda C-ABS. VIII.   R  EALIZATION  The advantages of ABS are realized under the following:  A.    Braking Distance Apart from significantly reducing the distance and time taken by the motorcycle to arrive to a complete halt, ABS has proved highly beneficial and complementing to the riders skill in safe braking especially during wet and emergency conditions. The Honda C-ABS minimizes the irregularities in the  braking distances among riders having different braking habits owing to the linked braking control between tyres. This is illustrated in Fig. 9. Fig. 9. Reduction in difference in Braking distance.
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