Introduction
The ABS (Anti-lock Brake System) monitors the speed of each wheel to detect locking. When it detects sudden braking, it will release braking pressure for a moment and then provide optimum braking pressure to each wheel. By repeating this process in a short period of time, it enhances steering control during sudden stops. As a result, it will also help improve the ability of stopping the vehicle.
The role of the ABS is to control the wheel speed in order to prevent the wheels from locking and to assure a maximum braking force. This is of major importance when the runway is slippery or very short. Wheel moment of inertia, Rotational wheel speed, Friction coefficient, Wheel Radius Normal force per wheel Friction force, Aircraft speed are analogous to the above vehicle wheel shown. Forces acting on a braked wheel .The ABS commands the brake pressure as a function of the difference between the measured and the reference wheel speed.
The latter is calculated from the measured aircraft speed and the desired wheel slips, using equation. At the moment, when the pilot pushes the brake pedal the brake pressure and the wheel slip increase provoking a ground force between tyre and runway. Assuming the case of full braking, the ABS will control the wheel speed to its reference value. To achieve a maximum braking force the reference slip should be chosen close to the optimum slip. However, when the pressure level in the brakes becomes too high, the wheel slip slides beyond the optimum of the adhesion curve and the system tyre/runway becomes instable. The slope of u (s) being negative, the wheel immediately starts to lock. In this case, the ABS rapidly releases the brake pressure to force the wheel speed back to the stable side of the adhesion curve. In fact, this situation occurs, when either the desired slip s, has been chosen on the instable side of the friction characteristic, or when a sudden change in ground force is encountered (e.g. a transition from a dry to a wet runway surface). The principle problem in ABS design is that the optimum slip and the exact shape of the adhesion characteristic depend on the runway surface and further parameters, which cannot be measured, such as the condition of the tyres or the dynamics of the normal forces. Bearing in mind that the optimum slip value may vary between 3% and 20%, it is clear that the choice of the reference slip value is crucial for a safe and efficient ABS. If it is too small the braking force might become insufficient, if it is too high, wheel lockup occurs.
Recently there has been a growing interest in intelligent control techniques for the design of aircraft and road vehicle Antilock Brake Systems (ABS). In particular, rule-based, fuzzy logic controllers have been applied to this problem and successfully tested in simulation. In fact, the use of non-linear, fuzzy control techniques appears to be particularly appropriate for the ABS control problem because of the high non-linearity of the system and the lack of a precise physical model of the friction force between tyre and runway. In addition to that, the controller must operate at an unstable equilibrium point to achieve an optimal braking performance. The most important problem in ABS control design - fuzzy or conventional - is that the optimum adhesion coefficient varies significantly with the surface condition (i.e. dry, wet, icy, etc.) of the runway. Because the latter is unknown, it is extremely difficult to define a controller that guaranties an optimal braking performance for all types of runway conditions.
How Do Wheels Lock?
During braking, wheels lock if the brake force applied is more than the friction between the road and tyre. This often happens in a panic braking situation, especially on a slippery road. When the front wheels lock, the vehicle slides in direction of motion. When the rear wheels locks, the vehicle swings around. It is impossible to steer around an obstacle with wheels locked. Locked wheels can thus result in accident. Skidding also reduce tyre life.
What Does ABS Do?
The system detects when the wheel are about to lock and momentarily release the pressure on locking wheel. The brakes are reapplied as soon as the wheels have recovered.
A toothed wheel (pole wheel) is fitted to the rotating wheel hub. A magnetic sensor mounted on each wheel in close in close proximity to the teeth, generates electrical pulses when the pole wheel rotates. The rate at which the pulses are generated (frequency) is a measure of wheel speed. This signal is read by electronic control unit (ECU). When a wheel is lock, the ECU (electronic control unit) sends an electrical signal to the modulator valve solenoid, which release pressure from the brake chamber. When the wheel recovers sufficiently, the brake pressure is reapplied again by the switch off signal to the modulator valve.
The modulator valve has an addition ‘hold’ state which maintains pressure. In break in the chamber, thus optimizing the braking process. The cycling of modulator valve (5 to 6 times per second) is continued till the vehicle comes to a controlled stop. With ABS, the vehicle remains completely stable even when the driver continues to press the brake pedal during braking, thus avoiding accidents.
Uses in : Apache RTR , Swift Dzire, BMW
Reference video : https://youtu.be/mKiTAcXK6M4
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