25-08-2017, 09:32 PM
VEHICLE SKID CONTROL
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INTRODUCTION
Vehicle skid can be defined as the loss of traction between a vehicle’s tyres and the road surface due to the forces acting on the vehicle. Most skids are caused by driver error, although only about 15% of accidents are the direct result of a vehicle skidding. Skids occurring in other accidents are usually the result of last minute action, by the driver, when faced with a crisis ahead rather than actually causing an accident. Skids can occur both in the dry and wet as well as icy conditions, however, the chances of losing control and having an accident increases by 50% in the wet. The most common type of skid we will be confronted with is when the rear end of the car slides out, causing an oversteer or when the front of the car plows toward the outside of a turn without following the curve of the turn causing an understeer. Usually, oversteer occurs as a result of going into a corner too fast or incorrectly hitting a slick area, causing the rear wheels to oversteer. A third skid called the four wheel skid can also occur, where all the four wheels lock up and the vehicle slides in the direction where the forward momentum is carrying it, with no directional control.
To counter these skids and to prevent accidents from happening, Vehicle Skid Control (VSC) is incorporated in the vehicle. Vehicle Skid Control (VSC) takes the safety aspects of the driver and the vehicle to the next level. It comes under the category of “Passive Technology”, which helps you to avoid a crash. Vehicle Skid Control (VSC) senses the onset of traction loss and helps the driver stay on track. This is achieved via the system's ability to reduce engine power and to control the brake actuator. VSC helps the driver maintain vehicle traction under demanding conditions by detecting and helping to correct the wheel spin.
CAUSES
The main causes of skidding are as follows:
a) Harsh or sudden acceleration.
b) Excessive or sudden braking.
c) Coarse or jerky steering movements.
d) Oversteer and understeer.
The effects of the above will be enhanced by speed. Combining these effects with non-recognition of adverse road and weather conditions will create problems for the driver.
TYPES OF SKID
The main types of skid that a driver could encounter on the public highway fall into three categories.
1) The front wheel skid.
2) The rear wheel skid.
3) The four wheel skid.
Characteristics
The rear of vehicle swings out of line and gives the impression of trying to overtake the front (oversteer); see figure 2. If the rear tyres approach their traction limit more rapidly than the front, then the effect is for the rear of the car to steer a wider path than the front wheels. This rotates the car more than the driver intended and, if nothing is done, leads to the car turning a smaller radius corner. When this occurs the car is said to oversteer.
UNDERSTEER AND OVERSTEER
Understeer
As the name implies, understeer occurs when the front slip angle is greater than the rear and the car goes straighter rather than following the intended turn. The slip angle, or yaw angle in technical terminology, is the angle between where the car is pointing and the intended path. The yaw moment is the rate at which the yaw angle is changing. The higher the yaw moment, the more likely it is that the driver is losing control. At the same point, the front wheel may start to grip less even when the steering is turned sharply and as a result the car continues in more of a straight line than a sharp turn. Here in this case, the skid control system brakes the inside rear wheel, effectively tightening the car’s line. By applying the brakes, the car slows down which further helps stabilise it.
Oversteer
Oversteer, on the other hand, occurs when the rear tyres have a greater slip angle than the front tyres and the back threatens to overtake the front, causing the vehicle to spin. In other words, if the rear tyres approach their traction limit more rapidly than the front, then the effect is for the rear of the car to steer a wider path than the front wheels. This rotates the car more than the driver intended and, if nothing is done, leads to the car turning a smaller radius corner. When this occurs the car is said to oversteer. Here the skid control system brakes the outside front wheel to reduce oversteer, effectively pulling the tail back into line.
SKID CONTROL
Stability control systems or skid control systems with names like StabiliTrak, Dynamic Stability Control, Stability Management, and Vehicle Skid Control are the latest advancement in vehicle safety. Regardless of the different names, they all perform the same task – to sense the onset of traction loss and keep the driver on track. These systems are designed to deliver transparent intervention the moment the situation becomes unstable. A vehicle skid control system actually detects when a driver has lost some degree of control. It then automatically stabilizes the vehicle to help the driver regain control. Vehicle Skid Control (VSC) takes the safety aspects of the driver and the vehicle to a completely new level. These skid control systems are often integrated with the engine management system to cut power in even more tricky situations. This scenario is a complex system of sensors and microprocessors that continually monitor the vehicle for any signs of instability. Once detected (usually in the form of a slide or skid), the system automatically applies selective braking to specific wheels thereby stabilizing the vehicle. This split-second intervention often happens so quickly that it is over before drivers even realize they were in danger of losing control. By gently stabilizing the car at the critical moment, control is returned to the driver with minimal fuss and alarm. Luxury cars, such as the Mercedes Benzes, BMW, Lexus, etc. now sold in India, have stability systems installed that are designed to remove oversteer or understeer.
WORKING
The heart of all these systems is a central processor that takes information from a number of sensors, and then determines whether the car is in a stable or unstable state. By combining the datas from ABS sensors (for wheel speed), steering angle sensors, yaw sensors (measuring the amount a car fishtails, or rotates around its vertical center axis), and lateral force sensors (measuring the amount of sideways g-force generated by the car), the central processing unit can actually detect when a vehicle is behaving in a way contrary to how the driver intends. VSC also includes a slip indicator with a warning sound and light to alert the driver that the tyres are about to exceed the grip limit.
WHEN DOES IT HELP?
Like the safety systems that preceded it, Vehicle Skid Control is designed to step in when human input is incapable of effectively controlling the vehicle. In most cases, critical situations are the result of human error in the first place-driving too quickly, inattention, misjudgment or simply panicking in an emergency situation. In these situations, everyone can benefit from a safety system that occasionally helps regain vehicle stability, while never taking full control out of the driver's hands. After the introduction of ABS, no safety advancement has added such a high level of driving security as VSC. When used with ABS and traction control, Vehicle Skid Control significantly increases a driver's chances of recovering from potentially dangerous situations. But no matter how advanced the safety aid, the ultimate fate of a vehicle and its occupants remains in the hands of the driver. No safety system should ever be expected to protect unconditionally. So while the latest generation of stability control systems offer drivers increased protection from both themselves and the unexpected, they can never overcome poor judgement or the laws of physics.
CONCLUSION
Driving has become more and more dangerous with the ever increasing population of man and vehicles. It is estimated that 25% of all accidents are caused by driver distractions. Automotive technology is being developed everyday to make our lives on the roads much safer. Vehicle Skid Control is one such instance. Safety is the principal benefit of this technology. But no matter how advanced the safety aid, we should never forget that the ultimate fate of a vehicle and its occupants remains in the hands of the driver. No safety system should ever be expected to protect unconditionally. So while the latest generation of stability control systems offer drivers increased protection for both themselves and the vehicle, they can never overcome poor judgment or the laws of physics.