The braking systems, in addition to several general conditions that must be met, certain decelerations imposed, progressive braking, no shocks, minimal drive effort, high reliability, rapid start-up, simple and cheap construction, should prevent the wheels from locking or keep sliding between certain limits.
Performing the above functions is related to the importance that is given to ensuring stability during running. In the same measure, the aim is to minimize the braking distance and reduce excessive tire wear.
Why do we need ABS in cars?
In the case of wheels locking when braking the car, the following shortcomings may occur:
- loss of stability when locking the rear axle wheels;
- loss of steering control when the front wheels are locked;
- increased braking space, because the coefficient of grip at the wheel slip is lower than when the wheel rotates.
To increase the braking efficiency and improve the stability and maneuverability of the vehicles, automatic braking control systems are used to avoid locking the wheels regardless of the applied braking moment and the coefficient of adhesion.
The ABS must keep the wheel slip in the stable range to use the optimum friction coefficient. In case the wheel locks the slide tends to 100% of the unstable domain and the braking distance increases due to a smaller friction force.
When braking a vehicle, the center of gravity moves to the front axle, they do not take on equal loads especially when braking in turns. For this reason, the rear axle tires can lose grip much easier than those on the front axle. If the rear wheels skids, the direction in which the car is moving can no longer be controlled by the steering system.
The automatic control system allows the adjustment of the braking within the following limits:
- when braking below the road grip limit, the automatic control system does not intervene, the braking moment being maintained at the maximum value ordered by the driver.
- In the case of braking at the road grip limit, the automatic control system notifies the tendency to lock the braked wheel and controls the maintenance or decrease of pressure in the braking system so that maximum road adhesion is used. When the wheel locking tendency appears, the ABS controls the isolation of the corresponding brake cylinder from the rest of the braking system. Depending on the accentuation or disappearance of the tendency to lock the wheels, the reduction or increase of pressure in the brake cylinder is ordered, thus executing a succession of braking-defrosting cycles that will keep the wheel in the optimum grip area.
- combined braking takes place when traveling through areas with a different grip, the abs ensuring the prevention of wheel locking, on the parts with low grip, and maximum braking, on the parts with high grip. Also, the automatic control system works efficiently even if part of the car is wheeled on-road sections with different friction coefficients.
History of the ABS braking system for cars
The application of automatic braking control systems is not new. They were used for the first time in 1948 in airplanes, whereby blocking the wheels, the tires were almost instantaneously damaged. The introduction in the automotive field occurred later in the 1970s in the USA. Here also appeared the first legal regulation regarding cars equipped with an automatic braking control system.
In 1973 almost all such systems were built in this country using logic circuits and electronic components. In Europe, although research into the creation of an automatic braking control system has been reported since 1959 at Daimler-Benz, only in 1977 a prototype created by Bosch was tested on 150 Mercedes cars and in 1978 began the series production. This is after 50 million German marks were spent for this purpose, an amount supported by the companies Daimler-Benz, Bosch, and Teldix.
And other companies have created such systems: Honda, Teves, and Girling. Each of them named their creation with its logo: Bosch-ABS (Anti-lock Braking System), Honda-ALB (AntiLockBrake), Teves-ATE (from the creator’s name Alfred Teves), Girling-SCS (Stop Central System ).
Most systems differ radically only by the adjustment mode. This can be achieved by adjusting the differential pressure on each wheel, depending on the local adhesion. Although effective, the system is expensive and therefore, most of the time, it only applies to the wheels of one axle.
All research so far has convinced that such systems substantially reduce the braking space. If, for example, on a wet asphalt a car fitted with ABS brakes within 40 meters, without this system the immobilization of the car occurs only after 58 meters.
The system comes into operation automatically, when suddenly passing on a part of the road with a low grip or when the wheels on one side of the car are running in different conditions than the others.
Initially, automatic braking control systems were developed based solely on mechanical principles. Because of this and due to the complexity of the anti-skid control, which requires a sophisticated logic, we could not move to series production.
In the next development phase, control systems with analog signal processing were developed. The development of electronics has led to the emergence of digital systems of automatic control having a central unit of calculation the microprocessor.
What is an ABS made of?
The main components of an anti-lock system are 4 in number: speed sensors for wheels, valves for brake pipes (pressure modulator), special pump, electronic controller.
The speed sensors are currently mounted on each wheel separately. In the past, a single sensor was mounted, which read speed from the gearbox or the flywheel. Later there were two sensors for the front wheels, but the most modern and best system includes one sensor for each wheel. These sensors transmit the speed of each wheel to the control module, which acts differently on the brake fluid pressure on each wheel.
The valves are now embedded in a distributor connected to the brake pump. This is called a modulator. The modulator receives information from the electronic controller, which in turn receives information from the 4-speed sensors. Depending on the speed of each wheel, the controller transmits to the modulator what pressure to exert on the brakes.
How does the ABS work?
- – Speed sensors read constantly how fast each wheel spins. A fast, instantaneous deceleration, as happens when you suddenly brake, transmits to the sensor the information that the wheel tends to lock.
- – The sensor transmits the information that the wheel wants to lock completely to the electronic controller.
- – The controller gives the information further to the pressure modulator, which reduces the pressure in that wheel using valves.
- – The result is that the wheel will have the same speed as the car, it will not stop faster and it will not lock.
Why is the brake pedal shaking when we suddenly brake and have ABS?
Many people recognize that their car has ABS or not if the brake pedal vibrates strongly when exerting high pressure. That means you want to lock the wheels, but the pedal shakes and swings. That pedal shake comes from the action of the valves that let the pressure of the liquid in the pipes decrease, little by little.
The valves are released and locked quickly, so as not to let the pressure drop suddenly. In turn, the valves receive impulses from the electronic module, when to release the pressure and when not.
Why is ABS braking better than the total wheel locking?
Studies of auto safety centers have shown that the wheels have an optimal braking point. A wheel can have 0 grips, when it turns normally, or 100% when it is completely stopped. Those studies have shown that braking is optimal when its friction is somewhere around 15-20%.
This is what the ABS does: to adhere perfectly to the asphalt, only 15-20% of the time, locking and unlocking the wheels very often (the pedal pulses I mentioned above).
In addition to the maximum braking efficiency, the ABS still allows drivers something to handle even in maximum braking. It is known that a car without ABS if it has locked wheels, will not allow the driver to drive. With the ABS, which blocks the wheel from time to time, the driver can avoid an obstacle by pulling the steering wheel.
Logically and without a doubt, the best braking would be one with constant and progressive pressure on the wheels, “from the foot”. But even the most skilled racers can’t brake as well as the ABS. That is why it is not surprising that all cars today come with such systems in the standard version.
What are the weaknesses of a car with ABS?
Like any other electronic system, ABS also has its weaknesses. However, less than benefits. It’s about ice or snow braking. Usually, a car with ABS will have a longer braking distance on ice or snow than a car without ABS. And that’s because in these conditions, winter tires, work better at 100% friction. A wheel locked on the snow will brake faster than one with the ABS braking system. So it is worth remembering that ABS works a little harder in winter.
Besides, for example, if we encounter a situation where one of the wheels is in the air, when braking, it locks automatically, transmits through sensors the way that wheel is stopped, the pressure decreases and when the wheel returns to the road, braking is very weak. There are rare situations, but they are encountered when the car does not have the proper suspensions.
I hope you are convinced that an ABS is a technological marvel, meant to help the driver and save lives. ABS safety consists of better braking, as well as avoiding different obstacles, pedestrians, cars, etc.
Today’s cars have standard ABS equipment, including used ones even if they are older. And from now on, when you brake with any car, you know how this system works which gives you added safety.