How ABS Works: ABS Dynamics. Part 1
The first serial car equipped with ABS was Mercedes S-Class in 1979. The system has been offered as an option for a long time and only in 1992 it was included on the list of basic features. At the beginning of the 1980s ABS could be ordered as an option with BNW 7-series.
What is ABS for?
Have you ever happened to detour a suddenly emerged hindrance and brake simultaneously? Of course, you have. It would seem quite easy – push the brakes and turn the steering wheel thus correcting the trajectory. However, everything is easy until a certain moment. If you press brakes to hard than necessary the wheel can block and…
Further there can be two possible variant of the succession of events. Both of them depend on the presence and the absence of ABS (Anti-lock Brake System). There thing is that blocked wheels sliding deprive the driver of the ability to maneuver – the car will simply drift as if the steering wheel is switched off. Only an experienced driver will be able to unblock the wheels in cold blood by releasing brakes for a moment. And then, using the impulse braking gain control and decrease the speed. The second variant is for a car equipped with ABS. The driver only has to push brakes harder and worl with the steering wheel calmly. Do you feel the difference?
During 30 years of existence the system has undergone significant changes. The rapidity of action has increased tenfold. Thus, for instance, the first control units weighed over 7 kg. The modern ones reach not more than 1.5 kg.
Blocking is even more dangerous because is can cause sliding. This can happen when the surface under the wheels is heterogeneous, the load is shifted on the axes due to the previous maneuver or the wheels are equipped with different tires. Moreover, with blocked wheels a car can change the trajectory under some side pressure (road grade or crash). In this case it is almost impossible to correct the trajectory.
In ABS for the estimation of speed inductive sensors of frequency are used. Each new generation of frequency sensors becomes smaller, more precise and reliable. Initially only one sensor was installed (on the speed changer of the rear axis or transmission). Later it was added by two more – on the front wheels. And only the latest ABS include sensors on each wheel with individual modulators. By the way, the first and the most primitive ABS affected the work of all braking systems.
Another negative effect of wheel blocking is the increase of stopping distance. The thing is that static friction force is usually bigger than sliding friction force. Thus for the most effective braking the braking system must generate such a force that the wheels rotate on the edge of blocking. There is an important factor as relative slip. It can vary (depending on the degree of wheel’s dormancy) from 0 to 100% (when wheel in completely blocked).
Experiments show that the maximal effectiveness of braking is reached at 15-20% sliding – it means that the speed wheel’s rotation is by 15-20% lower than the speed of a free-wheeling one. Nipping on ahead I would tell you that during braking the electronics maintains this very rate by periodically blocking and unblocking wheels.
to be continued…
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