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The system of interest to us took root in cars back in the late 1970s, so it passed the test of time. Currently, the lack of ABS as standard is a rarity. It significantly improves road safety and partly reduces the requirements for driver skills. In any case, under the control of ABS, even an inexperienced person is more likely to avoid an emergency.
IN THE POWER OF INTERESTS
The task of ABS is to maintain controllability during emergency braking. It is known that the grip on the blocked wheel is lower than on the rolling wheel - the braking forces it creates are less, and the steering is completely absent. At best, the car slides straight, at worst - along an uncontrolled trajectory with unpredictable results. ABS, on the other hand, controls the operation of the wheel on the border between the maximum possible (in specific conditions) clutch and a break in the lock, preventing it from developing. Of course, the tire grip coefficient itself does not depend on ABS. On ice, it may turn out to be ten times lower than on dry asphalt, which means that the car's handling will be different. But in both cases, ABS provides the maximum possible. With a fairly fine tuning, it is able to act even more efficiently than the driver ace.
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The work of ABS is based on the coefficient of wheel slip - the ratio of the difference between the speed of the car and the peripheral speed of the wheel to the speed of the car. In different driving modes, the translational speed of the car and the peripheral speed of the wheel may not coincide. With intensive acceleration, the peripheral speed of the drive wheel is higher than the speed of the machine, while decelerating, the opposite. Naturally, two modes correspond to 100% slippage - locking the wheel during braking or slipping in place. Meanwhile, the best grip of the tire with the coating and, therefore, the maximum transmission of braking forces are achieved with a degree of slippage of the wheels of about 20%. That ABS and maintains this value at the level of 15-20%.
ANATOMY
The hydraulic circuit of the ABS module includes solenoid valves and a pump. During normal braking, the valves are not engaged; the driver’s foot controls the required pressure. But when slippage occurs with the risk of wheel lock, ABS is activated.
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Modern ABS four-channel: this arrangement makes it possible to control the pressure in the brake system separately for each wheel. All system circuits work in a similar way in three modes - pressure retention, pressure reduction and increase. When the wheel is close to the lock, the system goes into pressure retention mode. Valves cut off the caliper from the brake master cylinder - now the fluid pressure on the pistons is constant regardless of the pressure on the pedal. But when slipping above 20%, the system reduces the pressure with the pump, relieving part of the fluid from the caliper to the brake master cylinder. When the slippage drops below a certain threshold, the system proceeds to increase the pressure: the valves open - and when the pedal is depressed, the pressure rises. These modes alternate until the situation changes: braking is interrupted or significantly weakened and there is no slippage, or the vehicle speed has dropped below 5-15 km / h (depending on the system settings). This alternate change of operating modes causes itching on the brake pedals. Frequency is high - the foot of even the best pro driver cannot compete in speed with ABS! When braking, ABS supports the slipping of all wheels at the same level to maintain directional stability. On a mix (for example, the left wheels of a car on asphalt, and the right wheels on ice), the system will maintain a linear motion, adjusting the pressure in the contour of each wheel depending on the adhesion of this wheel to the coating. Braking without ABS will lead the car to the side of the cover with better grip, and when the wheels lock, it will reach a U-turn.
Perhaps the most important elements of ABS are wheel speed sensors. According to their impulses, the speed of each wheel is calculated and compared with the speed of the car. Based on this information, the ABS module calculates and keeps the slippage of each wheel at the desired level.
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At the choice of the designer, passive or active sensors are used. Passive is easily identified by a gear ring (comb) on the wheel drive. It is very simple: when the comb rotates, the sensor generates an analog voltage signal. But, alas, with a low wheel speed, such a sensor does not give a clear signal, it may be wrong.
The active sensor reads the magnetic ring marks on the wheel bearing. It is characterized by a clear digital signal in the form of successive voltage pulses, the value of which does not depend on the speed of rotation of the wheel. But the pulse frequency reflects this speed.
For all-wheel drive vehicles, an ABS additional G-sensor with a longitudinal accelerometer is included in the ABS. It sends an acceleration or deceleration signal to the ABS module, which is taken into account when calculating the vehicle speed correction coefficient. Indeed, under certain circumstances it is impossible to measure the speed with the necessary accuracy.
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FORCE MAJEURE
There is nothing perfect in the world, and ABS is no exception. Maintaining control is sometimes paid for by increasing the braking distance. If ABS is effective with good adhesion of all four wheels to the road, then abnormal situations are possible on a problematic coating. Roughnesses in the roadway (comb, tram tracks, etc.) cause the wheels to bounce, and if the suspension fails, even a temporary separation of the wheel from the coating is possible. At such moments, the wheels are heavily unloaded, which leads to their early blocking during forced braking and, accordingly, to the early response of ABS. The same effect of early response is observed in areas of asphalt covered with sand, mud, gravel or on bare ice. The worst case scenario is a departure from the road. Without ABS, the locked wheels could bite into the coating, at least somehow damping the speed. With ABS, the braking distance is greatly increased, and in the event of braking in a skid, the car will take you away in an arc. In the December issue of the ZR for 2012, a special test is described that compares braking performance at a speed of 60 km / h on flat asphalt and on a comb. For two of the three tested machines, the braking distance on the comb increased by 40%!
BETTER NOT RISK
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Disabling ABS is not provided. But you can get rid of it by removing the fuse. Most often they do this when they go to work out on the ice track. However, it should be remembered that modern ABS is also responsible for the distribution of braking forces along the axes during normal braking (previously independent mechanical regulators controlled this). In the case of ABS shutdown, any normal braking can lead to blocking of the rear wheels with all the ensuing consequences.
OFFICE OF THE THERAPIST
ABS has a failure indicator lamp. Read fault codes. You can also monitor the parameters of elements and control some of them - for example, valves and the pump of the ABS module. It is best to use dealer diagnostic equipment. The system is quite reliable and does not include too many elements. Most ABS faults are related to external influences.
Control module errors
Most often these are internal electronic module failures. Sometimes such errors are random in nature, that is, after removal they no longer occur. If the errors are not deleted or reappear, the control module must be replaced: repair is not provided.
Errors in wheel speed sensors
Possible causes are from a wiring malfunction to a sensor failure. If an active sensor is used, the malfunction may be due to increased play of the wheel bearing (too large air gap between the sensor and the magnetic ring on the bearing) or the fact that when replacing the bearing it was simply placed on the wrong side. When using a passive sensor, a comb on the drive can create a problem: during the replacement of the wheel bearing or when removing and installing the drive, it could be slightly displaced from the seat. The signal from this sensor sometimes weakens due to accumulated dirt or metal particles on the comb. Both sensors are afraid of strong vibrations, but especially - active. Because of this, the sensor can sometimes be removed without damage, because hammer blows - not even on it, but next to it! - able to destroy it.
