The sensor has two output terminals, one of which is connected to the ground and the other (signal) is connected to the ECU. When one of cogwheel’s teeth falls into the magnetic field of the sensor, a positive voltage peak on the output signal is present, and after leaving the magnetic field a negative voltage peak is present, equal to the positive one. ![]() Speed at which the cogwheel passes along the magnetic field of the sensor.Įach of these factors plays a significant role in forming the output signal from the sensor. ![]() distance between the sensor and the rotating cogwheel.number of windings of the sensor’s coil.internal magnetic field strength of the sensor.The shape of the output signal depends on several factors: Sensor output is AC, and generates a voltage pulse each time any of the teeth of the rotating cogwheel passes through the sensor’s magnetic field. The sensor consists of a coil with a core magnetization. Operation principle of passive wheel speed sensorsĭuring its operation the ABS ECU receives signal from four identical in design sensors, one for each wheel (Fig. Such systems are used to be called four-channel systems. In its full configuration ABS includes four sensors and four pairs of valves, which allows individual control of each wheel to achieve maximum effect and allows keeping the diagonal distribution of brake actuators. It is determined by the ECU and depends on the condition of road surface. When ABS operates the average pressure in the brake chamber does not depend on the applied pressure to the brake pedal. If the wheel tries to block again, next cycle is being started. System frequency is 5Hz - 10Hz. This completes one cycle of operation of ABS. The pressure in the chamber increases and equalizes to the pressure in the master cylinder. When the wheel speed increases, the ECU turns off the input valve which opens and the brake chamber is connected to the master cylinder again. The brake chamber is disconnected from the master cylinder and from the hydraulic accumulator, and the pressure in the chamber remains constant and less than the master cylinder. When the possibility of wheel lock disappears, the ECU closes the output valve. Brake chamber pressure continues to decrease. Simultaneously, the ECU turns on the hydraulic pump motor to return liquid from the hydraulic accumulator in the master cylinder. The input valve releases the brake chamber from the master cylinder and the output valve connects it with the hydraulic accumulator, thus reducing the pressure. During an emergency brake (possibility of wheel “block”) the ECU controls the electromagnets of two valves simultaneously causing them to operate. In this case ABS does not affect the brake system. Brake chamber is connected to the master cylinder through the open output valve and the input valve is closed.Īt normal operating pressures (no wheel “block”) brake fluid passes from the master cylinder to the brake chamber without restriction as the pressure of the fluid in the cylinder and the chamber is equal and proportional to the amount of pressure applied on the brake pedal. In the initial state of the hydro-pneumatic unit, the two electromagnetic valves and the hydraulic pump motor are empty. By controlling these valves, the ECU increases, decreases or maintains constant pressure in the brake chamber. Individually adjustable valves are fitted in pair at each wheel: normally open input valve and normally closed output valve. Hydro-pneumatic unit incorporates hydraulic accumulator, electro-hydraulic pump and valves. ABS controller).Įach controlled wheel is fitted with pinion and inductive sensor comprising a permanent magnet and a coil. Rotation of the pinion induces AC voltage in the coil sensor, which frequency is proportional to the angular speed and the number of teeth of the wheel. During this time, the electronic control unit changes the torque. The ABS indicator light does not flash before, during and after the above action, i.e. control unit does not perceive the situation as an error, but as the normal operation of ABS.Īn analog ABS sensor is shown in fig.1 and a digital ABS sensor is shown in fig.2. ![]() If there is some increase to the individual rate of any of the wheels when accelerating, this wheel receives breaking pressure to reduce the difference. New cars are fitted with anti skid system, which basically works opposite to the ABS. This means that there’s a possibility of a wheel "block". The modern equivalent is now called "Stability Control" and is much more advanced than the standard ABS. The main function of the ABS system is to control the speed of the wheels when the brake pedal is pressed by checking whether any of the wheels decreases its speed faster than the others.
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