15 ANTI-LOCK BRAKES ABS BRAKINGWITH ABS INTOOPERATION DESCRIPTION OF OPERATION OF THE ANTI-LOCK BRAKING SYSTEM On the basis of the signals received from the rpm sensors located on the front and rear wheels, the electronic control unit instructs the hydraulic unit, so as to vary the pressure of the brake fluid delivered to the brake calipers & wheel cylinders passing through the three stages described below: A. Pressure Maintaining (Hold) Phase During any panic braking situation, whenever the wheel tends to lock (because of the higher brake pressure than optimum). ECU instructs Hydraulic Unit to block the brake pressure coming from Master cylinder, so that no further pressure gets into wheel cylinder. (Refer Fig.6) By default, the inlet solenoid valve remains open & the out let solenoid valve remains closed. These are the positions of the valves under unexcited condition. During the pressure maintaining phase, ABS ECU actuates only the inlet solenoid valve (EV) & there by closes it. The outlet solenoid valve remains closed in its default position. Since both the solenoid valves are shut the liquid channel is interrupted. The pressure channel is closed, so that further increase of pressure in the brake circui t is obstructed. Non return valve mounted in parallel to the inlet valve helps in speedy reduction of pressure when brake pedal is released. Solenoid valve Electricity status Valve open-close Open-close channel INLET ON CLOSE Master cylinder-Wheel cylinder OUTLET OFF CLOSE Wheel cylinder-reservoir Fig. 6
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