Abs Control Cycle Curves

Depressing the brake pedal increases the brake fluid pressure in each wheel cylinder, which in turn decreases the wheel speed (or increases the wheel deceleration rate). When the brake fluid pressure is increased to a level of point "A" of the brake fluid pressure curve in the diagram (at which the wheel deceleration rate exceeds threshold "-b₀ "), the ABSCM makes a pressure "hold" control. At the same time, the ABSCM calculates a "dummy" vehicle speed which is a reference speed it uses in the next stage of control.

When the wheel speed then drops below the slip ratio setting, i.e., a speed lower than the "dummy" vehicle speed by the predetermined value (at point "B" of the pressure curve), the ABSCM makes a control to prevent the wheel from locking, or a pressure "decrease" control.

As the wheel cylinder pressure decreases, the wheel speed starts increasing (or the wheel acceleration rate starts rising). When the wheel acceleration rate exceeds threshold "+b₁₀ " (at point "C" of the pressure curve), the ABSCM makes a pressure "hold" control. When the wheel acceleration rate exceeds threshold "+b₂₀ " (at point "D" of the pressure curve), the ABSCM recognizes that wheel lock-up will not occur and then makes a pressure "increase" control.

When the wheel acceleration rate drops below threshold "+b₂₀ ", (at point "E" of the pressure curve), the ABSCM starts pressure "hold" and "increase" control cycles at a given interval.

When the wheel deceleration rate then exceeds threshold "-b₀ " (at point "F" of the pressure curve), the ABSCM immediately makes a pressure "decrease" control.

Fig 1: ABS Control Cycle Curves

Courtesy of SUBARU OF AMERICA, INC.