This paper presents a new scheme of four-phase control and its design method to accommodate all road conditions for an anti-skid brake system. This control law differs from the conventional two-phase control by adding a high-pressure holding mode and a low-pressure holding mode. The elapsed time interval and the angular acceleration of the wheel are used to control the switching of these two additional modes. Firstly, a piecewise linear tyre model with two segments is used to approximate all tyre-road characteristics. Next, the threshold values for the two additional modes are determined to ensure the desired performance under all road conditions for anti-skid systems with a quick convergence rate towards the desired operating region, so that a large brake force and desirable directional stability for the vehicle can be obtained. Finally, computer simulations are performed for two selected cases with dry and wet roads, with the results showing that the desired operation is achieved in only a few cycles.

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