In automotive driving, hard braking may sometimes induce a skid, which significantly impairs the dynamic performance of a vehicle. This study is concerned with a method to control braking pressure of rear wheels for preventing the skid.
Detected signals are the longitudinal acceleration of a vehicle and the mean velocity of the both of rear wheels. The acceleration signal is put into an integrator, which gives the computed vehicle velocity as an output. The difference between the computed vehicle velocity and the measured wheel velocity is amplified. While the amplified signal is small, it is negatively fed back to the integrator through a low-pass filter.
Once a skid occurs, the output from the differential amplifier increases, because the controller is designed not to follow such a rapid change in the wheel velocity. Immediately after the output signal exceeds a predetermined level, the feedback loop is shut off. A capacitance incorporated in the lowpass filter, begins to send to the integrator a constant signal corresponding to the feedback signal just before the shutoff. The signal memorized in the capacitance almost cancels various kinds of noises involved in the acceleration signal, resulting in computation of the accurate vehicle velocity. At the time of the shutoff, an actuator releases the brake pressure in order to recover the wheels from the skid condition. When the signal from the amplifier decreases below the predetermined level, the brake pressure is increased again. The vehicle comes to stop through repetitive control of the brake pressure, keeping the slip ratio close to the optimum level.
The performance of the anti-skid control system has been experimentally investigated under several conditions such as on roads with various frictional coefficients and in engine braking operation. Improvement such as reduction in deflected angle and stopping distance of a vehicle has been confirmed.
