Variable duty cycle antilock braking system with accelerometer and fail- safe

摘要

Improvements in vehicle antilock braking systems of the type having an operator controlled master cylinder (11) and a second source (55, 57) of pressurized hydraulic fluid for selectively supplying rebuild pressure after an antilock event are disclosed. The improved system is selectively operable in one of three braking modes, a normal braking mode (119) where braking force is proportional to an operator brake pedal pressure, an enhanced anti-skid braking mode (69) where braking force may be maintained at a maximum nonskid level, and a conventional anti-skid braking mode (71) where braking force follows a cyclic pattern of fluid pressure bleed and build. The system includes circuitry (17, 19, 35, 37, 93, 95) for determining the speed of each wheel and an arrangement (59) operable independently of any vehicle wheels for determining vehicle deceleration. This comparison normally invokes the enhanced anti-skid braking mode. In the enhanced mode, the system is continually searching for an optimum wheel speed reference (39) by determining (119) on which side of the peak of a mu-slip curve the vehicle is operating, for example, by calculating the sign of the slope of the mu-slip curve at the current vehicle operating point, and then incrementing (88) a prior wheel speed reference by a value (79) which changes the wheel speed reference toward the peak (75) of the mu-slip curve thereby calculating an updated wheel speed reference to control appropriately the braking system. The accuracy of the determined vehicle deceleration, and in particular, of the accelerometer is confirmed by comparison (63) to an expected deceleration based on operator brake pressure, and the conventional anti-skid braking mode is substituted for the enhanced anti-skid braking mode in the event the determined vehicle deceleration is determined to be inaccurate. The enhanced anti-skid mode utilizes a pulse width modulated solenoid actuated two position valve (41, 43) operable in one position to provide a fluid flow path from the braking device, and in the other position to provide a fluid flow path from a second source to the braking device.