Anti-Lock Braking Mode: Notes
ABS VI has been designed to improve the controllability and steerability of a vehicle during braking. ABS VI accomplishes this by controlling the hydraulic brake pressure applied to each front caliper and the rear wheel cylinders. Anti-lock braking occurs only when the stoplamp switch is closed and a microprocessor, located in the electronic brake control module (EBCM), determines that at least one wheel is about to lose traction during braking. The EBCM will then allow the hydraulic modulator to change the brake pressures several times each second to keep the wheel(s) from locking and provide the driver with maximum vehicle controllability. ABS VI cannot increase the brake pressure above the master cylinder pressure applied by the driver and cannot apply the brakes by itself.
The result is greatly improved braking, enabling the driver to maintain steerability more easily and to bring the vehicle to a controlled stop. ABS VI provides effective braking and directional control over a wide range of read surfaces and driving conditions.
If any wheel begins to approach lockup, the EBCM will control the three motors and two solenoids, appropriately, to control brake pressure to the affected wheel(s). During front wheel ABS operation, the solenoids are turned on to isolate that brake pressure path to the caliper.
The EBCM the provides controlled current to the motors to regulate the speed and amount of movement. When the motor is activated and tries to drive the ball screw nut, the end of the ESB in contact with the motor drive dog rotates inwards, causing the spring to contract in its housing and allowing the motor to rotate the modulator gear.
The most common application of this principle is in window crank mechanisms, where the weight of the window or the force of the window to be lowered or raised. For the ESB, brake pressure on the top of the pistons corresponds to the weight of the window and the motor corresponds to the window regulator handle. As the motor moves backwards, the piston follows the nut downward, allowing the check valve to seat. The brake pressure to the caliper is now a function of the controlled volume within the piston chamber. To reduce brake pressure, the motor drives the nut further downwards. To reapply or increase brake pressure, the motor drives the nut and piston upward. If ABS were entered during low brake pressure, such as in ice, and dry pavement is then encountered during reapply, the piston is driven all the way to the top. This results in the unseating of the check valve and a return to base braking until sufficient brake pressure exists to cause the wheel to approach lockup again. At this point, the ABS cycle would start again. Total brake pressure during ABS is limited to the brake pressure present when ABS was entered. Also, any time wheel brake pressure exceeds the brake pressure at the master cylinder (caused by reduced force on the brake pedal), the check valve unseats and a small volume of brake fluid is returned to the master cylinder to equalize pressure. In this manner, ABS VI cannot increase the brake pressure above the master cylinder pressure applied by the driver and cannot apply the brakes by itself. When ABS is no longer required, the pistons are returned to their upmost position, and the solenoids on the front channels are opened to provide an optional base braking path again. See Fig 1 . The rear channel operates in a similar manner except that no solenoid is used. Both rear brakes' pressures are controlled by the same motor, and both rear brakes' pressures are maintained at nearly equal levels. See Fig 2 .