Fluid Pressure Control of Hydraulic Unit For ABS Control

FLUID PRESSURE CONTROL OF HYDRAULIC UNIT FOR ABS CONTROL





1) Normal Conditions (non-ABS control operation)
When the H/U (Hydraulic control Unit) solenoid valves and motor are not operating, the section from the M/C (Master Cylinder) to the W/C (Wheel Cylinder) is open (inlet valve open) and the section from the W/C to the M/C reservoir is closed (outlet valve closed) Also, the special valve is open. When the brake pedal is depressed, the fluid pressure produced by the master cylinder is applied in the primary system from TCS1 to FL W/C via FLI and to RR W/C via RRI. At the same time, in the secondary system, the fluid pressure is applied from TCS2 to FR W/C via FRI and to RL W/C via RLI. TCS valves (TCS1, 2) are OFF (open).
When the brake pedal is released, the fluid pressure at each W/C flows back through the same channels and is released. Since the each FRI RRI channels constrict the oil flow, they are provided with check valves from which the pressure is also released at this time.





2) Pressure-drop conditions
In order to eliminate the fluid pressure acting on the W/C and release excessive brake force, the inlet valve is ON (closed) and the outlet valve is ON (open), resulting in pressure-drop conditions. Figure shows hydraulic oil channels under these conditions (FRI, FRO RRI, RHO all under pressure-drop conditions).
Under these conditions the section between the M/C and W/C is closed and the section between the W/C and M/C reservoir is opened by solenoid valve switching. The fluid pressure entering the W/C from the M/C pressure chamber is released to the M/C reservoir, and the W/C pressure drops because of the pressure-drop conditions illustrated below.





3) Pressure-hold conditions
During ABS control, the fluid pressure is kept constant and pressure-hold conditions are maintained by the valves before or after the pressure rise/drop control, i.e.,the inlet valve is ON (closed) and the outlet valve is OFF (closed). The figure shows hydraulic oil channels under these conditions (all channels under pressure-hold conditions). Under these conditions the sections between the M/C and W/C and between the W/C and M/C reservoir are closed by solenoid valves (FRI, FRO - RRI, RRO), and the W/C fluid pressure is kept constant.





4) Pressure-rise conditions
Once wheel lock can be avoided due to the drop in pressure, the inlet and outlet valves both go OFF in order to increase the W/C fluid pressure again and secure braking force. Hydraulic oil channels under these conditions are shown in Figure. FRI, FRO - RRI, RRO are all shown under pressure-rise conditions. These conditions are the same as normal hydraulic oil channel conditions, i.e., the section between the M/C pressure chamber and W/C is open and the section between the W/C and M/C reservoir is closed. By keeping FRI - RRI open between the M/C pressure chamber side (brake pedal is depressed, so pressure is produced) and the W/C side, the M/C pressure is conducted to the W/C. The actual control at this time increases the pressure in steps, repeatedly increasing and holding the pressure, until the tendency for the wheels to lock re-occurs. At that time, since the M/C fluid enters the W/C, the brake pedal is stroked in stages. The stroke rate is detected by the stroke sensor, and when it has reached the predetermined value, the motor pump operates and fluid is pressure-fed from the M/C reservoir to the M/C pressure chamber, and the brake pedal returns to its original position. At this time the driver will experience kickback.