ABS 5 (3-Channel System)
NoteOperation, installation space requirements and weight of the ABS 5 have been optimized over the ABS 2 system.
ABS operation
The ABS control unit receives the stop light switch signal and the AC signals supplied by four speed sensors. Two microprocessors then convert these signals independently into corresponding digital wheel speed signals. The wheel speed signals are used to calculate wheel slip (nearly proportional to the calculated vehicle reference speed).
If both a vehicle deceleration condition and excessive slip at one wheel are detected, a pressure retaining period is initiated, i.e. the inlet solenoid valve on the corresponding wheel is closed in order to avoid any further pressure buildup. If the wheel continues to tend to lock even though the pressure is held constant, the pressure in the wheel cylinder is reduced. For this purpose, the outlet solenoid valve opens and the return pump returns the brake fluid to the master cylinder (pressure reduction phase) until the wheel starts rotating again. Further cycles adapted to the cycle just completed are then initiated.
To avoid faulty control due to bumps in the road surface, the instability threshold is made more insensitive as a function of the road surface condition.
This control design offers the following advantages over the ABS 2 generation:
^ Reduced braking distance when braking on uneven road surfaces
^ Improved control quality, felt by driver as equal deceleration during ABS braking operations
^ Improved pedal feedback comfort thanks to finer tuning of pressure pulse control.
This function, i.e. the input signals, is constantly monitored. If a fault is detected, the control unit switches the ABS feature off, activates the ABS warning lamp and stores the fault in a non-volatile memory in the control unit. As an additional measure, a test program is invoked whenever a speed of 6 km/h is exceeded after starting the vehicle. This program triggers the solenoids and the pump motor electrically and checks them. If a fault is detected, the control unit switches off the ABS function, the ABS warning lamp lights up and the fault is stored.
Schematic diagram - ABS 5
1 - ABS control unit
2 - Tandem brake master cylinder
3 - ABS hydraulic unit (3 hydraulic outputs)
4 - ABS sensors
5 - Central Information System
6 - Proportioning valve (1x)
7 - Brake light
8 - Stop light switch
9 - ABS warning lamp