General System Description
ANTI-LOCK BRAKING SYSTEM (WHERE FITTED)Introduction
On vehicles fitted with conventional braking systems, hydraulic pressure developed by the master cylinder forces pads brake calipers against a disc attached to each road wheel. Instances can occur, however, especially on wet, gravel or snow covered roads, when the friction of pad against the disc is sufficient to use the road wheels to lock while the vehicle is still in motion: a hazardous situation in which steering control may be lost.
The anti-lock braking system eliminates wheel lock by modulating the hydraulic pressure applied to the brake calipers so that the adhesion between tire and road is maintained. The system, which is assisted by the power hydraulic system, also reduces tire wear and improves vehicle stability during severe braking conditions.
Modulation of the hydraulic pressure is achieved by a modulator containing three solenoid actuated valves; one valve in each hydraulic line to the front calipers and one in a common hydraulic line to the rear calipers. Each valve is controlled by the Electronic Control Unit (ECU) which also activates a hydraulic pump to return used fluid back to the modulator input.
The ECU contains electronic circuits which actuate the valve solenoids from inputs generated by speed sensors comprising variable reluctance coils located close to an exciter disc attached to each wheel hub. Each wheel deceleration rate is thus calculated and the pressure applied to the calipers is cycled through the following control sequence so that optimum brake efficiency is achieved:
- Closes inlet and outlet ports of the appropriate modulator valve so that the pressure applied to the caliper is held at the level at which brake efficiency was lost.
- Opens then re-closes the valve outlet port so that the caliper pressure is reduced sufficiently for the wheel to accelerate back towards its correct speed, excess hydraulic fluid displaced from the caliper being returned to the modulator by the pump.
- Opens and closes the valve inlet port in rapid succession so that the caliper pressure is re-applied in a gradual (stepped) manner. Should a loss of brake efficiency be detected as the pressure is increased, the entire control sequence is repeated.
Operation
Each wheel speed sensor generates an A.C. signal, the frequency and amplitude of which is directly proportional to the rotational speed of the wheel sensed. These signals are processed by the ECU when the brakes are applied with the engine running.
Engine running and braking indication is achieved by monitoring the alternator output and detecting the condition of a brake light simulation relay. An input from the alternator is therefore provided and applied to the ECU. The brake light simulation relay, which is energized when the footbrake is operated, routes ignition power to the ECU.
Power for the pump and valves is derived via relays which are energized, through contacts of a common over-voltage relay, when the ignition is ON. Due to the electrical currents involved, the pump and valve supplies are unfused.
Each valve is capable of performing any one of three functions; pressure increase, pressure hold and pressure decrease. These functions are selected by the ECU and are implemented as follows:
Pressure increase permits hydraulic pressure developed by the master cylinder to pass to the brake caliper via the valve inlet and outlet ports. Selected when no current is passing through the solenoid.
Pressure hold maintains the hydraulic pressure at the caliper with the valve inlet and outlet ports closed. Selected when a current of 2.1A ±0.2A is passing through the solenoid.
Pressure decrease permits hydraulic depressurization of the caliper through the outlet port with the inlet port closed. Selected when a current of 5.1A ±0.6A is passing through the solenoid.