Lexus has for 2001 expanded availability of Brake Assist, the dramatic safety advance it introduced last year. Brake Assist is standard on all 2001 Lexus models except the IS 300, which uses electronic brake distribution to optimize performance of its anti-lock brake system (ABS).
Brake Assist can help reduce stopping distance during emergency maneuvers or "panic" braking. The system is designed to determine if the driver is attempting emergency braking and, if the driver has not stepped hard enough on the brake pedal to engage the ABS, it supplements the applied braking power.
Brake Assist joins Vehicle Skid Control (VSC), which Lexus introduced on some models for 1999, in addition to the standard anti-lock brake system (ABS). VSC can help control vehicle skids caused by loss of traction at the front or rear wheels. For 2001, VSC is newly standard on the RX 300 sport-utility vehicle. The system is also standard on the GS 300, GS 430, and LS 430 sedans and the LX 470 sport-utility vehicle. It is an option for the ES 300 sedan.
Most Drivers Don't Brake Hard Enough
While Lexus passenger vehicles provide outstanding braking performance, research has shown that many drivers - especially those with little experience - might not apply sufficient brake force when needed, such as during emergencies. Brake Assist is designed to measure the speed and force with which the driver applies the brake pedal to determine whether the driver is attempting to brake rapidly. If the system determines that is the case, it immediately applies additional pressure to take full advantage of the standard Anti-Lock Brake System (ABS).
The driver will therefore benefit from the vehicle's full braking power, while ABS will prevent brake lockup to help preserve steering control. Brake Assist intervenes only when the driver's action indicates an emergency-stopping situation and immediately reduces brake pressure when the driver lets up on the brake pedal.
How Brake Assist Works
When the driver depresses the brake pedal, the Skid Control ECU measures the depressing speed and force from the master cylinder sensor signals. Based on this data and the vehicle speed (calculated from the ABS wheel speed sensors), the ECU determines whether the vehicle is in a state of emergency braking.
If the ECU does determine a state of emergency braking, it causes the solenoid valves in the brake actuators and pump motor to operate. Hydraulic pressure generated by the pump goes to the individual brakes, generating higher brake pressure than called for by the brake pedal alone. The ECU controls the solenoid valves to provide the proper amount of assist force based on the driver's pressure on the brake pedal.
Vehicle Skid Control (VSC)
Many skids that cause a loss of vehicle control are caused by understeer, where the vehicle "plows" ahead even with the steering wheel turned sharply, or oversteer, where the vehicle turns more than the steering wheel position requires, causing it to swerve at the rear.
VSC helps control lateral skids on slippery or dry road surfaces by detecting and correcting understeer and oversteer conditions. The VSC system electronically monitors vehicle speed and direction, and compares the vehicle's direction of travel with the driver's steering, acceleration, and braking inputs. VSC can use throttle intervention and/or apply individual wheel braking to help control any discrepancy between where the driver is steering and where the vehicle is heading. The system integrates traction control to limit spinning of the drive wheels on slippery surfaces.
VSC uses some components shared with the anti-lock braking system (ABS) and an electronically controlled engine throttle, as well as a dedicated computer and sensors. These include: