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Downhill, Hill-Start Assist Control Systems Enhance Toyota Off-Road Capability


September 2006
 Filed under: TOYOTA Car News | TOYOTA Headlines
September 19, 2006 – Torrance, CA - The fourth-generation Toyota 4Runner sport utility vehicle (SUV) and first-generation Lexus GX 470 luxury utility vehicle, both launched for the 2003 model year, introduced numerous technological advancements. Among these all-new technologies were Hill-Start Assist Control (HAC) and Downhill Assist Control (DAC). Through one of the world's first uses of active wheel speed sensors, HAC and DAC helped the 4Runner and GX 470 improve their off-road capability.
In vehicles equipped with both HAC and DAC systems, an active wheel speed sensor is located in each wheel. The sensor uses a rotating magnetic encoder located in the wheel hub to detect a magnetic field. Once detected, it delivers digital signals based on the change of magnetic poles. The detection of the magnetic fields allows the use of magneto-resistive (MR) effects. The active wheel speed sensor is then able to measure the vehicle's extra low speed and rotating wheel direction.

Extra low speed detection is possible because the output signal of the active wheel sensor is proportional to the amplitude of a magnetic field. Because the magnetic field encoder does not depend upon the basic wheel rotation speed, the active wheel sensor can deliver a precise signal, even if the wheel speed is extra low.

The active wheel sensors detect rotation direction through the use of two MR elements. If the magnetic encoder detects a change in rotation, the phase between the two MR elements are shifted upside-down. This allows the active wheel sensors to digitally process the signals and provide wheel direction information. The sensor output is provided in three levels of current via a two-wired interface. In the case of a forward movement, the output is a two-level pulse signal. A three-level output pulse signal is provided in the event of backward movement.

The basic function of the HAC system is to help increase control on steep upgrades and stopping and starting on slippery surfaces. The HAC system is designed to help prevent the vehicle from rolling backward or slipping sideways during transition from a stopped position to climbing an upgrade. The system is automatically activated when the transmission is in any forward gear. The HAC system controls the brakes to help stop the individual wheel or wheels, which helps prevent the vehicle from rolling backward or slipping sideways.

The HAC system determines the driver's intended vehicle direction from the present gearshift position of the transmission, the moving direction of the vehicle wheels, and the wheel speed of the four wheels through the use of active wheel sensors. When backward rolling or slippage occurs, the HAC system is automatically activated. Upon activation the HAC system gently applies the brakes. The HAC system measures the necessary brake pressure of each wheel to help prevent individual wheels from locking and causing vehicle slippage.

The DAC system assists engine braking to improve directional control during descent on steep or slippery surfaces. When in 4WD low range and with DAC activated, if any one wheel accelerates beyond or below a two-to-four MPH low-speed setting, the DAC system will control the brakes to help ensure the vehicle continues in a straight position when going down a hill. The DAC system is automatically de-activated if the driver depresses the brake pedal, accelerator pedal, or the DAC button.

The active wheel speed sensors enable the operation of the DAC system by determining current road conditions. The system designed to determine current driver operation through an accelerator pedal angle sensor, master cylinder pressure sensor, and the setting of the L4 position switch. DAC next determines its target vehicle speed of two-to-four MPH based on road surface and vehicle driving conditions. When the vehicle accelerates during descent, allowing the vehicle speed to exceed the predetermined speed, DAC is activated. The DAC system will calculate the required brake hydraulic pressure for each wheel, allowing each individual wheel to meet the target vehicle speed.

The HAC system comes standard on all two- and four-wheel-drive Toyota 4Runners and all Lexus GX 470 SUV's. HAC is also available as an option on two- and four-wheel-drive Tacoma pickup trucks. All Toyota 4Runner models with 4WD and the Lexus GX 470 are standard-equipped with the DAC system. The DAC system is also available as an option on the four-wheel-drive Tacoma.

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Source: Toyota

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