With the launch of first generation Lexus GX 470 luxury utility vehicle and the fourth-generation Toyota 4Runner sport utility vehicle (SUV), the world has witnessed innumerous technological advancements. Topping the list were Hill-Start Assist Control (HAC) and Downhill Assist Control (DAC). The world’s first use of active wheel speed sensors improved the off-road capability.
When it comes to the functioning both HAC and DAC follow same approach. Vehicles equipped with both HAC and DAC systems has an active wheel speed sensor located in each wheel. The sensor detects the magnetic field using a rotating magnetic encoder located in the wheel hub. Once detected, based on change of magnetic poles, it delivers digital signals. With the detection of magnetic fields, magneto resistive effect came into play. The vehicle’s extra low speed and rotating wheel direction can then be measured with the active wheel speed sensor.
This is possible due to the proportionality between the output signal of the active wheel sensor and the amplitude of a magnetic field. Now as 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 direction of rotation is detected by active wheel sensor through the use of two MR elements. If there happens to be a change in rotation as detected by magnetic encoder, the phase between the two MR elements is shifted upside-down. This in turn enables the digital processing of signals by active wheel sensors and provides information on wheel direction. Through a two wired interface, the sensor output is provided in three levels of current. In the case of a forward movement, the output is a two-level pulse signal and if the event is of backward movement a three-level output pulse signal is provided.
The basic function of the HAC system is to help increase control on steep upgrades and stopping and starting on slippery surfaces. Designed to prevent the vehicle from rolling backward or slipping sideways during transition from a stopped position, HAC also helps to climb an upgrade. The system is automatically activated when the transmission is in any forward gear. It controls the brakes to help stop the individual wheel or wheels, which helps prevent the vehicle from rolling backward or slipping sideways.
The DAC improve directional control during descent on steep or slippery surfaces by assisting engine braking. The DAC system will control the brakes to help ensure the vehicle continues in a straight position when going down a hill. If the driver depresses the brake pedal, accelerator pedal, or the DAC button, the DAC system is automatically de-activated. 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 Toyota Prius, Land Cruiser, RAV4 V6 and 4Runner. HAC is also available as an option on select four-wheel-drive automatic transmission Tacoma pickup trucks. All Toyota Land Cruiser, RAV4 V6, and 4Runner models with 4WD are standard-equipped with the DAC system. The DAC system is also available as an option on select four-wheel-drive automatic transmission Tacoma models.

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