BRAKE CONTROL SYSTEM

Anti-lock Brake System (ABS)

1. The ABS prevents the wheels from locking during sudden braking or braking on a slippery surface. The ABS provides the proper braking force when the vehicle slips, thus ensuring vehicle stability and excellent braking performance.

   

   *A	With ABS	*B	Without ABS
   *a	Brake Operation	-	-

Electronic Brake Force Distribution (EBD)

1. The EBD controls the brake force that acts on the rear wheels in accordance with the changes in the vehicle conditions such as load factors or deceleration in order to ensure excellent braking performance.

   

2. During braking while cornering, this function controls the brake force that acts on the left and right wheels in accordance with the vehicle conditions at that time. This ensures vehicle stability and excellent braking performance.

   

   Brake Force

   Control Moment

Brake Assist

1. In the brake assist, the skid control ECU calculates the speed and the amount of the brake pedal application based on the signals from the master cylinder pressure sensor and then determines the intention of the driver to make an emergency brake application. If the skid control ECU determines that the driver intends to make an emergency brake application, this function activates the brake actuator to increase the brake fluid pressure, which increases the brake force.

   

Auto LSD

1. The Auto LSD achieves the equivalent functions of a Limited Slip Differential (LSD) through the use of a traction control function. For this reason, the contents of the brake control are the same in both the Auto LSD and TRC. When the driver presses the Auto LSD switch, this function achieves the LSD effect by regulating the hydraulic pressure that acts on the drive wheels and controlling the engine output in accordance with the amount of pedal effort applied on the accelerator.

2. The TRC enhances the start-off performance of the vehicle during low-resistance surface conditions, such as snow or mud, by restricting the acceleration effort during a start-off in order to prevent the wheels from spinning.

3. On the other hand, the Auto LSD tends to enhance the acceleration effort somewhat in order to apply greater drive torque to the wheel when it is in contact with the ground. Thus, this function helps the vehicle to become unstuck if a wheel loses its grip, and enhances the vehicle's start-off performance on high-resistance surface such as gravel roads.

   

   *A	With Auto LSD	*B	Without Auto LSD
   *1	Brake Actuator	*2	Skid Control ECU
   *3	ECM	*4	VSC OFF Switch
   *a	Traction	*b	No Traction
   *c	Braking	*d	Slipping

Traction Control (TRC)

1. The TRC helps prevent the drive wheels from slipping if the driver depresses the accelerator pedal excessively when starting off or accelerating on a slippery surface. Simultaneously with the hydraulic brake control of the drive wheels, the skid control ECU makes a request to the ECM to effect engine output control. This produces the drive force that suits the driving conditions in order to ensure the proper start-off acceleration.

   

   *A	Without TRC	*B	With TRC
   *C	4WD/AWD Models	-	-
   *1	Engine	*2	ECM
   *3	Skid Control ECU	*4	Brake Actuator
   *a	Slippery Surface	*b	Engine Output Control
   *c	Brake Slipping Drive Wheel	-	-

Vehicle Stability Control (VSC)

1. The following are 2 examples that can be considered circumstances in which the tires exceed their lateral grip limit. The VSC is designed to help control the vehicle behavior by controlling the engine output and the brakes at each wheel when the vehicle is under one of the conditions indicated below:

   

   *a

   Front Wheel Skid Tendency

   *b

   Rear Wheel Skid Tendency

2. To determine the condition of the vehicle, sensors detect the steering angle, vehicle speed, vehicle's yaw rate and vehicle's lateral acceleration, which are then calculated by the skid control ECU.

3. Whether or not the vehicle is experiencing a front wheel skid is determined by the difference between the target yaw rate and the vehicle's actual yaw rate. When the vehicle's actual yaw rate is smaller than the target yaw rate (a target yaw rate is determined based on the vehicle speed and steering angle) that should be generated when the driver operates the steering wheel, it means the vehicle is making a turn at a greater angle than the target locus of travel. Thus, the skid control ECU determines that there is a large front wheel skid tendency.

   

   *a

   Actual Locus of Travel (Actual Yaw Rate)

   *b

   Locus of Travel Based on Target Yaw Rate

4. Whether or not the vehicle is experiencing a rear wheel skid is determined by the values of the vehicle's slip angle and the vehicle's slip angular velocity (time-dependent changes in the vehicle's slip angle). When the vehicle's slip angle is large and the slip angular velocity is also large, the skid control ECU determines that the vehicle has a large rear wheel skid tendency.

   

   *a	Direction of Travel of Vehicle's Center of Gravity	*b	Movement of Vehicle
   	Slip Angle	-	-

5. When the skid control ECU determines that the vehicle exhibits a tendency to experience a front wheel skid or a rear wheel skid, it decreases the engine output and applies the brakes of the front or rear wheels to control the vehicle's yaw moment. The basic operation of the VSC is described below. However, the control method differs depending on the vehicle's characteristics and driving conditions.

6. When the skid control ECU determines that there is a large front wheel skid tendency, it takes countermeasures in accordance with the extent of that tendency. The skid control ECU controls the engine output and applies the brakes of the front wheels and the rear wheel on the inner circle of the turn in order to help restrain the front wheel skid tendency.

   Figure 3. Front Wheel Skid Tendency

   

   *a	Making a Right Turn	-	-
   	Brake Force		Control Moment

7. When the skid control ECU determines that there is a large rear wheel skid tendency, it takes countermeasures in accordance with the extent of that tendency. The skid control ECU applies the brakes of the wheels on the outer circle of the turn and generates an outward moment of inertia in the vehicle in order to restrain the rear wheel skid tendency. Along with the reduction in the vehicle speed caused by the braking force, excellent vehicle stability is ensured.

   Figure 4. Rear Wheel Skid Tendency

   

   *a	Making a Right Turn	-	-
   	Brake Force		Control Moment

Cooperative Control with EPS

1. Braking when Surface Resistance Differs between Both Sides of Wheels

   
   

   • If the driver suddenly applies the brakes on a road surface with a considerable difference in friction coefficient between the right and left wheels, the difference in the brake force between the right and left wheels will cause the vehicle posture to become unstable and create a yaw moment. In this state, the skid control ECU controls the VSC to stabilize the vehicle posture. At the same time, the skid control ECU performs cooperative control with the EPS to provide steering torque assist, which facilitates the driver's steering maneuvers to stabilize the vehicle posture.

   

   *a	Assist Direction	*b	Yaw Moment during Brake Control
   *c	Low μ Road	*d	High μ Road
   	Brake Force	-	-

2. Accelerating when Surface Resistance Differs between Both Sides of Wheels

   
   

   • If the driver suddenly starts off or accelerates on a road surface with a considerable difference in friction coefficient between the right and left wheels, the slippage of a drive wheel will cause the vehicle posture to become unstable and negatively affect its acceleration performance. In this state, the skid control ECU causes the TRC to control the hydraulic brake of the slipping drive wheel, and requests the ECM to effect engine output control. At the same time, the skid control ECU performs cooperative control with the EPS to provide steering torque assist, which facilitates the driver's steering maneuvers to stabilize the vehicle posture.

   

   *a	Assist Direction	*b	Yaw Moment during Acceleration
   *c	Low μ Road	*d	High μ Road
   	Drive Force	-	-

3. Front Wheel Skid Tendency

   
   

   • When the skid control ECU determines a front wheel skid tendency, it controls the VSC to dampen the front wheel skid. At the same time, the skid control ECU performs the cooperative control with the EPS to provide steering torque assists, which controls the driver's steering maneuvers to stabilize the vehicle posture. Steering torque assists are provided to inform the driver of the front wheel skid (a), and to prevent the driver's excessive turning of the steering wheel (b). In the assist for preventing excessive turning (b), the skid control ECU increases the resistance to counter the driver's steering effort, if the driver turns the steering wheel excessively.

   

   *a	(a) Assist Direction for Informing Driver of Front Wheel Skid	*b	(b) Assist Direction for Preventing Excessive Turning
   *c	Making a Right Turn	-	-
   	Brake Force		Control Moment

4. Rear Wheel Skid Tendency

   
   

   • When the skid control ECU determines a rear wheel skid tendency, it controls the VSC to dampen the rear wheel skid. At the same time, the skid control ECU performs cooperative control with the EPS to provide steering torque assist, which facilitates the driver's steering maneuvers in the direction to correct the rear wheel skid.

   

   *a	Assist Direction	*b	Making a Right Turn
   	Brake Force		Control Moment

Cooperative Control with 4WD/AWD System

1. Acceleration During Cornering

   
   

   • A sudden acceleration of the vehicle during cornering may cause a drive wheel to freewheel, which could cause the front wheels or rear wheels to skid. If the skid control ECU determines the freewheeling of a drive wheel, a front wheel skid tendency or a rear wheel skid tendency, it performs cooperative control with the 4WD/AWD system to optimally control the drive torque distribution to the front and rear wheels. Furthermore, the skid control ECU controls the TRC and the VSC as needed to ensure driving stability and acceleration performance.

   

   *a	Making a Right Turn	-	-
   	Drive Torque Distribution to Rear Wheel		Drive Force

   Figure 5. Rear Wheel Skid Tendency

   

   *a	Making a Right Turn	-	-
   	Drive Torque Distribution to Rear Wheel		Drive Force

Hill-start Assist Control

1. When the vehicle starts off on a steep or slippery hill, it could descend backward while the driver switches from the brake pedal to the accelerator pedal, thus making it difficult for the vehicle to start off. To prevent this from occurring, the hill-start assist control temporarily (approximately 2 seconds at the maximum) applies the brakes to all the wheels in order to prevent the vehicle from descending backward.

2. Without the hill-start assist control, the driver must quickly and precisely switch from the brake pedal to the accelerator pedal. With the hill-start assist control, however, the driver can start off easily and operate the pedal in a relaxed manner because the hill-start assist control prevents the vehicle from descending backward.

   

   *A	With Hill-start Assist Control	*B	Without Hill-start Assist Control
   *a	Easy Control	*b	Difficult Control
   *c	Prevent Vehicle from Descending Backward	*d	Increase Backward Speed of Vehicle

Downhill Assist Control

1. When the vehicle is descending a steep hill and engine brake alone cannot provide a sufficient deceleration force while the transaxle is in the L or R range, the downhill assist control performs 4-wheel brake control to maintain a constant, low vehicle speed. Thus, the vehicle is able to descend in a stable manner without causing the wheels to become locked.

2. When the vehicle descends a steep hill without the downhill assist control, the driver must pay close attention to the brake and accelerator pedal operations. However, with the downhill assist control, the driver can concentrate on the steering operation without accelerator and brake pedal operations.

3. The downhill assist control enables the vehicle to achieve a high level of stability because it can descend a slippery hill at low speeds without causing the wheels to become locked.

   

   *A	With Downhill Assist Control	*B	Without Downhill Assist Control
   *a	Easy Control (No Braking and No Accelerating)	*b	Difficult Control
   *c	Stable Speed	*d	Unstable Speed