Electric Servo Brake System Description
Overview
The electric servo brake system is used to convert as much of the vehicle's kinetic energy as possible into electricity and store it in the battery. The system enables to maximize the regenerative brake force depending on the brake pedal pressure available. The braking force is determined by the amount of regenerative electricity and the brake. When vehicle speed is low, the brake creates most of the braking force.
Electric servo brake system performs the following control:
- Servo control: Reduces the driver's effort for operating the brake pedal.
- Regeneration control: Coordinated control with the regenerative brake by the traction motor.
- Stall management: Protective control of the traction motor.
- Brake assist control: Increases the braking force when emergency braking is detected.
- Hill start assist control: Startup assist on a slope road.
- Automatic brake hold: Reduces the driver's load for operating the brake pedal.
On vehicles equipped with ACC and CMBS, the electric servo brake system performs brake control for the ACC and the CMBS.
Brake Control
If vehicle speed is low, the amount of regeneration by the traction motor decreases, thus causing a decline in braking effort. The electric servo brake control unit drives the motor of the tandem motor cylinder to increase fluid pressure while minimizing the change in braking effort, enabling a smooth transition from regeneration brake to hydraulic brake.
Servo Control
System Shutdown State
When the electric servo brake system is shut down, the master cylinder cut valve (MCV) opens and the pedal feel simulator cut valve (PFSV) is closed while the brake pedal is not in use.
System Startup State
The electric servo brake system activates when the driver's door is opened. When the system activates, the motor starts its initial operation. The master cylinder cut valve (MCV) opens and the pedal feel simulator cut valve (PFSV) is open until the brake pedal is pressed.
Servo Control Operations
During normal operation, the master cylinder cut valve (MCV) closes and the pedal feel simulator cut valve (PFSV) is open. Under these conditions, the system can prevent the brake hydraulic pressure created by the braking pedal force from transmitting to the VSA modulator-control unit. Pedal feedback is created virtually by the pedal feel simulator. The hydraulic pressure for the VSA modulator-control unit is created when the piston in the slave cylinder gets pushed by a thrust force. The thrust force is transmitted from the rotary torque of the motor in the tandem motor cylinder using a gearbox. The required hydraulic pressure is calculated by the electric servo brake control unit using the stroke sensor signal and controlled by the rotary angle of the motor in the tandem motor cylinder.
Regeneration Control Operations
When the brake and regeneration system work together, the master cylinder cut valve (MCV) closes and the pedal feel simulator cut valve (PFSV) is open. The electric servo brake control unit uses the information on the amount of charge in the regenerative electricity system to activate the motor in the tandem motor cylinder and reduce the hydraulic pressure. At the same time, the unit also increases the amount of charge in regenerative electricity system.
Fail-Safe
During the fail-safe mode, the master cylinder cut valve (MCV) opens and the pedal feel simulator cut valve (PFSV) is closed. The hydraulic pressure created by the brake pedal force activates the brake caliper.
Stall Management
When driving uphill, the driving force of the vehicle's drive motor balances against the gravitational force that pulls the vehicle back. Electric current continues to flow only to specific motor drive circuits, generating heat in them. If driving uphill for an extended period of time, the amount of heat generated could damage the circuit and motor. To prevent overheating, the stall cooperative control activates the electric servo brake, which prevents the vehicle drive circuit from overheating. When the brake or the accelerator pedal are pressed, the stall cooperative control stops.
System Wake-Up and Shutdown
The electric servo brake system is activated when the driver's door is opened while the vehicle is in the OFF (LOCK) mode or when the vehicle is turned to the ON mode.
When the electric servo brake system is not activated (the vehicle is turned to the ON mode while pressing the brake pedal) the brake pedal may travel down as the pedal feel simulator cut valve (PFSV) opens. This is normal and not a system fault. To prevent the driver from feeling this condition at system start-up, the electric servo brake system is activated for 3 minutes, triggered by the driver's door switch signal when the driver's door is opened. To prevent excessive battery discharge, the system will automatically shut down after 3 minutes from when the driver's door is opened.
Activating and shutting down the system by the driver's door operation is as follows.
Open the driver's door
The electric servo brake system is activated when the driver's door is opened, and will shut down automatically 3 minutes after the driver's door was opened with the vehicle in the OFF (LOCK) mode.
Turn the vehicle to the ON mode after opening the driver's door
If the vehicle is turned to the ON mode within 3 minutes after opening the driver's door, the electric servo brake system will continue to operate.
Open the driver's door again after the system is activated by opening the door
Once the driver's door is opened and the system is activated, the system's activation time will not be extended even if the door was opened again.
Open the driver's door after turning the vehicle to the OFF (LOCK) mode from ON mode
The system will shut down automatically 3 minutes after the vehicle is turned to the OFF (LOCK) mode, and the system's activation time will not be extended even if the door is opened.