ESP(Electronic Stability Program) System
Description of ESP
Optimum driving safety now has a name : ESC, the Electronic Stability Control.
ESC is based on the MK 25 ABS Hydraulic System. ESC recognizes critical driving conditions, such as panic reactions in dangerous situations, and stabilizes the vehicle by wheel-individual braking and engine control intervention with no need for actuating the brake or the gas pedal.
ESC adds a further function known as Active Yaw Control (AYC) to the ABS, TCS, EBD and EDC functions. Whereas the ABS/TCS function controls wheel slip during braking and acceleration and, thus, mainly intervenes in the longitudinal dynamics of the vehicle, active yaw control stabilizes the vehicle about its vertical axis.
This is achieved by wheel individual brake intervention and adaptation of the momentary engine torque with no need for any action to be taken by the driver.
ESC essentially consists of three assemblies : the sensors, the electronic control unit and the actuators.
The electronic control unit incorporates the technological experience accumulated in connection with the MK 20 system, but has been substantially expanded in terms of capacity and monitoring concept in order to permit the additional sensor signals and arithmetic operations to be processed and converted into corresponding valve, pump and engine control commands. Two 16-bit processors and one 8-bit processor, which monitor each other, cooperate to handle these requirements.
Of course, the stability control feature works under all driving and operating conditions. Under certain driving conditions, the ABS/TCS function can be activated simultaneously with the ESC function in response to a command by the driver.
In the event of a failure of the stability control function, the basic safety function, ABS, is still maintained.
DESCRIPTION OF ESC CONTROL
ESC system includes ABS/EBD, TCS and AYC function.
ABS/EBD function The ECU changes the active sensor signal (current shift) coming from the four wheel sensors to the square wave.
By using the input of above signals, the ECU calculates the vehicle speed and the acceleration & deceleration of the four wheels.
And, the ECU judges whether the ABS/EBD should be actuated or not.
TCS function prevents the wheel slip of drive direction by adding the brake pressure and engine torque reduction via CAN communication.
TCS function uses the wheel speed sensor signal to determine the wheel slip as far as ABS function.
AYC function prevents unstable maneuver of the vehicle. To determine the vehicle maneuver, AYC function uses the maneuver sensor signals
(Yaw Rate Sensor, Lateral Acceleration Sensor, Steering Wheel Angle Sensor).
If vehicle maneuver is unstable (Over Steer or Under Steer), AYC function applies the brake pressure on certain wheel, and send engine torque reduction signal by CAN.
After the key-on, the ECU continually diagnoses the system failure. (self-diagnosis)
If the system failure is detected, the ECU informs driver of the system failure through the BRAKE/ABS/ESC warning lamp. (fail-safe warning)
ESC CONTROL CYCLES
WHEEL PRESSURE MODULATION :
The ESC control cycles may be inserted into the ABS/TCS load collective at any point as long as the duty cycle for the electric motor does not exceed 20%.
INPUT AND OUTPUT DIAGRAM
INTERNAL BLOCK DIAGRAM
ESP OPERATION MODE
1. ESC Non-operation-Normal braking
Operation
In this position, the inlet valve and the TCS valve are open, the electrically operated shuttle valve and the outlet valve are closed.
* ESV : Electric reversing valve.
2. ESC operation
Operation
The on/off booster builds up a pressure of approx. 10 bar in order to enable the ESC pump to suck brake fluid at low temperatures. In this position, the inlet valve is driven in a pulsed cycle. The TCS valve is closed. The outlet valve remains closed. The electrically operated shuttle valve is opened. The hydraulic pressure is led to the wheel brakes which are to be applied for a brief period of time.
