ESP Control Unit, Function
ESP control unit, Function
The functions of the ESP control module are divided as follows:
- Signal conditioning
- Function logic component
- Safety circuit
- Auxiliary functions
Signal conditioning
The following input signals are processed by the function logic component for calculations or as information:
- Calculation of vehicle speed/slip
- Wheelspeeds, through CAN and direct
- Calculation of lateral forces
- Steering angle sensor signal (through CAN)
- Lateral acceleration sensor signal
- Signal from yaw rate sensor
- Calculation of longitudinal forces
- CAN data on engine torque
- CAN data on current gear
- CAN data on braking pressures
- Information signals (control)
- Brake light switch signal (through CAN)
- Parking brake switch signal (through CAN)
- ESP OFF switch signal (through CAN)
- Pedal travel sensor signal (through CAN)
Function logic component
The conditioned input signals are processed in the logic component and converted into output signals.
- Wheel speed sensor signals:
The 4 conditioned wheel speed signals are constantly compared with each other and with specified slip thresholds on the front and rear wheels in relation to the speed of the vehicle.
This comparison is used to determine the following values and control variables:
- Vehicle speed
- Acceleration/deceleration
- Brake slip (normal and ABS braking)
- Acceleration slip (ASR)
- Deceleration slip (EBR)
- Signal from steering angle sensor
If the vehicle changes direction or is being driven on a curve, this is detected using the signal from the steering angle sensor and on the basis of the different rotational speeds of the front wheels. These two recognition variables in combination with the vehicle speed provide the ESP control module with information on the driver's desired vehicle behavior.
- Lateral acceleration sensor signal:
The ESP control module uses the signal from the lateral acceleration sensor to determine the lateral forces occurring during cornering. The status controller in the ESP control module can detect whether the vehicle is oversteering from the lateral acceleration signal together with the yaw rate signal.
- Yaw rate sensor signal:
The ESP control module uses the yaw rate sensor to detect the movements (yaw rate) that the vehicle attempts to make about the vertical axis (e.g. when the vehicle is accelerated). Using the signals from the lateral acceleration sensor and yaw rate sensor, the ESP control module determines the actual handling characteristics of the vehicle.
- CAN data on engine torque:
The ESP control module is informed about the engine torque output by the engine control module via the CAN data bus.
In the case of ESP control mode, the function logic component tells the engine control unit to set a reduced engine torque.
- CAN data on current gear (automatic transmission only):
Via the CAN data bus the function logic component (ESP) is constantly informed about the currently engaged gear by the ETC control module. The current gear is used to calculate the drive forces acting on the drive wheels and for drive torque control.
- CAN data on braking pressures
The SBC control unit provides data on current braking pressures to the ESP control unit through the CAN data bus.
The brake pressure sensors detect the brake pressure which is used by the logic component to calculate the wheel brake forces (longitudinal forces). If ESP control becomes necessary, the existing wheel-brake forces (longitudinal forces) are taken into consideration when the cornering forces (lateral forces) are calculated.
- Brake light switch signal (S9/1through CAN)
Transmitted through CAN by SBC control unit. There it is generated based on data for pedal travel, pressure and brake light switch status. The ESP logic recognizes and processes these data. Transmission of the signal from the ESP control unit to the engine-management CAN for use by other systems.
- Signal of parking brake switch (S12via CAN):
If a signal is issued from the parking brake switch, EBR control mode is not permitted.
- ESP OFF switch signal (N72s14 through CAN)
When a signal from the ESP OFF switch is present, the circuitry for controlling the engine's output torque is deactivated, and the ESP and ABS indicator lamps remain on continuously.
Safety circuit
The task of the safety circuit is to identify erroneous signals from the sensors and faults in the control module and in the wiring system. During the control sequence, it monitors actuation of the high-pressure/return pump. The solenoid valves are monitored permanently. The system responds to recognition of malfunctions with self-deactivation accompanied by a display to alert the driver. In addition, a fault code is stored in the control module.
The safety circuit also constantly monitors the battery voltage. If the voltage falls below 10.5 V or exceeds 17.5 V, the system is switched off until the voltage returns to the specified range.
Indicator lamp actuation
The ESP control unit registers the signals for the ABS indicator lamp, the ESP and ABS warning lamps and the multifunction display, which it then relays to the instrument cluster through the CAN data wire.
Auxiliary functions
Speed-sensitive power steering subfunction
The control module uses the ground speed and status signal to calculate the current required to control the speed-sensitive power steering P-valve (Y10).
Wheel speeds and speed status signal outputs
- Wheelspeed output: The ESP control unit transmits data on instantaneous wheelspeed to all systems that require a wheelspeed signal for operation. The left front wheelpseed is also generated as a discrete output.
- Wheel speed status signal output: the ESP control unit transmits the wheelspeed status signal to all systems that require information on current vehicle status (vehicle stationary, in motion, wheelspeed sensor defective).