Electric Power Steering (ES), Function - GF46.35-P-0001FR
Model 257, 290 up to model year 2021
Block diagram
Function requirements, general
- Status of "Circuit 15 ON"
The status of circuit 15 ON is sent by the electronic ignition lock control unit over the suspension FlexRay to the electrical power steering. The engine speed is sent to the electrical power steering control unit over the suspension FlexRay from drivetrain control unit.
Electric power steering (ES), general
The electric power steering consists of the steering gear for the rack-and-pinion steering, the torque sensor the electrical power steering, the actuator motor the electrical power steering and the electrical power steering control unit. The ES performs continuously variable, speed-dependent regulation of the power steering.
The advantages compared to hydraulic power steering are:
- Improved steering feel
- Fuel saving
- Hydraulic fluid not required
- Compact design
- Speed-dependent power assistance
- Steering return is assisted.
- Diagnosis capability
System diagnosis takes place via the electronic ignition lock control unit, the diagnostic CAN and the diagnostic connector using Xentry Diagnostics.
The ES comprises the following subfunctions:
- Function sequence for electric power steering
- Steer Assist function sequence
- Function sequence for reducing holding torque
- Function sequence for limp-home mode
- Display on instrument cluster function sequence
Function sequence for electric power steering
The electrical power steering control unit requires the following information to calculate the required amount of assistance:
- Current steering torque from torque sensor for electrical power steering
- The current wheel speeds as calculated by the left front axle rpm sensor, right front axle rpm sensor, left rear axle rpm sensor and right rear axle rpm sensor and sent via the suspension FlexRay by the ESP® control unit.
- Steering wheel angle and steering angle rate of change, via the chassis FlexRay from the steering wheel angle sensor in the steering column tube module control unit
- Current engine speed > 400 rpm, over the suspension FlexRay and engine CAN with the powertrain control unit interface from the CDI control unit (with diesel engine) or the ME-SFI [ME] control unit (for gasoline engine)
The electrical power steering control unit uses this information to calculate the level of assistance to be applied for the electric power steering from the stored characteristics map and activates the electric power steering actuator motor accordingly. The direction of rotation of the electric power steering actuator motor depends on the direction of movement of the steering wheel. The assistance provided by the electric power steering is transmitted from the electric power steering actuator motor to the rack-and-pinion steering gear, thereby supporting the steering torque applied by the driver.
When the vehicle is at a standstill, no power steering is available if the engine and ignition are switched off. With the ECO start/stop function, reduced power steering is available at standstill if the engine was switched off using the ECO start/stop function. The steering power is deactivated after the ignition is switched off. Power steering is active if the vehicle is maneuvered when the engine is off and the ignition is switched on.
Steer Assist function sequence
The steering assistance function helps the driver to achieve an optimum steering reaction in critical driving situations. The Steer Assist function is realized using the electrical power steering control unit and the Electronic Stability Program control unit.
The following functions are performed here:
- Driver support through countersteering for a vehicle with oversteer
- Driver support for braking on varying road surfaces
The current driving status is detected by the Electronic Stability Program control unit.
If required, the "Steering assistance torque request" message is sent over the suspension FlexRay to the electrical power steering control unit, which actuates the electric power steering actuator motor accordingly.
When parking, the parking system control unit sends the following signals over the suspension FlexRay to the electrical power steering control unit:
- Current status of parking system control unit
- Vehicle stop signal, at which the vehicle stops at a maneuver point during the parking process.
- Requested toothed rack position
The following signals are sent by the electrical power steering control unit to the parking system control unit:
- Current status of the electrical power steering control unit
Function sequence for reducing holding torque
If a holding torque is exhausted by the driver when driving the vehicle straight ahead e.g. in a crosswind or on a roadway inclined on one side, it is detected by the electrical power steering control unit. The applied holding torque is partially compensated for by the electric power steering.
To rule out any faulty compensation (e.g. highway corner with large radius), the electrical power steering control unit receives the turn rate (vehicle speed around its vertical axis) as well as the vehicle's lateral acceleration from the Supplemental Restraint System control unit over the vehicle dynamics CAN and the suspension FlexRay with the Electronic Stability Program control unit interface. The reduction of the vehicle's holding torque depends on the speed.
Function sequence for limp-home mode
When the electric power steering is switched on (over circuit 15 on), the electrical power steering control unit runs a self-test. If a fault is detected, the power steering is not switched on. The vehicle can be steered with increased force of the hand.
While driving the electrical power steering control unit checks the signals from the electric power steering torque sensor. If these signals are outside the defined upper or lower limit, the electric power steering is switched off by the electrical power steering control unit. The vehicle can be steered with increased force of the hand.
Display on instrument cluster function sequence
The electrical power steering control unit sends the request for a warning message over the suspension FlexRay and the user interface CAN with the electronic ignition lock control unit interface to the instrument cluster. The warning message "Power Steering Defective! Service Required!" is then displayed.
| Electrical function schematic for electric power steering | PE46.35-P-2050-97XBA | ||
| Overview of system components of electric power steering (ES) | GF46.35-P-9997FR |