Electric power steering (ES), function - GF46.35-P-0001GN
MODEL 166 as of model year 2016
MODEL 292
Block diagram
Function requirements, general
- Status "Circuit 15 ON"
The status "Circuit 15 ON" is transmitted by the electronic ignition lock control unit via chassis CAN 2 to the electrical power steering control unit.
Electric power steering (ES), general
The ES consists of the rack-and-pinion steering gear, the electric power steering torque sensor, the electric power steering actuator motor and the electrical power steering control unit.
The ES performs continuous, speed-proportional power steering control.
The advantages compared to hydraulic power steering are:
- Improved steering feel
- Fuel saving
- Hydraulic fluid not required
- Compact design
- Speed proportional steering
- 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 steering assistance:
- Current steering moment, from electric power steering torque sensor
- Current wheel speeds, determined via the left front axle rpm sensor, right front axle rpm sensor, left rear axle rpm sensor and right rear axle rpm sensor and made available via chassis CAN 1, electronic ignition lock control unit and chassis CAN 2 by the Electronic Stability Program control unit
- Steering wheel angle and steering wheel angle rate of change, from the steering wheel angle sensor in steering column tube module control unit over chassis CAN 1, the electronic ignition lock control unit and chassis CAN 2
- Current engine speed > 400 rpm via chassis CAN 1, electronic ignition lock control unit and chassis CAN 2 from the ME-SFI [ME] control unit (with gasoline engine) or CDI control unit (with diesel engine)
The electrical power steering control unit uses this information to calculate the steering assistance to be set from the stored characteristics map and actuates the electric power steering actuator motor accordingly.
The steering support provided by the electric power steering actuator motor is transferred to the rack-and-pinion steering gear via a belt drive. The direction of rotation of the electric power steering actuator motor depends on the direction of movement of the steering wheel.
The steering assistance is transmitted to the rack-and-pinion steering gear by the electric power steering actuator motor, thereby assisting the steering torque applied by the driver.
When the vehicle is stationary, no electric power steering is available if the engine and the ignition are switched off. With the ECO start/stop function, reduced power steering is available when the vehicle is at a standstill if the engine was switched off via the ECO start/stop function. The steering power is deactivated after the ignition is switched off. Electric power steering is active if the vehicle is maneuvered with the engine switched off and the ignition switched on.
Steer Assist function sequence
The steering assistance function helps the driver to achieve an optimum steering reaction in critical driving situations. Steering assistance is provided with the aid of the electric power steering control unit and 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 vehicle status is sensed by the Electronic Stability Program control unit.
If necessary, the message "Steer Assist torque request" is sent via the chassis CAN 1 and chassis CAN 2 with the electronic ignition lock control unit interface to the electrical power steering control unit, which actuates the electric power steering actuator motor accordingly. For this purpose, the parking system control unit (with CODE 235 (Active Parking Assist)) sends the following signals over the chassis CAN 2 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 (with CODE 235 (Active Parking Assist)):
- Current status of the electrical power steering control unit
Function sequence for reducing holding torque
If a holding torque is applied by the driver when driving the vehicle straight ahead, e.g. for a crosswind or on a road surface inclined on one side, the electrical power steering control unit detects it. The applied holding torque is partially compensated for by the electric power steering.
To rule out any faulty compensation (e.g. highway cornering with large radius), the electrical power steering control unit receives the rotational speed (vehicle speed about the vertical axis) and the lateral acceleration of the vehicle over the chassis CAN 1 and chassis CAN 2 with the electronic ignition lock interface from the Electronic Stability Program control unit. The reduction of the vehicle's holding torque depends on the speed.
Function sequence for limp-home mode
When the ES is switched on (through "circuit 15 ON"), the electric power steering control unit runs a self-diagnosis procedure. If a fault is detected, the electric power steering is not switched on. The vehicle can be steered with increased force of the hand. During driving operation, 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 ES 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 transmits the request for a warning message via chassis CAN 2 to the instrument cluster.
The instrument cluster's multifunction display then shows the warning message "Power steering defective! Service Required!" is then displayed in the instrument cluster (IC) control unit.
| Electrical function schematic for electric power steering | PE46.35-P-2050-97NBA | ||
| Overview of system components of electric power steering (ES) | GF46.35-P-9997GN |